Driving 6V relay from 6 to 12 Volt

Thread Starter

memotronics

Joined Apr 9, 2021
1
I have a circuit (on paper, at least) that drives a relay via a transistor. The relay coil is nominal 6V (works from 4V to 8V). The supply voltage that is being switched into the relay coil by the transistor is is unknown, somewhere between 4 and 15 Volts. Obviously that becomes a problem if it happens to be over 8 Volts.

I have to find a way to "regulate" the voltage so that it doesn't exceed the 8 Volts. My first thought is to put a resistor in series with the coil, and then bypass that resistor if the supply voltage is low. Is there some other way? The solution has to be dirt cheap, like under 50 cents.
 

Ian0

Joined Aug 7, 2020
3,268
1) Drive the relay with an LP2951 voltage regulator (or some other regulator with a shutdown/enable pin)
Use the regulator enable pin to switch it. That gives you a logic input, which might be handy.
LP2951 is about 40p from RS.

2) Drive the relay with a current sinkrelay.pngR1 = 0.6*coil resistance / Nominal coil voltage

Use a dual transistor if space is a problem
Cost = 2p for the extra transistor, 1p for the extra resistor.
Q1 might need to dissipate some heat at maximum input voltage.
 

Ian0

Joined Aug 7, 2020
3,268

BobTPH

Joined Jun 5, 2013
3,682
Well, I would question any designer who tries tries to activate a 6V relay with it's minimum operating voltage of 4V, especially in a mass produced commercial product, which is the only place a 50 cent cost increase is going to hurt.

Bob
 

ElectricSpidey

Joined Dec 2, 2017
1,814
If the drive voltage of the transistor is between 5 and 9 volts a emitter follower configuration may work.

Of course if the collector voltage is actually 4 volts then it wont work.
 

BobTPH

Joined Jun 5, 2013
3,682
Any resistance will reduce the voltage to the relay.
As will the Vcesat of the transistor (assuming bipolar.) So the product is going to be out of spec when powered by 4V. He will not get 4V on the coil which is the min to activate the relay.

Bob
 

crutschow

Joined Mar 14, 2008
27,751
If he really needs 4V at the coil from a 4V supply then below is a circuit that will do that while limiting the voltage to 6V.
It's basically a simple 6V LDO regulator with the drop-out voltage determined by the MOSFET on-resistance.
But it will likely go over his cost limit.

The circuit can also act as the relay switch if the Q1 base voltage is pulled to ground.

The MOSFET needs to be a logic-level type device.

At the bottom is the circuit with a bipolar transistor, If he can tolerate a 100mV or so Vce(sat) reduction in the relay voltage..

1618073351601.png

1618073895471.png
 
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