My First PCB

I finally made my first AOG PCB!! Yay!

Attached my 12v battery to it, saw sparks and the earth wire promptly melted. Boo!

I’ve double checked the circuit and everything appears to be the right way around. Inspecting the bottom, I saw that a tiny bit of solder on the underside pad for the 12v battery in may have been touching the other pad.

. It wasn’t touching the pin, just the pad (see image).


Trying to desolder this has just made things worse so I’m probably going to have to start again, but what’s confusing me is that these pads for the large two and three pin connectors all seem joined on the PCB. It looks like one continuous metal plate with three holes in it. I’ve checked images in here and vids on YouTube and most people seem to have exactly the same boards.

Am I missing something here,

Sure you’ve got all the diodes the right way round? Soldering is fine there as far as I can see. The two pins circled are connected to each other anyway.

First thing I checked.

I am using one non standard part, but I can’t see that making a difference as it’s just a socket:

I’ll check the diodes again shortly, but they all looked right to me.

Please post a picture of the component side

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Thanks guys.

These are the caps I had to get from a third party supplier. 35v 1000uf, think they’re the right ones.


Finally, and for the purposes of transparency, I did solder the socket to this the wrong way around. Just the socket though, I put the component in the correct way. I don’t think this should make any difference?


Diodes looks good.
The big diode is not burnt so you did not lead the current through that one.

Take out relay to rule out that as a problem. Power up.
Next still relay out power up with 12v plus direct to where it say 12v out (bypass relay)
That is how I use my PCB as I have relay elsewhere.
You should use 10 A fuse

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  1. Remove relay.
    1.1. Connect battery to 12v Battery In and see if I get sparks?

  2. Leave the 12v earth connected to 12v Battery In, but connect 12v live to the live of 12 Out.

Sorry to ask a really dumb question, but where am I putting the 10amp fuse?


In this schematic you can see the 10amp Fuse between the Powersource (Battery) and the 12V Battery In Socket.

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Yes that was the idea.

I would put fuse somewhere between battery 12V + pole and 12V + in
Did the non standard relay socket fit right into the holes?
Optocoupler socket is correct no matter how it is turned.
Please photo of back side also.

Ok, thanks, I’ll give that a try as soon as I get done tonight.

Photo of underside of PCB below. Beware, there are some ugly joints, but they are all solid and none are shorting as far as I can see :slight_smile:

Where are the sparks coming from?

With the on of switch disconnected you should get nothing. If you have that disconnected and there are sparks then assume the relay is not working correctly.

I recommend removing all the pluggable components from the board (nano, cytron, ads ect) to save on blowing something up.

Also check the 12v to 5v ICs (black box’s) are the right way round as they are directional as well.

You can put a lower voltage in after the relay to test should make less sparks.

Just noticed that your on off switch connection is a loose wire bridge… Is that what’s sparking?

That’s really useful, you obviously understand the level to which I understand electronics at this point!

The sparks are coming from the 12V Battery In as soon as I connect the battery wires. I connected the positive and as soon as I touched the ground wire, sparks. The first wire I connected just melted in a second.

If you look closely at the top of the ground screw in my photo above, you can see the burn mark from when I tried with a better gauge wire…

It’s like the battery in + and - are immediately shorting which is why I asked about the fact that all of the large connectors on the PCB look like one continuous plate of metal. I can’t bend my head around that.

Good tips about removing components and applying less voltage. Obvious now you say it, but less obvious to a keen newbie.


Burn mark ringed.

Touched the ground for a split second, sparks from here.



Oh sorry. Just to add, I’ve shorted the on / off switch as I want it always on.

You’re not connecting - ve wire to that post?

+ve wire to the burned terminal (12v battery)
-ve to the GND terminal

If you are connecting both battery terminals to 12v battery then you’ll have a dead short and melt things.

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Rubbish drawing but red = +ve, black = - ve

Also if you want to be always on then wire +ve to the 12v out terminal. It’ll bypass the relay and you can save yourself the expense of buying one for you next board.

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Also, the red wire that goes from PWM2 to Cytron shouldn’t be reattached normally I think.


And that answers all of my questions.

Makes perfect sense, many thanks.

Ok, I’ve tested as you suggest and no sparks : shorting. Great.

But all I get is the sound of the relay switching, a lot of fast clicks, no LEDs illuminate. I’ve tested with another relay, same result. Reckon I’ve blown something?