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Driving a 12V relay with 18V

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Adhith:
hello everyone...
I working with a project where im using a 12 VDC as the coil voltage for a SPDT relay to trigger another circuit. But the problem is that a voltage source of 18V is only available. So what would be a good method to use the relay??
I cant find a 18V relay from shops and buying one online would take minimum of 2 weeks and the project is supposed to be finished before it. So  could I use a linear regulator 7812 to bring down the 18v to 12v or could I use a 24VDC relay assuming that the 18V could drive the 24v one or any other suggestions??

Dave_C:
Hi!

24V relay should work at 18V, but it's contacts might not be opening/closing as fast as they should. That might increase arcing during switching heavy loads and reduce lifespan of the relay.
To use 12V relays you can reduce the voltage across the relay coil by adding a resistor in series with it.

First, measure the coil resistance.
To get the current you want the relay to work at, divide 12V by that resistance.
Next, Divide the voltage that you want to drop ( 18V - 12V = 6V ) by the current you calculated
You get resistance that you need.

Faster method:
The nominal coil voltage is 2/3 of supply voltage.
So you want the ratio between coil resistance and total resistance (of both the coil and the resistor ) to be the same

Example: If coil resistance is 200Ohm, then choose a 100Ohm resistor.


digsys:
.. or throw in a say 5V1-6.0V 1W zener in series

Wolfgang:
... or measure the coil resistance of your relay and add a simple resistor  :)

Ian.M:
... or if its being controlled by a signal from a MCU, PWM it.
See http://www.te.com/commerce/DocumentDelivery/DDEController?Action=srchrtrv&DocNm=PWM_and_Auto_Relays&DocType=CS&DocLang=EN

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