negative trigger relay backfeeding problem
negative trigger relay backfeeding problem I have a negative trigger relay setup as follows: 30+86 to 12v, 87 to output, 85 to switched ground trigger from another device. The problem is that when the trigger wire is not grounded, it is getting 12v and burning up the device. How can I fix this? If I use a diode on the negative trigger wire will it allow the relay to still see the ground? Which way do I put it in and what size? Thanks
Ryan
Ryan
Joined: Nov 2003
Posts: 11,328
Likes: 345
From: Jacksonville, FL (originally from Toronto Canada)
Sounds like you have a bad relay. That's assuming you have the output on 87 (normally open) and not on 87a (normally closed). A diode won't help unless your switched ground is producing power on the same wire when it's not switched on.
Tried a couple of relays. Same result. I was given some advice to put the diode across the relay i believe 85 and 86 in a certain direction which I wrote down but dont have in front of me, to fix the problem. What do you think? Thanks
Ryan
Ryan
Joined: Nov 2003
Posts: 11,328
Likes: 345
From: Jacksonville, FL (originally from Toronto Canada)
That could help but only if there was a possibility of current backfeeding from your switching device. Can you be more specific about what you're connecting (especially what you're using as the switching device)?
If you had a simple ground switch connected to 85 it shouldn't be possible for this to happen. You can test by just attaching a piece of wire to 85 then touching it to ground. The relay should trip when the wire is touched to ground and release when the wire is taken away. If that works as it should, you should suspect the switching device is not just a simple grounding circuit - it's feeding current into the circuit when it's not grounding. Headlight and foglight circuits often work that way.
If you had a simple ground switch connected to 85 it shouldn't be possible for this to happen. You can test by just attaching a piece of wire to 85 then touching it to ground. The relay should trip when the wire is touched to ground and release when the wire is taken away. If that works as it should, you should suspect the switching device is not just a simple grounding circuit - it's feeding current into the circuit when it's not grounding. Headlight and foglight circuits often work that way.
The device is a 2-step. It has a switch ground output when it is activated. I tested this wire and it has no voltage at any time whether active or not. When active it grounds as it is supposed to and the relay switches. The problem is that the 85 pin on the relay has a 12v charge when the 2-step wire is not active(grounded) I tested it with and without the wire attached to pin 85. The weird thing is that even with no ground attached to the relay, only the two 12v sources connected, there is a 12v charge on pin 85.
All that the relay needs to do is switch the neg output to a pos output. But the problem is that whenever the 2-step is not active it is feeding 12v back into the circuit and frying it. It doesn't happen right away, but over time. So I am not sure if it is the voltage that is a problem or the spike when the relay closes. Thanks
Ryan
All that the relay needs to do is switch the neg output to a pos output. But the problem is that whenever the 2-step is not active it is feeding 12v back into the circuit and frying it. It doesn't happen right away, but over time. So I am not sure if it is the voltage that is a problem or the spike when the relay closes. Thanks
Ryan
Joined: Nov 2003
Posts: 11,328
Likes: 345
From: Jacksonville, FL (originally from Toronto Canada)
You will have power at pin 85 as long as your power source connected to pin 86 is live. 85 and 86 are connected by a electromagnetic coil so pin 85 will have input voltage minus the voltage drop across the coil (which wouldn't be very big).
Everything sounds correct. I'm suspect that you are right about the surge as the relay releases. A diode in parallel across the 85 and 86 terminals will stop that. Just about any rectifier diode will work. Mount it with the stripe toward the input power end (terminal 86). This will help absorb the surge that occurs as the magnetic field collapses in the coil when you remove ground from pin 85.
Everything sounds correct. I'm suspect that you are right about the surge as the relay releases. A diode in parallel across the 85 and 86 terminals will stop that. Just about any rectifier diode will work. Mount it with the stripe toward the input power end (terminal 86). This will help absorb the surge that occurs as the magnetic field collapses in the coil when you remove ground from pin 85.
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Joined: Nov 2003
Posts: 11,328
Likes: 345
From: Jacksonville, FL (originally from Toronto Canada)
You can just solder it across the external terminals - no need to take it apart and mount internally.
As a side note, Bosch-style relays have recently started coming with internal diodes so they now recommend that terminal 86 connect to positive and 85 to ground (as you have done) - it used to make no difference if you had those terminals reversed.
As a side note, Bosch-style relays have recently started coming with internal diodes so they now recommend that terminal 86 connect to positive and 85 to ground (as you have done) - it used to make no difference if you had those terminals reversed.
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