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Old 31st August 2012, 11:38 AM   #1
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Default Found perfect relay replacement - but coil resistance is off. Which one is better?


I need to replace a relay in an old amplifier, and I found two almost perfect replacements, but none is entirely right. The original relay has a coil resistance of 200Ω, whereas the replacements have 170 and 270Ω. Which one should I choose? Does it even matter? The voltage is 12V if that is of any relevance.

Thanks a lot, readers.
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Old 31st August 2012, 12:05 PM   #2
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Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina
Coil resistance normally don't care so much, be sure that the driver (FET, Transistor, ULN200x, etc.) can manage the current, its all. Between them, I would prefer the lower resistance one, because surely, it has more important magnetic field in the gap, an then the catch force is higher, and low microphonics and better contact, usually, but non always.
Osvaldo F. Zappacosta. Electronic Engineer UTN FRA from 2001.
Argentine Ham Radio LW1DSE since 1987.
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Old 31st August 2012, 12:39 PM   #3
Mooly is offline Mooly  United Kingdom
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It would help to see the circuit although both those relays mentioned are probably near enough not to worry over.

If the relay coil has an external series resistor to "loose" some unwanted voltage (for example running a 12 volt relay on say a 30 volt supply) then you will need to replace that resistor with one suited to the new coil value so that the new relay runs with 12 volts across the coil.

And just to complicate things... some designs may use say a 12 volt relay which is "kicked" by a higher voltage initially and then allowed to run at a reduced voltage to minimise power loss and heat dissipation. So if possible measure the old relay volts with it actually working.
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Old 31st August 2012, 02:51 PM   #4
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Hi guys, thanks for the help. It shouldn't be a problem, the relay is running on a 12V supply. The specs for my replacement perfectly matches the old one, minus the coil resistance (I have datasheets for both), so I think everything will work out in the end.

Thanks again.

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Old 31st August 2012, 03:32 PM   #5
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When series fed from a higher voltage rail i usually pick the higher resistance relay and and add a parallel resistor to get the right coil resistans.

170 seems like a close enough value to work as is.
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Old 1st September 2012, 05:38 AM   #6
jitter is offline jitter  Netherlands
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Originally Posted by flatheadmurre View Post
170 seems like a close enough value to work as is.
Agreed, the voltage printed on a relay is a nominal voltage. Therefore it doesn't need to be exactly that, so if there were a series resistance, it might not need to be replaced if coil resisntace isn't too far off.

The datasheet of the relay will tell you the reliable operating range (the graphs on coil voltage with U/Unom on the vertical axis and temp on the horizontal axis).
E.g. the 200 mW range of the Axicom V23105-series has a reliable operating voltage (U/Unom) of approx. 0.7 at 20 degrees C. If the coil is specified at 12 V, then it will still reliably operate at 8.4 V (0.7 * 12). The max coil voltage depends also on the currents carried by the contacts, at 2 * 2 A you can go as high as U/Unom of 1.5 at 20 C, i.e. 18 V in case of the 12 V version (even higher if contact-current is less).

Last edited by jitter; 1st September 2012 at 05:52 AM.
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