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24v relays
I ordered one 12v relay, two 24v relays for spares so I replaced R78 & R79 fan relays with the new 24v relays, When I ordered those they said 24v relays have a resistor in them, can’t tell looking at the old one, here is a couple of pictures is this correct, Resistor?? New on top Old on the bottom
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7 Replies
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The relay in the top is the built in resistor type, no flyback diode.
The relay in the bottom picture has a flyback diode and I do not see a resistor in the relay schematic.
A flyback diode or freewheeling diode is placed with reverse polarity from the power supply and in parallel to the relay’s inductance coil. The use of a diode in a relay circuit prevents huge voltage spikes from arising when the power supply is disconnected.
I’m not sure about the inter changeability between the 2 types. We’ll need someone smarter than me to chime in for that answer.
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I don’t know if I am smart, but I order the resistor type from Prevost, last bag was 10 relays for $40.
I use them interchangeably with everything else.
So far so good. The bus is still running and everything works.
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So I dug a little deeper into the relay quandary. I looked up the relays used in your coach for the R78 and R79 fan relay’s and found that the Prevost part number that I came up with was 561707, which is listed in the parts manual as, 561707, RELAY 24V S.P.D.T. W/RESISTOR.
Jon was right. I hope this helps.
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I called back Friday and they both said this is the relay that they sell for my coach?? doesn’t mean it’s correct but the new one they said it needs a resistor and I take it that the R means
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Both the fly-back diode and the resistor are cost effective transient suppression schemes for relays. The transient voltage can be several hundred volts potentially damaging the ECM or other electronics.
The resistor is cheap and reduces the transient to around 70V (depending on resistor value) but the resistor will always have current flowing through it and thus increases power consumption.
The diode clamps to 0.7 volts but increases the release time of the relay by a factor of 5 and can lead to “tack” welding of the relay contacts. So, diode and resistor are not quite interchangeable. I would stick with what the manufacturer put there.
1999 XLV X-6872, liberty #440 -
So your saying I’m good with what Prevost sold me??
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I’m sure it will be fine. If Prevost sells the resistor type now as a replacement, they are obviously not concerned about the higher transient and the additional power consumption when active.
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