Having recently read about the advantage of using diode supressed relays, I am think about adding diodes to all the little "cube" relays I have on our coach. Most are 12 volt and a few are 24 volt. What type and rating diode shoulde I use? Several years ago, Gary sent me some really big diodes to install on my solenoids. I know these would work, but I would like something a little smaller if possible. Thanks in advance. Jack
General purpose 4001 (1A @ 50V) is very popular & low cost in electronic supply or Radio Shack. Hook band to negative side.
Sojourn for Christ, Gerald
I think I use 4004 diodes on mine. Basically the same as what Gerald said.
How many do you need? I'll send you some.
craig
I think that diodes can be very good at extending the life of switches and contacts in relays. Howver, I don't think that it's as simple as "hook the band end to the negative side".
If the diode is connected across a winding, the band end should be connected to the source of a negative going pulse. This is going to occur when the switch opens, but it will be at the positive end of the winding. The rest of the time, no current will flow through the diode.
If the diode is connected with the band to the negative end of the winding, the fuse or circuit breaker will blow and the coil will never be powered up, if the diode does not burn out first.
We used the diodes extensively with some troublesome concrete pumping controls and it worked very well.
For what it's worth.
Tom Caffrey
Good point. I missed that on Gerald's post.
Band end goes to the positive side.
Many cube relays already have either diodes or MOV varistor suppressors built into them. You can usually see on the diagram printed on the side... if you see a little thing that looks like this (https://busconversionmagazine.com/forum/proxy.php?request=http%3A%2F%2Fwww.heartmagic.com%2Fcube.jpg&hash=d5ec15b534e93f8daa35a9a9d795fec24d54bee7) , a diode or anything else shown in parallel with the relay's coil, it probably has protection built in. The MOV version is nice because polarity doesn't affect it...
Jack,
Are you wanting to protect the relay drive electronics, or the relay contacts? Two separate issues with different solutions. The 400X series diodes mentioned are fine for eliminating the inductive kick of the collapsing field of the relay coil. The point here is to restrict this voltage surge going back into the driving solid state electronics. In most cases, in our buses that is, we are not driving the "automotive" or "cube" type relay from solid state circuitry. Hence if a switch or another relay is actuating a downstream relay, a suppression diode is not required. That said, I put 4007 diodes across all my relays. Just good design practice. By the way, the band on the diode represents the cathode of the diode. It ALWAYS is connected to the +12/+24 vdc source end of the relay coil. To do otherwise (connect anode to positive voltage), will forward bias the diode and destroy it. Their are time when you want to forward bias a diode. This ain't one of them.
Most relays don't have MOV's across the contacts because MOV's are voltage selective which negates the value of the voltage/current translation effect of a relay. In other words, they become limited where they can be used. The largest variable of relay and/or switch contact life is dictated by voltage level, whether AC or DC, and the type of plating on the relay contacts. All this is more important in high current applications, more so with DC than AC. This is why all relay catalogs talk about coil voltage as one parameter, but denote contact ratings and plating as a separate parameter.
Feel safe with diodes across all your relay coils, regardless of driving source. Just make sure you never get reversed polarity at the batteries or you will fry many of them.
Forgot to mention, Jameco electronics (www.jameco.com) has relays, and pre-installed diode relay sockets with wire leads. So you don't have to piece them together. Very reasonable prices. Have many in my trucks and bus.
Quote from: gumpy on April 14, 2009, 05:09:04 PM
I think I use 4004 diodes on mine. Basically the same as what Gerald said.
How many do you need? I'll send you some.
craig
Thanks Craig,
Probably about 20. I will be glad to pay you for them. Ijust thought this would be cheaper than replacing all those relays with the "new improved" diode suppressed relays. Jack
Jack, I just want to point out one downside to placing diodes across dc relay coils. In some cases the diode will cause a delay in the opening of the relay or slow the opening speed of the contacts. This can cause burned contacts etc. Most bus circuits with switch or plain contact driven relay activation need no suppresion. The reason for the slow speed of the relay is the ringing caused by the diode as the field collapses then reversing direction until the energy dissipates. I still have the detroit intake parts if You need any. Regards John
Slow relay release is a possibility, but for that to happen in the practical world, it'd have to be a very large coil. Cube relays won't know the difference. Arcing contacts from "slow releases" will give you a lot less trouble than poofed doodahs from spikes.
In my opinion, ANY relay coil needs suppression. Not so much for the thing driving that coil as much as for other sensitive electronic things on the bus. Spikes from un-protected relay coils can easily reach hundreds of volts and kill things you wouldn't think were even near the circuit.
Case in point, I have a 100A contactor that joins my house batteries to the starting battery. I used to not have the coil protected. One night I switched it off and noticed that as I did, the LED that told me my Jakes were engaged flashed! Totally different circuit, getting hit with a pretty decent spike. I thought "oh well no biggie" until a month later, again at night, I switched that contactor off as I was listening to my $600 stereo..which instantly took a dump from the spike and had to be replaced.
Do what you wanna, but after that, EVERY relay on my bus got a diode and I've not had a problem since....
Cheers
Quote from: JackConrad on April 15, 2009, 04:53:26 AM
Thanks Craig,
Probably about 20. I will be glad to pay you for them. Ijust thought this would be cheaper than replacing all those relays with the "new improved" diode suppressed relays. Jack
Jack,
I'll put them in the mail this morning.
Craig