Driving a 12V relay with 18V

[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

[Seekonk]

If not using a micro, I usually figure coil voltage to be half of rated value once closed.  To speed and insure relay pull in, put a 470uF capacitor across the resistor. This gives full voltage while the relay is in transition. If this is a high vibration application, voltage on the coil should be higher to prevent unlatching.  A relay needs the high voltage to have a strong enough magnetic field to overcome the air gap. Many 12V relays will not drop out until the voltage drops close to 3V.

[Adhith]

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.

Thank you very much Sir for the help could you please check the calculation that I have done
the coil resistance is 395 ohms
so the current through the coil is 12/395 = 0.0303 A
so the resistance = voltage drop/current through the coil =6/0.0303 = 198.01 approx 200 ohm
on looking the watt dissipation = 6x0.0303= approx 182mW So a normal 200 ohm quarter watt resistor will do the job right??

[Dave_C]

Yeah 

[james_s]

Look at the datasheets for some 24V relays, 18V may be within the acceptable range for some of them and certainly it will work in many cases. You can use a resistor on a 12V relay but you may be burning up a significant amount of excess power depending on the coil current.

[Adhith]

Look at the datasheets for some 24V relays, 18V may be within the acceptable range for some of them and certainly it will work in many cases. You can use a resistor on a 12V relay but you may be burning up a significant amount of excess power depending on the coil current.

I checked some datasheets and what I could find is that, a voltage above 19V is necessary for a firm switching. So i guess I should move with a 12V relay with resistor in series. The relay is used for charging a battery pack in my project and it wont be used more than 2 hours during a charging cycle. So would that be a significant amount of power wastage if its only used less than 2 hours??

[Dave_C]

The wasted power that james_s talked about is the power dissipated by the resistor ( 0,182W according to your calculations )
It depends on your requierments if that's acceptable or not.
If you powered the relay from a battery that would matter a lot, otherwise it's not really too much of a problem.

[Adhith]

The wasted power that james_s talked about is the power dissipated by the resistor ( 0,182W according to your calculations )
It depends on your requierments if that's acceptable or not.
If you powered the relay from a battery that would matter a lot, otherwise it's not really too much of a problem.

the relay is use to charge a battery pack and its charged with a wall adapter. so I guess it wont matter that much

[Raj]

Just use an src instead of relay