Why did my capacitor in my transmitter blow

[Mastermime]

Hey everyone, as I was testing my circuit with a 12v SLA 7.5 Ahr battery (dont have the actual batteries yet), a capacitor in my transmitter blew and smoke.  I have no idea why.  If there is a flaw in my circuit below, could you let me know.  I dont know if this was coincidence or not, but right as I was testing voltage at transmitter line the cap blew.  The caps in the transmitter are 470uf 16v.  Please help. 

Thanks

Edit: I wonder if I accidentally touched the two probes together when checking the voltage.. hmm maybe.  If you guys dont see anything else wrong, I bet thats what happened

[ebrady]

Assuming you did not inadvertently touch anything with your probes...

#1 - Did you connect the battery backwards?  That might blow the 470uf capacitor in front of the 7812 if it is polarized.

#2 - Did you build this circuit?  If so, do you have either of the 470uf caps in backwards?

There might be other things, but those are the two that come to mind immediately.

[waltr]

What is the voltage rating of the cap that blew?
Over voltage and reverse voltage (cap in backwards) are the two easiest ways to blow an electrolytic cap.

[Mastermime]

Well the caps are rated for 16v.  So that shouldn't have been an issue. And I didn't put them in backwards.

[waltr]

Looking at your circuit the pass transistor is backwards and should be an NPN.
Look at the example circuits in a National Semiconductor data sheet for a 78xx regulator with a pass transistor.
A smaller, 0.1uF, cap on the 7812's output pin is also recommended. This helps prevent the regulator from oscillating.

[Mastermime]

When building this boosting regulator, I was just following the schematic on this website http://www.zen22142.zen.co.uk/Circuits/Power/boosti.htm

[waltr]

When building this boosting regulator, I was just following the schematic on this website http://www.zen22142.zen.co.uk/Circuits/Power/boosti.htm

This circuit is correct. It uses an PNP transistor with the Emitter on the higher (input) voltage side.
The circuit you posted above is not. The transistor is backwards (Emitter arrow should point to the low voltage side (output).

[Mastermime]

Ahh sorry about that.  When I was soldering, I was following the correct circuit.  I just made that schematic so you guys would be better informed.

So the conclusion I'm coming to i that accidentally shorted that power line by touching the two probes of my multimeter together.  Waltr, can you confirm this?

[ebrady]

Other than the transistor, is the schematic correct?   If so, I am not seeing how a short circuit across your probes should result in a capacitor blowing up.   

It could have been a bad cap.

[Mastermime]

Other than the transistor, is the schematic correct?

Yes everything else is correct.

[Soeren]

Hi,

Personally, I'd up the value of the base-emitter resistor (actually, I am, as I'm just making a small simple lab-supply - to be posted later, if anyone would like).
You don't wanna get the 7812 working very hard (and needing a lot of heat sinking), so make it 10..33 Ohm. You just need the regulation from the 7812 and enough current to properly control the pass transistor, not the grunt work, which should be handled by the power PNP.

And do not, I repeat do not, use a Darlington for this, it has too high a voltage drop!

The 7812 needs an input voltage of at least 14.5V to 15V (or even higher, depending on who made the 7812) and to that you need to add the base-emitter drop of the pass transistor, which will be ~0.7+ for a non-Darlington and up to 2.5V for a Darlington.

Question you need to ask yoursefl... What voltage does your two batteries span over their discharge?

[Mastermime]

Ok thanks for the help.

10..33 Ohm

I'm confused on what this means? Switch to 33 ohm resistor?

And do not, I repeat do not, use a Darlington for this, it has too high a voltage drop!

Ok thanks for the heads up.  Luckily I'm using a tip2955.  This has a drop of 0.6v

Question you need to ask yoursefl... What voltage does your two batteries span over their discharge?

You know that is a good question.  I bought the batteries off Ebay from a Chinese dealer http://www.ebay.com/itm/1-pcs-7-2V-5300mAH-Ni-MH-Rechargeable-Battery-Pack-RC-/270614422848

They haven't arrived yet.

[Soeren]

Hi,

10..33 Ohm

I'm confused on what this means? Switch to 33 ohm resistor?

It means: Change the 1 Ohm resistor to something higher (and the 10 Ohm basis resistor you can leave out).
(10..33 Ohm is "from 10 Ohm to 33 Ohm", but that was before I realized that you're gonna use a TIP2955).

Luckily I'm using a tip2955.  This has a drop of 0.6v

The drop depends on the current through it - how much current are you hoping for?
(This will determine the value of the resistor discussed above).

You know that is a good question.  I bought the batteries off Ebay from a Chinese dealer [...]

OK, they're 6 cells each, 12 cells total of the NiMH variety.
NiMH cells are 1.40 to 1.45 fresh out of the charger and, with a large drain, around 0.9V when flat
So, freshly charged they're around 17V down to approx. 11V when flat.

Since you need minimum 12V+2.5+0.7V=15.2V (and more likely around 1.0..1.5V more) at the input of the regulator and the fact is, that this isn't going to work for more than a tiny fraction of the available discharge, I'd recommend you to look into switching regulators, either a SEPIC or a Buck/Boost, to get a steady 12V over the full discharge.
Dimension Engineering has got a couple that may work, depending on your current demand.

[Mastermime]

OK, they're 6 cells each, 12 cells total of the NiMH variety.
NiMH cells are 1.40 to 1.45 fresh out of the charger and, with a large drain, around 0.9V when flat
So, freshly charged they're around 17V down to approx. 11V when flat.

Since you need minimum 12V+2.5+0.7V=15.2V (and more likely around 1.0..1.5V more) at the input of the regulator and the fact is, that this isn't going to work for more than a tiny fraction of the available discharge, I'd recommend you to look into switching regulators, either a SEPIC or a Buck/Boost, to get a steady 12V over the full discharge.
Dimension Engineering has got a couple that may work, depending on your current demand.

Ahh shoot.  The current demand is approximately 2.5 amps.  If I buy this http://www.dimensionengineering.com/products/anyvolt3
I can just ditch the circuitryfor the regulator booster I have, right?

[Soeren]

Yes!

[Mastermime]

Thanks for your help Soeren! I owe you.