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Old 08-25-2012, 12:50 AM   #1
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Default How to test a relay



Does anyone know how to test a relay? It's the blue one in the upper left hand corner. Thanks.
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Old 08-25-2012, 02:26 AM   #2
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I'll tell you how not to test a relay. If it clicks, that doesn't mean it is working. Learned that the hard way.
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Old 08-25-2012, 04:16 AM   #3
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Remove the relay.

Measure the resistance between pins 1 and 2. It should be more than 10 Ohms and less than 300.

Measure the resistance between pins 3 and 5. It should be infinite.

Apply 12V on pin 1 and ground on pin 2. The relay should click. Now measure the resistance between pins 3 and 5, with the power still applied to pins 1 and 2. It should be less than 1 Ohm.
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Old 08-25-2012, 10:40 AM   #4
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Quote:
Originally Posted by Reverant View Post
Remove the relay.

Measure the resistance between pins 1 and 2. It should be more than 10 Ohms and less than 300.

Measure the resistance between pins 3 and 5. It should be infinite.

Apply 12V on pin 1 and ground on pin 2. The relay should click. Now measure the resistance between pins 3 and 5, with the power still applied to pins 1 and 2. It should be less than 1 Ohm.
This dude is a ******* bauce.
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Old 08-25-2012, 03:28 PM   #5
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Thanks Reverant. ^Agreed Huslter.

Quote:
Originally Posted by Reverant View Post
Remove the relay.

Measure the resistance between pins 1 and 2. It should be more than 10 Ohms and less than 300.

Measure the resistance between pins 3 and 5. It should be infinite.

Apply 12V on pin 1 and ground on pin 2. The relay should click. Now measure the resistance between pins 3 and 5, with the power still applied to pins 1 and 2. It should be less than 1 Ohm.
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Old 08-25-2012, 03:33 PM   #6
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Oh, just so I don't blow anything up - can I get 12v from the battery or is there too much current? Electronics = FML.
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Old 08-25-2012, 03:47 PM   #7
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Yes you can, provided that test 1 doesn't fail.
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Old 08-25-2012, 07:04 PM   #8
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Quote:
Originally Posted by kenzo42 View Post
can I get 12v from the battery or is there too much current?
As a general tip, there is no such thing as "too much current."

You can apply too much voltage to something, however the amount of current that any given device will draw is self-limited. Specifically, the formula is I = E / R, where E is the voltage which you supply to the device, R is the internal resistance of the device (or impedance, in the case of inductive AC loads), and I is the amount of current that will pass.

A hypothetical device with a coil resistance of 500 ohms will thus draw 24 milliamps when 12 volts is applied to it (12 / 500).

When speaking of source current (such as from a battery, alternator, or other power supply) we mean only the maximum amount of current which you are permitted to draw from the supply before it either explodes or falls below rated voltage. So a power supply which is rated at 10 amps can supply any load (or combination of loads) whose total current requirement (as I=E/R) is any arbitrary value between 0 and 10 amps.

(Yes, for the EEs in the crowd, I'm aware that some supplies have a minimum load for guaranteed stability.)
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Old 08-25-2012, 07:07 PM   #9
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As an aside, this was the first thing that came to my mind when I saw this thread: http://jalopnik.com/5198663/advances...sch-relay-race
Advances In LeMons Cruelty: The Bobby Bosch Relay Race



The Taiwanese National Anthem wasn't the only new penalty for the 24 Hours Of LeMons South Spring; though the TNA turned out to be quite spectacular in practice, it isn't the most diabolical new penalty.



Welcome to German Electrical System Hell! The Bobby Bosch Relay Race is pretty simple, and it really separates the real BMW, Porsche, VW, and Audi mechanics from the pretenders: you take ten standard Bosch SPDT relays (pocketed at the junkyard, of course) and intentionally garboon one of them. When you've found it, the car can rejoin the race. Maybe the coil is bad, maybe the normally-open contacts are bad, or maybe I've been a total bastard and permanently jumped the normally-closed contacts. Whatever, because Bobby prints the schematic right on the relay cover, so picking out one bad one in a bucket of ten should take about five minutes, max. Sadly, some teams don't understand Relay Testing 101, and so it takes them a bit longer than that... and you'd think an E30 team would be really, really skilled at this, eh?
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Old 08-26-2012, 04:11 AM   #10
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I had to get the Porsche towed because I didn't have a multimeter onboard. Had I known the info in this thread, I would've known to swap the FP relay for the 'something else' relay to drive home, looking like a bauce.
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