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I have a few Arduino Pro Mini clones (cheap Chinese stuff) and would like to power them with 12V power supply (same as fan voltage). According to the Arduino Pro Mini spec the RAW pin can take 3.35-12V (3.3V model). In practice this means a 12V PSU cannot be used as they are almost always over 12V with low load. I read that the voltage regulator in this board is capable up to 16V input. I tried plugging in a cheap 12V power adapter that read 15.1V with no load but a component on the arduino clone board actually exploded instantly. The board seems to work still when powered from usb programmer. Component that burned out is just above VCC, RST and GND pins.

Why did this happen? What did I just damage and most importantly what is a safe voltage level to use? I already have a few 12V switching power supplies it would be a shame if I could not use them.

Solution:

In the end it seems to have been a faulty arduino clone, faulty or poor quality power adapter or the fact that I powered the power supply before plugging it to the board's RAW input pin. The cheap 12V switching power supplies work just fine even though my multimeter registers a voltage spike as high as 30V during power up.

  • There doesn't seem to be any Pro Micro board on the official site. I can only find it on Sparkfun. Could you verify which board do you have? – asheeshr Mar 16 '14 at 2:23
  • My mistake, it's actually Pro Mini (corrected now). – DominicM Mar 16 '14 at 4:07
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The genuine Pro Mini's use a MIC5205 regulator which should accept up to 16V at it's input normally, with an absolute maximum of 20V. It's unlikely that the regulator would be damaged by 15.1V.

However, the component you have indicated that has blown is a capacitor. SMD capacitors are available in different voltage ratings, typically 4V, 6.3V, 10V, 16V, 25V, 35V and 50V (and above, but let's ignore them for this). It's rare to see any intermediate values.

SMD capacitors are very intolerant of being used at a higher voltage than specified. This is especially true of electrolytic and tantalum capacitors. The blown capacitor on the genuine Pro Mini's is polarised (the grey band not he package and the schematic indicates this), so we can infer it is either electrolytic or tantalum. A 10µF SMD electrolytic is unlikely to be in this package, so it is almost certainly a tantalum.

Sparkfun sell 10µF tantalums, and they are rated at 16V. It's quite likely these are the same ones used on the Pro Mini. 15.1V is very close to 16V - in fact, if this is a poorly smoothed power supply, the meter might show 15.1V but the peaks could well be 16V or even 20V. It is recommended that you de-rate the voltage ratings on tantalums by 50% at least.

It's really important to realise that most low-cost DC power supplies are not regulated and produce a very bumpy output voltage with no regulation:

enter image description here

Tantalums also have no tolerance for reverse voltage, so if you did apply reverse voltage, it would pop.

So it is quite likely that you over-volted the cap and blew it. You could replace them with higher rated capacitors - I would go for 35V ones if you want to supply 15.1V. Desoldering and desoldering a single capacitor is relatively easy as long as the pads haven't been damaged. Realistically, you could use a normal leaded 10uF electrolytic between RAW and GND instead of the SMD one.

EDIT: However, looking at the data sheet for the regulator, it shouldn't really matter if that 10uF is there or not:

A 1μF capacitor should be placed from IN to GND if there is more than 10 inches of wire between the input and the ac filter capacitor or if a battery is used as the input.

Some regulators absolutely require an input capacitor, with others it is just advisable. This looks like it is just advisable. So if it isn't working on external power now, you may have blown the regulator as well.

Even if the clone board uses a different SMD regulator, their characteristics are all very similar. As stated, some do need a capacitor on the input to work, so replacing it might fix it.

There is a further consideration that dropping from 15.1V to 3.3V on a SMD regulator is not a great idea. You will need to burn off a lot of power.

The MC5205 can dissipate 455mW with a minimal PCB footprint (which the Pro Mini has) with a 25°C ambient:

Thermal calc

So, we know the maximum power we can dissipate, and we know the voltage drop - we can then calculate the current:

P = (Vin - Vout) * I

0.455 / (15.1 - 3.3) = I

I = 38.5mA

This isn't very much at all. You might want to consider regulating off the board, and probably with a switching regulator rather than linear regulator.

I also note your PS:

the ground of the 12V PSU is shared with Arduino ground pin, not sure if this might be the cause.

If they weren't shared, there would be no way that this could work. They need to be shared otherwise the Vcc from the power supply is not referenced to anything and may as well not be there.

  • Fantastically detailed answer! The psu I used could well have higher peak voltage. Could a switching supply have this problem too? The damaged capacitor has "a104C" written on it and "3A052" below that. Not sure what that means. The pads seem fine as I only contacted the RAW pin for less than half a sec. I will attempt to solder a normal 35V capacitor when my soldering iron arrives. I assume larger cap is ok? – DominicM Mar 16 '14 at 15:52
  • A blown tantalum would likely result in more damage than just a blown capacitor (like, say, a scorched PCB). – Ignacio Vazquez-Abrams Mar 16 '14 at 16:22
  • I only see damage to the capacitor, no marks on the PCB at all. Top of cap has a hole and some exposed metal inside also some black burn mark on the side but that's all. I only touched the contacts for a very short time so maybe that's why. – DominicM Mar 16 '14 at 16:57
  • I've definitely taken out tantalums and the damage has been limited to the cap and minor scorching. – Cybergibbons Mar 16 '14 at 17:10
  • @Cybergibbons I had the power adapter plugged in before connecting the arduino, does that rule out that the startup voltage was the cause? I have switching supplies but not sure if they are prone to higher voltages than rated or not..? – DominicM Mar 16 '14 at 18:15
3

The component that exploded during your test is a capacitor.

According to the position on the board you mentioned, it is one of the 2 electrolytic capacitors used on both sides of the 3.3V regulator (I would say that's probably the upstream capcitor).

There are a few reasons why a capacitor may explode:

  • apply an inverted voltage to it (polarized capacitors only)
  • apply a voltage above its rating
  • apply a current above its rating

Your situation is probably the 2nd one: too high voltage applied.

According to Sparkfun original schematics, this capacitor must be polarized with 10uF capacity. Unfortunately the schematics do not mention the maximum voltage.

According to the specification for the original board, the maximum voltage for this cap should be at least 12V, but there is no guarantee that it is anthing above than 12V. Hence, you have to consider 12V is the maximum voltage applicable to your board (on the RAW pin).

In addition, you have mentioned you are using a cheap chinese clone, it is perfectly possible (although not expected, as it would make the board operate differently than the original specs) that this one uses a lower voltage cap (eg 10V) which would be a bit cheaper.

If you do want to reuse your 12V PSU, you will have to ensure that its output voltage never exceeds 12V, for this you are left with a few options:

  • use a voltage divider made of 2 resistors: you must first know the max voltage output of your PSU and then calculate resistance values to get 12V based on that max voltage. That's easy but it can waste a lot of energy
  • use a 12V Zener diode outside your PSU
  • use a 12V voltage regulator circuit after your PSU: that one is the safest, but also the most expensive solution

Edit:

Of course, if you have already done it and are equipped for it, you can find replacements for the capacitor that has exploded and buy a higher-voltage one, then replace it on your boards; that will reauire good soldering/desoldering equipement usable for surface-mounted components.

  • Or replace the low-voltage parts with higher-rated ones. – Ignacio Vazquez-Abrams Mar 16 '14 at 8:24
  • Yes of course but that involves SMC desoldering which is not something everyone can do. – jfpoilpret Mar 16 '14 at 8:30
  • @Ignacio-Vazquez-Abrams I have edited my answer to include your suggestion, thanks. – jfpoilpret Mar 16 '14 at 8:35
  • I do have a few capacitors, just waiting for my hakko soldering iron to rrive as my el cheapo broke... Can I use any non smd capacitor with voltage rating of let's say 35V? – DominicM Mar 16 '14 at 15:44
  • Higher voltage caps is OK, as long as capacitance is the same as the original. You can go and remove the SMD cap (only one is needed to be replaced) and solder a "normal" cap instead, but beware of shorts as leads are much bigger! – jfpoilpret Mar 16 '14 at 16:12

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