Connect with us

Protection from reverse polarity

Discussion in 'General Electronics Discussion' started by adoggg, Oct 25, 2013.

Scroll to continue with content
  1. adoggg

    adoggg

    1
    0
    Oct 25, 2013
    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")?
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    11,160
    2,548
    Nov 17, 2011
    Better: ise a diode in series to the power supply. See for example here, first post, circuit top left in the image.
     
  3. JMW

    JMW

    90
    3
    Jan 30, 2012
    Build a "diode bridge" using suitable Schottky diodes. Polarity in becomes irrelevant.
     
    Last edited by a moderator: Oct 26, 2013
  4. davenn

    davenn Moderator

    13,786
    1,936
    Sep 5, 2009
    Hi adoggg
    welcome to Electronics Point forums
    enjoy your stay

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

    Dave
     
  5. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    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
     
  6. davenn

    davenn Moderator

    13,786
    1,936
    Sep 5, 2009
    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
     
  7. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    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
     
  8. JMW

    JMW

    90
    3
    Jan 30, 2012
    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.
     
  9. BobK

    BobK

    7,682
    1,688
    Jan 5, 2010
    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
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,480
    2,828
    Jan 21, 2010
    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.
     
  11. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,271
    Nov 28, 2011
    The OP seems to have seagulled this thread...
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-