Recently while trying to answer this myself, I found a lot of discussion on this subject, but no definitive answer.
M.2 B Key sockets have a variety of possible modes as defined by the NGFF specification:
The second to last mode “RFU” (Reserved for future use) is defined by Dell as:
I am unsure what HCA stands for, but have seen it printed on the silkscreen of some of my earlier latitude models. Clearly it is some kind of proprietary device that requires one PCIe lane. It may stand for Host Channel Adapter – Implying some kind of non-NVMe PCIe SSD (i.e. has an Option ROM).
None of this tell us anything about what Dell have actually implemented on thier WWAN slots, neither does their tech support, or any of their documentation. I found myself eyeballing the traces extending from the socket, but even this was inconclusive as a lot of them are fed up from vias under the socket.
It’s given that it’ll have SSIC/USB 2.0 because almost all WWAN cards use those interfaces, but what of the others?
Fortunately there’s no need for any further conjecture because the schematics for these models are floating around the internet, so let’s answer that question definitely.
The above diagram covers models 7280, 7380, 7480, 7490 and likely others too. So there we have it. The interfaces on the WWAN socket are:
SSIC (Chip-to-Chip USB 3.0)
PCIe (Permanently disabled in BIOS)
Just because we know what interfaces are there, we still don’t know what kind of peripherals will actually work. For example – if a socket can accept WWAN-PCIe – SSD-PCIe should also be no problem as the required connections are all there, however those devices will have their configuration pins tied differently, allowing the BIOS to determine exactly what’s attached.
This allows manufacturers (for example) to allow PCIe WWAN cards, but disallow PCIe SSDs. On top of that, just because an interface is there, it doesn’t mean that it’s actually enabled.
This turns out this is the case for my Dell Latitude. While the PCIe is there, it cannot be used because the port on the root complex is disabled in the BIOS, and there’s no way to enable it (without hacking the BIOS). I was able to confirm this by testing out a variety of B-Key PCIe devices, none of which were detected (even when strapped as WWAN-PCIe).
A dick move by dell, but given how rare WWAN cards requiring PCIe are, they had no reason to enable it, and my experience from owning previous models is that they’re pretty good at tying up loose ends like this.
Short of hacking the BIOS, or building a new type of USB 3.0 card from scratch, only WWAN cards will work in the WWAN slot.
If like me you happen to have a Thunderbolt 3 enabled laptop, and are finding yourself wanting to attach arbitrary PCIe peripherals to it, you may have found yourself looking for some kind of adapter board.
You may also be aware that such items are strictly forbidden under the Intel Thunderbolt licensing programme. This is because thunderbolt chips are only sold for use on certified products. As a device like this is only part of a product, it could never be certified.
But that doesn’t technically stop it from existing. In theory this problem should go away with the upcoming release of USB4, but that may be some time away.
You may have seen one of these on either eBay or AliExpress:
AR SP BPD PCIe_Rev_2p1
But what is it? How can it exist? It appears to have the same dark green hue and silk-screen font as an Intel reference board (of which I’ve got quite few). I’d be comfortable to say they did indeed make this.
Quite how they’ve ended up for sale is an interesting question. What’s even more interesting was the transaction its self. I bought this off AliExpress, paid in U.S. Dollar, apparently ship from China in 20 days… but…
It arrived the next day! Shipped from the Amazon UK warehouse?!
This certainly adds the the intrigue of who exactly it is that is selling them, not to mention how. The punchy price tag tells us they haven’t got an unlimited supply of them.
Sellers generally advertise them as for SSD use only. This is more because it only supplies 3.3V to the attached peripheral than anything else. Most PCIe cards also require 12V which this board does not supply. That doesn’t stop us from providing the 12V separately.
Plugging it in
The thunderbolt info dialog describes it as an “LT-LINK Node Lite” – a similar name to the AKiTiO Node Lite. We also see the warning there about a graphics device which “may not function properly” telling us that the firmware on this board was for some kind of eGPU enclosure.
To test it out I’ve decided to throw a rather curly scenario at it –
I’ll be attaching it to my ExSys PCI chassis. Fitted will be an Intel PRO/100B – a 24 year old conventional PCI adapter, which doesn’t support 64-bit DMA (nor does it have 64-bit drivers). Fortunately the source code for its driver is in the Windows DDK, so I’ve compiled it for 64-bit Windows for this test…
This test will also add another couple of PCI-to-PCI bridges onto the existing arrangement in the Thunderbolt chain.
The chassis is connected to the Thunderbolt adapter via the PCIe card option.
Even with a long chain of bridges, a transition to conventional PCI, and the rather legacy nature of what I’ve attached – it works, and passes traffic.
So there we have it. You can indeed use peripherals other than SSDs on these!
A little while back I wrote a post on silencing a Cisco 2911 for home use. You can read about that here. I accept that it’s unlikely anyone would be using 2951 at home however you may find yourself in a situation where you need to reduce the noise level of this router. In my 2911 guide I went to the extent of rebuilding the power supply as a 12V DC version to reduce heat output. We’re not going to be bothering with that here, instead we’ll be looking solely at the fans. The first task is to remove the four original fans. Remove all the screws you can see on the fan module, and release the plastic clips holding the face plate.
Next step is to unpick the contacts from the receptacle I personally used the Molex Mini-Fit Jr extraction tool (11-03-0044) – which is the correct tool for this, however you may find it more convenient to just cut the wires off the old fans and splice them onto the new fans.
In my case I’ve used the same gold-plated Mini-Fit Jr contacts (42815-0012) as were used on the original fans. You may not feel the need to bother with those. You can read more about these connectors here. Just in case it isn’t obvious – the pinout for that connector is as follows:
1 : GND (Fans 4, 3)
2 : +12V (Fans 4, 3)
3 : PWM 3
4 : TACH 3
5 : +12V (Fans 1, 2)
6 : GND (Fans 1, 2)
7 : Module presence strap
8 : PWM 4
9 : TACH 4
10 : PWM 2
11 : TACH 2
12 : PWM 1
13 : TACH 1
14 : Module presence strap
The next step is to re-install connections for fans 1 and 2 only, as we’re going to blanking up the holes for fans 3 and 4. If you’re not replacing the contacts entirely like me, you won’t need to be bothering with this.
As previously mentioned, connections for fans 3 and 4 are omitted, except we’ve strapped the tach signal from fans 1 and 2 to them to keep the software satisfied that all 4 are present. Now we have to assemble the new fan module. I’ve used two Delta AFB0612VHC fans. I don’t personally see the need to go splashing out on expensive “ultra silent” fans from the PC modding scene. You can if you like but they’re not going be much (if any) quieter than these.
I have also blanked up the holes for fans 3 and 4, to ensure airflow remains consistent. Next step is to re-assemble the fan module, and plug it back into the router – making darn sure you’ve wired it properly first. Now, the obligatory ‘show env’ to check that everything’s OK:
SYSTEM POWER SUPPLY STATUS
Internal Power Supply Type: AC
Internal Power Supply 12V Output Status: Normal
External Redundant Power Supply is absent or powered off
SYSTEM FAN STATUS
Fan 1 OK, Low speed setting
Fan 2 OK, Low speed setting
Fan 3 OK, Low speed setting
Fan 4 OK, Low speed setting
SYSTEM TEMPERATURE STATUS
Intake Left temperature: 21 Celsius, Normal
Intake Right temperature: 20 Celsius, Normal
Exhaust Left temperature: 34 Celsius, Normal
Exhaust Right temperature: 25 Celsius, Normal
CPU temperature: 59 Celsius, Normal
Power Supply Unit temperature: 49 Celsius, Normal
REAL TIME CLOCK BATTERY STATUS
Battery OK (checked at power up)
Motherboard Components Power consumption = 55.3 W
Total System Power consumption is: 55.3 W
Environmental information last updated 00:00:04 ago
As I stated in my previous 2911 guide, when we make modifications like this, we’ve got to consider the consequences. For the 2951 you’ll not be able to use it with any service modules. If you need to install service modules, I would recommend using 4 of these fans.
How much quieter is it?
To save you from asking – it now makes about as much noise as a middle of the range ATX power supply under a favourable conditions (i.e. small load, and room temp of 20 degrees Celsius.
We’ve all heard that they’re different (somehow?) I thought this was going to be straight forward, turns out I was wrong.
Before we get into the details, we need to define what exactly an “AMP” plug is. In the first instance an AMP connector is made not by AMP, but CommScope. The AMP company was dissolved many years ago and its products have gone through various rounds of divestment and acquisition. Other brands this type of connector has been sold under (historically) are “Tyco” and “TE Connectivity”.
Non-AMP plugs are generally referred to as Stewart Stamping (SS) or Western Electric (WE) Style.
The second thing to consider is that eBay is awash with counterfeit products described as AMP plugs, but are not the products of any of the aforementioned suppliers, nor are they even physically similar to the genuine articles. Most of these are low quality SS/WE style plugs, and should not be used with AMP tools.
There are two distinct varieties of “AMP” (ish) connector.
Traditional or “Line” style (as their documentation refers to them)
“High Performance” style
Example part numbers:
6-557315-3 (Line style, round, 8P8C)
6-569278-2 (High performance style, round, 8P8C)
Since you are here because of differences in tooling, let’s get straight into that. It turns out that these two require different tools:
Which one is the fabled “AMP” connector? Both varieties are quite different to regular SS/WE plugs, however from a tooling perspective, the “black dot” tool for “Line” style plugs is significantly different. It is this type which other manufacturers sell tools labelled as “AMP” style.
For comparison with SS/WE I’ll be using the Stewart 2990003-01 tool (2990006-01 Die, yellow dot).
When we look at the “black dot” die next to the one from the Stewart tool, the difference is clear. There is a third punch-down in the centre which crimps part of the plug onto the stripped wire, a feature that no other type of plug has. Because of this, you cannot use this tool to crimp most SS/WE plugs.
As for the “white dot” tool and associated “high performance” plugs, they’re not exactly the same as SS/WE plugs and associated tools but I’ve found that they are more-or-less interchangeable – I certainly could not see and problems with mixing them.
Since we’ve got all of this here – let’s put a clearly non AMP plug into the “black dot” die and see what kind of mess we end up with…
Not a great result, and unlikely to work very well.
But wait! Some non-AMP plugs are actually sort-of compatible with the “black dot” tool
I would’t be making a habit of this. As we can see the wire has been damaged by plastic being squashed by the middle punch-down. Not recommended!
The “high performance” feature
If you care enough to read on…
On the left is an example of one, and on the right we’ve got the bog standard alternative. Please excuse the rubbish bit of wire I’ve used here.
In the case of the “high performance” plug the cable is butted right up against the terminals, meaning that there is only a minuscule amount un-twisted cable, whereas the plug on the right has quite a lot.
When we’re running long distances or at 10GbE this matters, but not in any other case.
Some (line type) 8P8C AMP plugs crimp onto the cable at 3 points, whereas the rest crimp at 2 points.
I have only seen one other connector type which crimps at 3 points (also originally an AMP specific type – now CommScope) – a special variety of 6 position “long” modular plug which has the lengthwise dimensions of an 8 position plug.
These are normally used where a shielded 6 position type is required. Even without this requirement – if you are using 6 position plugs regularly these things are a heck of an improvement over the standard type. They fit standard 6 position sockets, crimp very easily onto round cable, and the longer dimensions and tab make them a lot easier to connect/disconnect.
If like me you’ve spent any amount of time searching for such a thing, you may have also noticed there is virtually bupkis in the way of such products to choose from.
I come from the land of piggy back plugs: New Zealand. I’ve very much missed their convenience since moving to the UK.
Okay, so they’re not as common in New Zealand as they used to be. Thanks to regulatory crackdowns and changes in consumption habits, we can say in retrospect the 1990s was zenith of piggy back plugs (or tap-ons as we apparently call them).
While the days of popping down the supermarket to buy one are unlikely to return, at least you can still get them on pre-made appliance and extension cords, and re-wirable ones can be purchased from electrical wholesalers.
A very long time there was a company called Clix who manufactured the first BS1363 piggyback plug (more information here). As those are now collectors items, a modern replacement is desperately desired.
Let’s take a look at the UK’s only purchasable piggy back plug. The seller describes it as a “Surged pass through”, somewhat diminishing the piggybackness of it. Let’s open it up and take look…
Fortunately there are a pair of screws on the underside which let us look at the guts of it. These don’t need to be undone to wire the plug.
We are first presented with a plastic spacer which surrounds the socket contacts, and we can see the surge protection gubbins waving at us down by the neutral pin. This spacer also holds the (pointless) neon light.
Quickly we can see my biggest concern with these plugs. That contact is only just barely on the fuse. I’ve purchased a number of these, and can say they vary from unit to unit. This one isn’t so great. If concerned I’ve found they can easily be bent back into a sensible position with pliers.
But regardless I’d pull that 13 amp fuse out and put either a 3 or 5 amp fuse in its place. I’d not be in the habit of using these plugs with 13 amps.
Lifting up the spacer we can see the socket contacts and surge gubbins clearly. Once again, quality is less than spectacular. I cannot comment on the efficacy of the surge protection. In my opinion surge protection is of little value, and in my case I have de-soldered all of these components, as well as the neon, because all I wanted was a plug.
The one last gripe I have is with two protruding corners on the cradle which catch your screwdriver when you are tightening the line and neutral screws. I’ve clipped them off with side cutters (circled).
Surge protection device? Even if the surge protection is effective, it’s not anything to get excited about. There are plenty of other better made surge protection devices to choose from.
Piggy back plug? Definitely. Why the hell the seller isn’t advertising it as this, is beyond me.
Apparently the idea of such a thing is so alien to the British that it has to have some useless surge protection jazz stuffed in it to make the sale?
As I’ve said the quality of the contacts isn’t amazing, but it is acceptable, as this is the UK’s only piggy back plug, you’ve not got another choice.
If you need something like this – buy a box of them now. Who knows how long these will be available for.
Recently while watching the YouTube channel of UK Electrician John Ward I came across a most interesting clip where an eager viewer from New Zealand has posted in a considerable collection of electrical bits and bobs. Myself originally being from New Zealand it was amusing to watch. Among the collection is a most interesting combination antenna & power socket, which certainly, I had not ever seen before.
One item our enthusiastic mailer of electrical articles has not included, but has made the host aware of, is the subject of this article: The long-discontinued PDL 40A – the de-facto symbol of Kiwi electrical innovation and nostalgia.
The key difference between these plugs and a regular tap-on is that the phase pin on the rear socket is not connected to the plug side, therefore, using a 4 core cable, the socket on the back can be switched via some kind of control device on the end of the lead.
Typical uses were:
Float switches for water pumps
Timer switches for lighting or heating devices
In engineering environments it is common to find them with a loop of wire attached to the phase pins for attaching inductive clamp meters
Anything else you can think of that has to switch a single appliance, without the desire to expend effort fitting a socket to that device
While they were designed for use with 4 core cable – ‘Kiwi ingenuity’ is actually another form of the phrase ‘Hook or by crook’ and not surprisingly I have not ever seen one wired like this (that wasn’t wired by myself). Typically 3-core cable is used, then the earth wire gets re-purposed as the phase return, and the switching device has to do without earth. In the case where an earth is connected to the switching device, it’s because the neutral has been done away with, or some other solution is devised that doesn’t involve purchasing a length of 4-core cable.
I find myself wondering if the practice of using these plugs with 3 core cable may have contributed to PDL’s decision to discontinue it. Certainly in the case of earlier versions of the plug which aren’t easily identifiable as interrupted phase versions, subsequently wired with 3 core cable in some unknown likely dangerous arrangement i.e. earth connected to the phase pin – that cable could be mistakenly re-wired onto a metal chassis appliance likely leading to a fatal electric shock.
The Australians have got their own version of this – made, of course, by Clipsal.
For anyone wanting this kind of plug, at least these are still made, and certainly, by the time I started wiring stuff it was the only one purchasable. I can say from experience it’s just not the same as using a 40A. While not quite of the same quality – It could be argued that the Clipsal is better, because both the line and neutral are “interrupted”, for the almost inconceivable scenario where an RCD is doing the switching perhaps? Making full use of this does require a rather unwieldy length of 5-core flex, which by the time we get to 1.5mm2 is pretty big stuff, typical for full load 10 amp applications.
The fact that we’re using one of these plugs at all indicates that we’re not exactly flush for time or money; and in practice I doubt anyone has ever bothered with two pole switching, typically bridging the neutral inside the plug, instead stuffing a couple of lengths of figure eight Christmas tree wire into it, getting us the minimum requisite four conductors.
In this day and age 40As are exceptionally difficult to come by. They were unheard of in domestic environments, and uncommon in industrial / commercial environments too. I got a taste of its rarity when entering an electrical wholesaler with one about 15 years ago, to ask where I could get another: “Whoa!” said the guy behind the counter – “Haven’t seen one of those for a while!” Apparently that day when a 40A was carried into their store was a special one.
The few that still exist are very precious and typically hoarded by obsessive people like myself, a very unusual item to be in possession of indeed considering that I now live in the UK. I can boast a very large collection of 1 (and a broken black one), which is about as many as I’ll ever have.
Will I ever find a use for it? Even if I moved back to New Zealand, probably not.
Recently while assisting with an Arduino project, I found myself needing a simple circuit which generates either a 1 KHz or or 100 Hz square wave. The reason for this was to connect to an interrupt pin to generate a timekeeping-level accurate 1ms or 10ms timestamp, which the Arduino its self cannot generate as its crystal is fixed at 16.000 MHz
This turns out to be a little more difficult than I expected. Because you can’t divide 100 down to 10 with flip-flops, whatever you end up building is going to do one frequency or the other, requiring a change in crystal to switch. So first of all, let’s look at which crystals can generate these frequencies:
A crystal that can divide down to 1 KHz must be a power of two, multiplied by 1000. Some examples (all of which are easy to come by) are:
32.000 KHz (divide by 32)
2.048 MHz (divide by 2048)
4.096 MHz (divide by 4096)
8.192 MHz (divide by 8192)
Likewise, a crystal that can divide down to 100 Hz must be a power of two multiplied by 100. These are not so common. Some examples I could find:
25.600 KHz (divide by 256) – I could only find one example from a single manufacturer, which is stocked by some vendors but no longer in production
1.6384 MHz (divide by 16384) – Once existed, but at the time of writing none appear to be in production or for sale
6.5536 MHz (divide by 65536) – Several examples in production at the time of writing, reasonably obtainable
My requirements are:
Must be easy to change from 100 Hz to 1 KHz
No expensive or obscure components
Must be all SMT
Vcc = 5V
The next headache
Now I have to find an IC which can divide two of the above frequencies down to 100 Hz or 1 KHz. The trusty old CD4060 immediately jumps out. If we switch between 25.600 KHz and 32.000 KHz crystals, also changing the output stage – we’ve got a solution. Problem is, this solution falls foul of two my objectives – that one-and-only obscure 25.600 KHz crystal, which is not SMT.
With the only practical primary clock (for me) for 100 Hz operation being 6.5536 MHz, that rules pretty much all CD4xxx timers, which according to their datasheets, can’t operate with such high input clocks.
So far as I could see that leaves two options: 74xx292 (Rare in SMT) and HEF4541. If we are to select 8.192 MHz for the 1 KHz option, both can divide by 8192 and 65536, and handle those input clocks.
One more bump in the road
Because of the obscurity of 74xx292 in SMT, I’ve gone for HEF4541. The HEF4541 can in theory have a crystal connected directly to it, but after hours of profanities I discover that running at Vcc = 5V it can’t quite self oscillate at 6-8 MHz. We can prove this by shorting RS and RTC, and we see that it self-oscillates (with no other components) at about 5.9 MHz, which reveals the shortest propagation time between those two pins.
Great, so now we need another IC. Fortunately that only needs to be a 74HC2G04 which is tiny and inexpensive, barely increasing the footprint of this circuit.
The final solution
First, the 100 Hz version. Note that R3 can also be a wire link.
And now the 1 KHz version. R3 is moved, and the crystal frequency is changed.
I recently purchased a Cisco 2911 to replace my 1921 for use at home, as I needed an extra WIC slot. Now that they’ve been obsoleted by the ISR 4000 series, they’re starting to appear on eBay for relatively palatable sums. For me, the 2911 was a good choice because it has four WIC slots and fits in a 450mm deep rack, whereas the 2901 requires at least a 600mm deep rack, which is far too large for my home office. The 1941 was another possibility, but it’s not enough of an upgrade, and quite frankly, too damn ugly.
Without even having to bother plugging it in and switching it on, I know this thing is going to be too noisy for a home environment. The good news is that the standard array of leaf-blower strength fans are only needed when this product is used in extreme situations, i.e. loaded up with a four WIC cards, a 24-port Gigabit switch service module, with PoE, all ports at full power, and roasting in a street cabinet on a searing hot day in Egypt.
As this does not remotely resemble my use case, I can do away with most of the cooling. First stop – the fan module:
Top is the original, which I am going deaf just looking at, and below is my modified module.
I’ve removed all four of the original fans and fitted a single 70mm 4-wire fan (Delta AFB0712HHB). In order to prevent the system log from filling up with warnings about failed / missing fans, I’ve connected the tach signal from that one fan to the input for the 3 fans.
A quick run of ‘show env’ reveals that this has done the trick. The router being none the wiser to three of the fans being absent.
SYSTEM FAN STATUS
Fan 1 OK, Low speed setting
Fan 2 OK, Low speed setting
Fan 3 OK, Low speed setting
Fan 4 OK, Low speed setting
Just in case it isn’t obvious – the pinout for that connector (Molex 44133-1208) is as follows:
In my setup, everything runs from a single battery backed regulated DC +12V source. This is no coincidence, as most I.T. equipment internally runs from +12V, meaning that in almost all cases my gear doesn’t require an internal power supply. This router is no exception, needing only a single +12V source (with 5V standby voltage), so I effectively don’t need the power supply here either.
Good news for this conversion, because that’s another source of heat done away with, in fact it means that I don’t need any cooling in the lower half of the router, so that inlet vent can be blanked up – focusing the cooling Mojo of my single 70mm fan solely on the top (mainboard) half of the router.
But it’s not quite that simple. On my previous router (a 1921) the +12V could be feed straight through to the mainboard with no extra components. On the 2911, we need a bit of extra stuff to satisfy it.
I whipped up a small emulator PCB which fits in place of the power supply’s original PCB, and has all the extra bits needed to satisfy the routers’ software / hardware – i.e. present its’ self as a PWR-2911-AC, leaving the router none-the-wiser to the fact that it is now powered by an impostor power supply. The downside is that there is nothing but empty wasted space in the lower half of the router.
I’m not going to go into the details of this, but you can download its schematic here. While I was at it, I moved the power switch and inlet to the rear and blanked up the front. A little more convenient, because it means I don’t have to grope around in the back of my rack. For anyone else with the desire and patience to construct an emulator board like mine, a 60W power brick can easily replace the internal power supply.
A quick check shows that IOS is happy with my phony power supply, with the temperature sensor working, serial number and model number still reading as per the original AC supply this replaces.
NAME: "C2911 AC Power Supply", DESCR: "C2911 AC Power Supply"
PID: PWR-2911-AC , VID: V05 , SN: DCA1647R2GF
SYSTEM TEMPERATURE STATUS
Power Supply Unit temperature: 28 Celsius, Normal
How it runs
The power consumption of an idle unloaded 2911 at the 12V stage is 1.8 Amps (about 23W) – show environment reports a lot higher (38W), I am assuming this takes into account inefficiency in the power supply.
If we are to assume that this is also the unit TDP – It’s practically bupkis given its large size. According to my scientific ‘finger on heatsink’ tests, all of my WIC cards run very cool. The mainboard ASIC also barely gets warm to the touch.
The only thing I need to keep an eye on is the CPU temperature. The CPU in my unit is a Cavium Octeon (MIPS64), which is fairly energy efficient, but still chucks out the loins share of the heat. It has an internal temperature sensor, which we can read out with the ‘show environment’ command.
SYSTEM TEMPERATURE STATUS
Intake Left(Bezel) temperature: 31 Celsius, Normal
Intake Left temperature: 23 Celsius, Normal
Exhaust Right(Bezel) temperature: 34 Celsius, Normal
Exhaust Right temperature: 27 Celsius, Normal
CPU temperature: 61 Celsius, Normal
Power Supply Unit temperature: 28 Celsius, Normal
At 61 degrees, it is 2 degrees hotter than it was with the stock hurricane grade array of fans, where it sat at 59 degrees. Suffice to say that for my light use case, those fans are indeed completely unnecessary.
For anyone thinking of attempting this…
Having a single fan is ideal, because there is no risk of irritating ‘beat patterns’ (which often occur when fans rotating at similar speeds are near each other) – but you can only get away with a single fan if also doing away with the power supply, there’s nothing in the service module bay, and the inlet for the lower half of the router is blanked up. As is the case with mine.
As the PWR-2911-AC does need a little bit of airflow at 30-40 watts, I would suggest replacing with three thinner 70mm fans (like the one I have used) and doing away with / blanking up the 40mm fan, because you are not going to find a quiet one, then strap the tach signal for the 40mm fan to one of the 70mm fans to eliminate software errors.
As a keen electronics hobbyist, I have designed some 50 or so PCBs to date. In each instance where a switching regulator is required, I’m typically reaching for one of two options: Where efficiency isn’t important – the trusty old LM2596, or when efficiency is required, I’ll be using a design from Linear Technology with synchronous regulation.
On my last two boards however, for reasons I am myself not entirely sure of (cost perhaps?) I used an MC34063. It’s been with us since the dinosaurs roamed the earth, and is unsurprisingly very primitive. It should have been designated to the dustbin of history, but thanks to the internet and the renascence of electronics in the hobbyist space, it has made an aggressive comeback, and for a simple reason: It’s dirt cheap.
My MC34063 was deployed on the PCB with the above circuit, lifted unchanged from the datasheet. It just so happened that I need 5V at 500mA max, from a 24-28V source. Perfect. What could possibly go wrong?
There is one very important thing we must consider when using this chip: It has absolutely no built-in thermal protection. The above circuit does have over-current protection, but this does not offer any protection from sustained short circuits. In many cases that isn’t a problem, but on this board it was.
From looking at the photo, we can see that there’s quite a bit of burned out stuff, making it a little difficult to piece together exactly what happened. Fortunately it all unfolded before my very eyes. The problem started with something that was nothing to do with the MC34063. See those two rectangular capacitors? One of them is particularly toasty indeed.
That capacitor is an AVX “TAJ” series 330uF 10V tantalum. It had developed an internal short circuit which caused the MC34063 to gradually heat up, eventually reaching a point where its internals melted, then becoming a short circuit its self.
Once the MC34063 became a short circuit, the 25V input voltage surged straight through to the 5V secondary, bear in mind that, that voltage is coming from a bank of large lead acid batteries.
Both pairs of batteries were protected with battery fuses, but those were 15A a piece, as this is a very high power setup, also on the PCB was a 30A maxi blade fuse. Surely one of those would have blown? Nope. When silicon melts to the point of becoming a short circuit, there is typically still a few ohms of resistance, which in this case was not enough to blow any fuses.
What happens next? BOOM! The short circuiting of the MC34063 unleashed 25V @ ~40A of potential at that shorted capacitor, which promptly exploded, ejecting a significant amount of fire and hot gasses in the process. In the picture you can clearly see the internals of it have become a melted blob of metal, transforming it into a very effective short circuit.
The last phase of destruction was the MC34063 its self burning to a cinder, as it is now the weakest part of the circuit, doing significant damage to the PCB in the process.
It’s at this point that you start recounting exactly what is attached to the 5V rail, because it is likely now toast. The tantalum capacitor must have briefly been open circuit because all 10 ICs fed from the 5V rail were completely destroyed, as well as all of the chips on a second PCB also fed from this regulator, requiring hours of rework to replace them all. Just as well there was nothing expensive connected to it.
When using an MC34063, or anything else without built-in protection – short out its output for a few minutes and see what happens. If you find yourself staring at a mess like the above, sort it out. Don’t ever assume it won’t happen.
In cases like this where the system is fed from batteries, protected by large fuses – add a second smaller fuse i.e. 500mA before small circuits like this.
In my case I have ditched the MC34063 and replaced with with a Wurth 173010542 7805 switching drop in replacement. It gets me a 5V output with 90% efficiency, over-current and over-temperature protection. Not cheap, but when you are talking about stuff that could start a fire…