Short circuit on this board somewhere- I am at a complete loss!

This is a simple H bridge implemented with TIP 122/127.

The base resistors (560R) terminate in a screw terminal block and the conjoined emitters also both terminate in a screw terminal block through the 4.7R resistor.

With only the power connector (on the right side) connected to 12V, but nothing else connected, it shorts my power supply.

1) I have checked for bad transistors and they are all good.
2) I have checked for solder dags.
3) I have sanded the heat sink around the holes to make sure there are no Al dags connecting to the back of the transistors through the holes.
3a) I have used those thermally conductive heat pad things behind the transistors along with the plastic eyelet things for the the bolts.
4) I have used my continuity checkers to look for connections between the back of the transistors and the heat sink - none.
5) I have checked for a bad cap, but it is good.
6) Can't find any soldering mistakes.

If I put my continuity across the power connection header then I get a brief beep but then it stops and I hear nothing else.

What am I missing here? What have I not considered?

If I put my continuity across the power connection header then I get a brief beep but then it stops and I hear nothing else.

Click to expand...

Errrmmmm..... the capacitor charging up perhaps!

Brian.

Hi,

your schematic shows TIP128... it should be TIP122...

Can we see a detailed picture of the backside. and a more detailed for the comonent side?

***
Test:
When your power supply is current limited to 100mA.. what voltage do you see? - measured across the supply rails

Klaus

But I actually removed that cap from the circuit board and re-tested it but it still shorted the power supply. Any other ideas?

I see I should be doing my h bridge something like this anyway:


If I wanted to power my current design from 20V then the load would never see more than the 12V input to the bases anyway......emitter follower.......

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Sorry Tip122 yes. 12V DC plug pack thing - it just shuts down due to a safety measure being triggered and it stays that way until I disconnect my board.

20V laptop plug pack - the voltage seems to oscillate around 200mV to 2V or something like that also due to a shorter lived safety measure being triggered I presume.

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How's this?

If you roll the top edge of the board down and towards you in this photo then the front of the TIPs will be facing you and I have the TIPs in this order: TIP122...TIP127...TIP122...TIP127.

Hi,

really hard to verify.
Please check for an accidental short at the power connector.:


but yes, there are complete H bridges in one IC. This is easy to use and there is less voltage drop and less heating.
You could do with a much smaller heatsink.

Klaus