Buzzing relays

[junkman42]

Just a thought.  Impedance does not mean a hill of beans when it comes to dc being applied to a relay coil.  With ac yes dc no.  The resistance of the coil does however.  A bridge rectifier does not increase the power applied to the coil.  If you look at the output of a bridge rectifier You will not see pure dc, You will see pulsating dc.  In short some relays will not care and some will.  The transfer relay in My coach, a commerical unit uses a bridge rectifier to power the relays.  The relays are a common heavy duty relay seen in many pieces of commerical equipment.  The reason for this is to reduce the hum being heard during normal operation.  All relays produce magnetic fields and by using dc metal panels,read ferrous panels will not resonate at the applied ac frequency.  Try it it might work.  Do not put a capacitor on the output side of the bridge in this application.  My 2 cnts.  John

[gumpy]

Thanks John,

That was kind of where I was headed. Try it. See what happens. I figure the worst thing that could happen is the relay coil burns out, and I have to replace it. At this point, I'm there already, and the bridge may just allow me to salvage the relay and keep it going for "awhile longer". That "awhile longer" might be a day, or a week, or it might be 20 years. Regardless, it's an improvement over declaring the relay dead today and replacing it. I figure I'm only out a few bucks and a little time to try, and I might actually learn something new here.

I was not planning on putting in a capacitor. If I were to replace the relay with a DC coil relay, then I'd probably add the capacitor and resistor to make a cleaner DC trace.  But that does lead me to wonder why the bridge output does not produce the same buzzing as the AC does. It's waveform also drops to zero between each rectified half-cycle, so shouldn't the coil begin  dropping out on each of the low voltage dips in the wave?

It sure seems to work, though. The test I ran with the 4 1N4004 diodes was very impressive with how quiet the relay became.

[gumpy]

Just thought I'd post an update to my buzzing relay problem.

When we returned home yesterday from our mini-rally at Devin's (awesome time, by the way), the bridge rectifiers I ordered from DigiKey were waiting for me. I soldered on some leads, and spent a bit of time connecting them into the coil wires on the two relays that were buzzing and chattering. Plugged in the coach and voila, no buzzing or chattering.

5 minutes later, I heard a relay click, and thought, "Hmmm, that's not supposed to happen."  I opened my transfer switch cover, and smoke rolled out.

The upshot of it is one relay coil fried itself and the other was just seconds away from doing the  same.

So, as usual, Richard was correct, and the relays ate themselves on DC in a very short time.

What really gets me is that after replacing the two bad relays, I plugged in the repaired box a couple times, and on the second plug-in, one of the remaining 2 original relays started chattering!!  So, I'll probably be replacing them all before long. I'm going to dissect one of the two I replaced and see if there are any indications of why it failed.

Also, one of my bus friends wrote and said he might have some 60amp relays that he had salvaged. I like to install them in place of these 30 amp relays. I don't think that has anything to do with the current problems, but I'd feel better if I didn't have to have my contacts tied together to get adequate amp throughput.