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Protection from reverse polarity

adoggg

Oct 25, 2013
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I am trying to do a little repair on a semi-complex system that contains a circuit board. I won't bore you with the details, but the bottom line is I have to wire in a 24Vdc supply in. I got myself a cheap 24Vdc 1A adapter (which is what is called for), from which I'll have to strip the barrel connector and test the wires for polarity. But I'm pretty green and I'm afeerd of getting the polarity wrong when I hook it up and blowing up the whole circuit board that is being supplied. My plan is to put an LED (or two) in between my adapter and the connection(s) to the circuit board before I plug it in to test it. Do you think that the diode will pop the connection before any damage is done to the PCB and that it will protect me? Or is there not enough information to tell? Or is there a better plan (besides "just be sure to test correctly for polarity")?
 

JMW

Jan 30, 2012
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Build a "diode bridge" using suitable Schottky diodes. Polarity in becomes irrelevant.
 
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davenn

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Hi adoggg
welcome to Electronics Point forums
enjoy your stay

Build a "diode bridge" using suitable Schottky diodes. Polarity in becomes irrelevant.

yup my thoughts exactly .... it works really well

Dave
 

BobK

Jan 5, 2010
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How does that help at all? Right now, has a positive lead and ground lead that he needs to connect to the pcb correctly. After he builds a bridge, he has a positive and ground lead that he has to connect to the pcb correctly. Nothing has changed except that more possible mistakes are introduced in making the bridge.

Bob
 

davenn

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How does that help at all? Right now, has a positive lead and ground lead that he needs to connect to the pcb correctly. After he builds a bridge, he has a positive and ground lead that he has to connect to the pcb correctly. Nothing has changed except that more possible mistakes are introduced in making the bridge.

Bob

I often incoporate a bridge rectifier into gear I build.... particularly amateur radio stuff that's being used out in the field ( hilltopping etc) where with lots of leads from lots of bits of gear
the possibility of accidentally reversing something can happen

adoggg

you cannot use a LED in series with the supply as it will only pass ~ 20mA, your bit of gear is likely to use much more than that.
did you check out the link that Harald gave you ?

cheers
Dave
 

BobK

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I often incoporate a bridge rectifier into gear I build.... particularly amateur radio stuff that's being used out in the field ( hilltopping etc) where with lots of leads from lots of bits of gear
the possibility of accidentally reversing something can happen
Yes, and that is a fine practice. If there was a bridge designed into the PCB then he could not connect it incorrectly.

But the problem the OP is asking about is how he can protect against connecting the wrong leads to the PCB. If he can connect the wrong leads coming out of the power supply, he can just as easily connect the wrong leads coming out of the bridge. So how does it help with his problem?

Bob
 

JMW

Jan 30, 2012
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How does that help at all? Right now, has a positive lead and ground lead that he needs to connect to the pcb correctly. After he builds a bridge, he has a positive and ground lead that he has to connect to the pcb correctly. Nothing has changed except that more possible mistakes are introduced in making the bridge.

Bob
The ground on PCB is easy to discern, Pin 7 on an IC, etc. That means the other connection is going to be the + output of the bridge.
At that point the input to the bridge could be AC/DC doesn't matter. That is the "magic" of a bridge. I suggested Schottkys as they minimize the .7 VDC voltage drop associated with others. This would only pose a problem if the walwart produced exactly 6.0 VDC and the board needed exactly 6 VDC to operate.
 

BobK

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The ground on PCB is easy to discern, Pin 7 on an IC, etc. That means the other connection is going to be the + output of the bridge.
At that point the input to the bridge could be AC/DC doesn't matter. That is the "magic" of a bridge. I suggested Schottkys as they minimize the .7 VDC voltage drop associated with others. This would only pose a problem if the walwart produced exactly 6.0 VDC and the board needed exactly 6 VDC to operate.
You really don't understand what I am saying.

The situation right now. The OP has + and - power supply leads that he is not sure how to connect to the PCB.

The sitution after inserting a bridge on the power supply: The OP has + and - power supply leads that he is not sure how to connect ot the PCB.

You may know how to identify ground and power on the PCB, but, apparently, the OP does not.

Bob
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Yes. The problem is that you need to determine both the polarity of the supply and the required polarity of the circuit before applying power.

JMW gives a method. There are others.

Personally, I would look for a large polarised capacitor near the input connector and see which input power connection was connected to the labelled -ve pin of the cap. I would then try to confirm that with other measurements.

*some* equipment will have a diode across the power supply, or parasitic diodes from semiconductors (or input protection diodes). If you have a multimeter with a diode test function you *may* be able to detect these by probing the power supply input. The correct way to power something is with these reverse biased.

However, as I said earlier, I would do several tests and look for a consensus.
 

KrisBlueNZ

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The OP seems to have seagulled this thread...
 
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