# Can I use 12 volts on a 5 volt coil relay?

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#### Dan Kuschill

##### New Member
I am using a JRC-21F 4100 relay. The electricity I am using for the coil may be 5 volt DC or may be 12 volt DC. If I use a relay that is rated at 5V for the coil, I have noticed that either 5V or 12V will activate it. My question is if I use a JRC-21F 4100 5 volts, can I run 12 volts though it will no long term problems? I am just asking about the coil part.

Thanks

##### Well-Known Member
You could eventually damage the relay running off 12v for a 5v relay, you could wire 3 diodes in series with the coil as a simple dropper.
Max.

#### MikeMl

##### Well-Known Member
Or a resistor. Look up the coil resistance on the relay data sheet, or measure the DC resistance of the existing relay. To find the required resistor, multiply the coil resistance by (12-5)/5 = 1.4. The power rating of the resistor would also need to be 1.4 times whatever the coil power is at 5V.

If you need to make the switch-over automatic, write back and we will come up with something...

#### Dan Kuschill

##### New Member
Thank you very much. Both of your answers took care of me. I was thinking it could cause long term problems, so I figured I would ask. The resistor solution works very well for me. Thanks for giving the formula also.

#### MikeMl

##### Well-Known Member
The power rating of the resistor would also need to be 1.4 times whatever the coil power is at 5V...
Second thought: or is it 1.4^2???

#### JimW

##### Member
Is the O.P. saying that at various times the activating voltage would be either 5V or 12V? If this is the case, then the diode or resistor voltage drop solution is not viable. Seems to me that you should know what the voltage will be, and then you pick the correctly spec-ed relay. But if it could be either, then you will need to be more creative on the solution.

JimW

#### Les Jones

##### Well-Known Member
If I understand your problem correctly you want the relay to opperate with either 5 volts or 12 volts without the relay being damaged when opperated from 12 volts. I did initialy think about suggesting an 5 volt LDO regulator but I do not know if they would have a low enough dropout voltage for the relay to work when the regulator was supplied with 5 volts. here is a suggested solution. It is just a modification of the resistor idea.

Provided the relay coil takes less than about 80 mA then a BC557 (Or similar) should be OK for Q1 and Q2. You will have to work out the value of R1 to suit the relay current. R2 would be 1K, R3 10K, R4 4.7K The zener diode could be either 5.6 or 6.8volts.
This is how the circuit works. When supplied wit 5 volts Q2 would not turn on as the zener diode would not conduct. As Q2 was not conducting Q1 would be turned on shorting out resistor R1. If the voltage was above the zener voltage plus vbe of Q2 (About 0.6V) then Q2 would turn on. This would short base to emitter of Q1 so it would not conduct and hence not short out R1. If the relay coil takes more than 80 mA then some components would need to be changed.

Les.

#### Diver300

##### Well-Known Member
Second thought: or is it 1.4^2???
No, because the current will be the same as the rated coil current. It is only the voltage that will be 1.4 times the rated coil voltage.

Of course, rating the power dissipation of the resistor 1.4 times larger won't matter.

#### Dan Kuschill

##### New Member
I had a PCB board designed, and it was designed around a 12 volt coil, because originally I was only planning on coming off of a 12V source. I later realized it may be good to come off of a 5 volt source which is also available for the coil. Most of the time using the 12v would be best, but sometimes 5v would be better. I will have about a 2 foot wire going between the relay and the coil power source, so I am feeling the best thing to do is put in a relay with 5 volt coil, and on the wire have the option to put in a resistor.

This is the relay that I am using with specifications listed, what ohm resistor should I use?

http://www.ebay.com/itm/10-50-100-p...hash=item2819e5045a:m:mdau_epFNUqdUqwSFW3CIdw

##### Well-Known Member
That shows a 12v coil relay?
To ensure you use the correct resistor, measure the coil resistance when delivered.
BTW, on these types of PCCT relays, make sure you open the sealed vent hole if it is fitted with one.
I don't have the PDF on this unit but can post if needed.
Max.

#### Tony Stewart

##### Well-Known Member
I am using a JRC-21F 4100 relay. The electricity I am using for the coil may be 5 volt DC or may be 12 volt DC. If I use a relay that is rated at 5V for the coil, I have noticed that either 5V or 12V will activate it. My question is if I use a JRC-21F 4100 5 volts, can I run 12 volts though it will no long term problems? I am just asking about the coil part.

Thanks
NO do not use a 5V coil on 12V. if it dissipates 0.25W at 5V ( according to,spec) then the coil,DCR must be 5*5/0.25=100Ω
so with 12V you will dissipate V^2/R=P= 144/100=1.44 W and it will be almost three times as hot.

If you need a power switch that works on both 5V and 12V, I suggest you replace the Relay with a high side smart power switch or NPN + PNP transistor and choose a base current of 2-5% of the output current

#### spec

##### Well-Known Member
NO do not use a 5V coil on 12V. if it dissipates 0.25W at 5V ( according to,spec) then the coil,DCR must be 5*5/0.25=100Ω
so with 12V you will dissipate V^2/R=P= 144/100=1.44 W and it will be almost three times as hot.

If you need a power switch that works on both 5V and 12V, I suggest you replace the Relay with a high side smart power switch or NPN + PNP transistor and choose a base current of 2-5% of the output current
Perhaps a bit more power dissipated going from 5V to 12V. Power ratio in a resistor = (V2*V2)/(V1*V1)= (12V*12V)/ (5V*5V)= 5.76

spec

#### Mosaic

##### Well-Known Member
This may be a little OT.
I have a similar situation driving both 12V and 5V relays via a 2n2222 driver using a dropping resistor. All's fine...EXCEPT that my uC used the Vcc as a Vref for a 4 wire +/.02V measurement system.
When those relays pulled, the circuit ground saw a ground bounce spike of around 0.1V and then settled back to 0.05V offset. Naturally this messes with any precision vref.

I had to use a split analog/digital ground backed by 100uF lo ESR decoupling caps and a star config to the main ground Buss. Kept the effects down to 0.005v

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