Using PicoScope to understand automotive relays

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Using PicoScope to understand automotive relays

Postby Alan » Tue Mar 06, 2012 5:35 am

Apprentices investigate the behaviour of a standard 12V, 30 Amp automotive relay as it is switched off. They use a two-channel PicoScope to observe that each time the relay is de-energised, a significant voltage spike is generated in the energising circuit, 86 - 85. They also discuss what way the field circuit should be connected in a relay. There is a mis-conception that it makes no difference which way the field is connected, the relay will work either way. This much is true, but the DIN standard identifies 86 as the start of coil and 85 end-of-coil. This standard must be followed because if the relay has a diode fitted to eliminate the voltage spike, it is always connected anode to 85 and cathode to 86 (facing back toward live). If the relay is connected incorrectly, it renders the diode useless, and the relay will put a voltage spike into the system everytime it is switched off. http://cpfitters.info

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Re: Using PicoScope to understand automotive relays

Postby AuthorMike » Fri Mar 09, 2012 3:08 pm

Great video, great find. Really good explanation of how, and why!
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Re: Using PicoScope to understand automotive relays

Postby Kev » Sun Mar 11, 2012 9:15 pm

I think I'ts important to understand that in practice you will find these 'standard type' of relays wired both ways,for example 'term 85' with positive polarity on it (Relay removed). I've encountered this on many occasions, the reason being that if the relay is being used for anything other than powering up an 'electronic' piece of equipment, it generally doesn't matter. The key thing to remember here is to be 'open minded' when diagnosing, look at it for what it is, is it powering a piece of electronic equipment? 'Yes or No'. Most relays have a small diagram shown on the body of the relay itself, if you see a clipper diode symbol across terms '85 & 86' then replace it with the same type of relay. Also note on these type of relays that it isn't unusual to see Term 87 with B+ on it instead of Term 30, (Relay removed of course).

Hope this helps Kev
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