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Posted by: Perry Kiehl Jul 11 2017, 11:46 AM

I'm making a harness that needs a power circuit switched via relay.

I've got generic Bosch style 5 pin relays, and I'm seeing a voltage drop on the power out side of the relay.

It's rare that I make a harness that needs a relay, and in the few that I've done with a relay circuit, this has never been reported as a problem.

I'm not an electrical engineer, but I always thought a relay was just an electro-mechanical switch and I should be getting the same voltage on the output.

Just to be clear for those who do know more about this than me, none of the wires that I'm using to test this are more than 24" long and none are less than 14 gauge wire.

The relay power is for a coil pack, so I'm sure that needs a good 12+ volts.

Posted by: 914Sixer Jul 11 2017, 12:03 PM

Are all the relays showing the same test?

Posted by: Perry Kiehl Jul 11 2017, 12:14 PM

Yes, Mark they do. I currently have only cheap ones, no real Bosch or Hella. I've subsequently read about release voltage of a relay, but I don't know why the ones I got would do that as none of the similar Bosch type relays say a thing about "release voltage"

Posted by: Bartlett 914 Jul 11 2017, 12:27 PM

If you can, measure at the relay input and output. There should be zero volts. If measuring from the ground, then both input and output should read the same voltage (12 volts). If there is a difference, then the relay is bad, There should be no drop at the relay contact. It the circuit has a large load, then there can be a voltage drop across the length of the wire. Shorter wires or larger gauge in the answer

Posted by: danschy Jul 11 2017, 12:54 PM

Pretty hard to get a significant voltage drop across relay contacts. Are you sure your relay has mechanical contacts and it is not some kind of solid-state relay? Do you hear a metallic click when activated?

Posted by: jd74914 Jul 11 2017, 12:55 PM

Do you know the circuit load (current)? How are you testing the drop? As Mark said there shouldn't be much drop across a good mechanical relay since there is minimal contact resistance. Could you be pulling down the power supply voltage?

Posted by: Perry Kiehl Jul 11 2017, 01:36 PM

This is all bench testing. I'm using a battery charger for the power supply, I'm measuring with a digital multi-meter, and I'm measuring output via 12" 14 gauge test leads on the relay socket

Posted by: Spoke Jul 11 2017, 02:16 PM

It is possible that the battery charger is full-wave rectified with no filter caps, not pure dc.Not good if used by itself.

Posted by: Spoke Jul 11 2017, 02:19 PM

What voltage do you measure at the output of the charger?

What is the part number of the relay?

Posted by: Dave_Darling Jul 11 2017, 05:59 PM

Could that show a voltage drop across a switch like in a relay?

PK could try using a battery instead of a charger/voltage supply box.

I'd be tempted to pry open a relay to see WTF is really going on in there--but I've got bit of the old "destructo" still in my soul.

The round Bosch relays literally have contacts like a set of points do. Should be zero resistance or voltage drop across them unless there is contamination/corrosion of the contacts.

--DD

Posted by: Perry Kiehl Jul 11 2017, 08:50 PM

I feel sooo stuuupid! It's the battery charger. Whenever the coil is energized the voltage drops.

Posted by: Spoke Jul 11 2017, 08:52 PM

It's called "fog in the cockpit". Happens to everyone at one time or another.

Bottom line, does the relay work?

Posted by: Perry Kiehl Jul 12 2017, 08:31 AM

Yes

Posted by: 914_teener Jul 12 2017, 07:06 PM

classic.....


Reminds me when I thought my compressor switch was bad.

Posted by: Perry Kiehl Jul 12 2017, 07:44 PM

The smart guys figured it out right away.