Mount started smoking!!

[choochoo_baloo]

I was testing the wiring I had done this afternoon for my pier with mount (Celestron Advanced VX). I was using a freshly charged 12V leisure battery BUT the polarity of the 12V male power plug was incorrect - a laspe of concentration......

So everything was plugged in, and when I switched on the mount using it's switch nothing happened apart from some smoking from the connection panel/RA motor area. There was also a faint plastic-y smell. Obviously I immeadiately then switched the mount off again. It was only switched on for about 2 seconds.

I realised that I had wired the power plug incorrectly when I checked it with a multimeter. So I re-wired it to the correct polarity. However I had to pop out after that, and I've just returned about 2 hours later, turned the mount back on, and now it seems to be working fine.

So, before I breath a complete sigh of relief, I'd like to ask if anyone can suggest what happened.

When I used the mount in the field last year with a battery, I attached the clamps to the wrong battery terminals and had a reversed polarity that time too. However on that occasion there was no smoking or smells, the mount simply wouldn't turn on.

Therefore all I can think is that the high battery voltage today (13+ V) due to not allwoing sufficient time for the surface charge to dissipate from it's charging, must have been too high for the mount electronics, which caused excessive internal heating. This would explain why everything appears fine now, since 2 hours have elapsed allowing the surface charge to dissipate.

Obviously I'm not convinced taht I've permanently damged something internally, so if anyone can reassure me otherwise from my above account, that would be brilliant!

Thanks for any information in advance.

[perfrej]

There is most likely a reverse protection diode over the input terminals in order to protect the rest of the electronics. Said diode is likely toast right now. It it didn't happen the last time you performed the feat because that battery didn't have enough current capacity to smoke it. A leisure battery can supply much more current.

/per

[gajjer]

I would have thought a reverse protection diode would be in series with the electronics. Across the terminals it would have a very short and unfruitful life and not provide protection. Don't know what the circuit in the drive is so can't really advise. It's possible any logic was protected by a regulator, but you are very lucky. All I can suggest is that you inspect the board for damaged components. It is possible that components have failed but the circuit keeps on working. It may not be reliable though.

good luck

cheers

gaj

[ollypenrice]

I do know that Tak mounts have no reverse polarity protection and are killed instantly by an error. I haven't tried this myself but a friend has!

Olly

[damnut]

All explained above, but you are playing russian roulette with your mount. If you are frequently changing polarity of plugs it is really worth putting a some sort of mark / sticker /label on it so you check before using. Or  that if it is a long time after previous use, check it.

[hughgilhespie]

Oh well, I expect it costs hundreds of pounds to include a reverse polarity protection diode so the board designers are told not to include them. I mean the diode alone must cost, what? say 5 pence? then theres the extra solder, etc etc.

Cynical, moi ?

[michael.h.f.wilkinson]

A reverse-voltage protection diode should not fail under higher output current capacity of the battery. It should only fail at higher voltage. If you can expect a reverse voltage of 12-13.8V, a fully charged battery should not be able to kill it. What might cause the smoke is insulation being fried by a high current. The wires carrying the current will still work, but the insulation may be unreliable.

[Astro_Baby]

Skywatcher mounts have no reverse polarity protection either for the record.  Thats one of the reasons I never use adapters that have any form or polarity switch and any lead is marked for its assigned purpose.

On a Skywatcher mount your motherboard would now be toast and you would be looking at a few hundred quid and a long wait for a spare to fix it.

[Alien 13]

Two things 1st does your battery/lead have a fuse if not it it vital that one be fitted and for good measure I would include an inline diode too (this will cause about 0.5v drop in voltage)

It could be that some capacitors have failed causing the smoke so its worth inspecting the board and find replacements.

Alan

[Paul M]

A parallel protection diode is normally used in conjunction with an in-line fuse (or other over-current protective device).

The diode only needs to live long enough for the current it shunts in an incorrect polarity situation to blow the fuse - which would be almost instantaneous. Much faster than you could throw a switch.

Series protection diodes are much less common in power circuits because in normal use they are passing forward current and their internal resistance will produce heat. They also produce a voltage drop of 0.7v so the equipment being supplied by a 12v battery would only see 11.3v. 

For this mount to have smoked and not blown a fuse somewhere in the supply side is a concern. If it was a protection diode that fried, it did really well to shunt the battery's full capacity even for only a few seconds. It would be highly likely that the diode would fail and therefore leave the rest of the electronics exposed to reverse polarity.

These mounts must have some very robust and well designed reverse polarity protection. And as others have said may now be compromised, as may some of the other electronics.

This cat is unlikely to have 9 lives...

Edit: I see Alien13 just beat me to it...

[choochoo_baloo]

I believe I've found the source of the smoking - attached are 2 photos of a blackened component on the connection panel circuit board!

Obviously it's component "C3". The white clip thing next to it with the soot mark is the on-off switch connection. So can anyone tell me what "C3" is likely to be, and should I replace it as a matter of urgency?

[Alien 13]

From the board component idents it is a small electrolytic capacitor most likely tantalum, voltage rating needs to be 16v min as to the value know idea but if its purely for decoupling then anything between 4.7 to 10 micro farads will most likely do.

Alan

[choochoo_baloo]

From the board component idents it is a small electrolytic capacitor most likely tantalum, voltage rating needs to be 16v min as to the value know idea but if its purely for decoupling then anything between 4.7 to 10 micro farads will most likely do.

Alan

Sorry for the nooby question, but how are you able to identify the circuit components? I've had a look for a circuit board diagram on the Internet but cannot seem to find one.

Do you think I should de-solder it from the board and look for some component marking (at least what may be left post frying!) or are you very confident that it's a capacitor? In the case of polarity reverses, is capacitor blowing a common symptom?

[Alien 13]

If you look at the comonent boxes some are rectangular some rectangular with the corners modified this is one of many ways to indicate that the component is polarity sensitive C3 is standard talk for capacitor number 3 from the manufacturers item list.

Most polar capacitors where space is at a premium are tantalum and they do have voltage ratings as you have discovered they do not like reversed voltages and will go short circuit and burn out.

Hope that helps

Alan

[Paul M]

Indeed, The product data sheet for that 5v regulator recommends a 10uF tantalum cap as a "input bypass".

It also notes that a protection diode is not required. The regulator will look after itself!

http://www.farnell.com/datasheets/10195.pdf

So it looks like all the electronics, except that poor C3, are protected behind that nifty little device.