Driving 6V relay from 6 to 12 Volt

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.

Delta Prime

Joined Nov 15, 2019
1,311
Hello there
welcome to AAC!
I have a circuit (on paper, at least) that drives a relay via a transistor
I have the advantage of a crystal ball
But perhaps you can share this piece of paper containing your circuit thank you if that's possible if not I apologize

crutschow

Joined Mar 14, 2008
32,926
What signal controls the transistor?

MrChips

Joined Oct 2, 2009
29,245
Put a resistor in series with the coil.
Then put a 7.5V zener diode across the coil.

AnalogKid

Joined Aug 1, 2013
10,586
The supply voltage that is being switched into the relay coil by the transistor is is unknown,
Why?

Delta Prime

Joined Nov 15, 2019
1,311
I think you guys scared him away

crutschow

Joined Mar 14, 2008
32,926
Put a resistor in series with the coil.
But that would make the 6V relay pull-in even more marginal at the 4V minimum supply the TS mentioned.

crutschow

Joined Mar 14, 2008
32,926
I think you guys scared him away
Don't see how.
He hasn't been back since his first post at 6:25PM.

BobTPH

Joined Jun 5, 2013
7,557
To really fix this, a buck boost coverter regulated to 6V would be the answer.

Bob

Ian0

Joined Aug 7, 2020
8,406
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 sinkR1 = 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.

DickCappels

Joined Aug 21, 2008
10,068
The buck converter might not satisfy his 50¢ criterion

Ian0

Joined Aug 7, 2020
8,406

Ian0

Joined Aug 7, 2020
8,406
Another idea, that might fly (especially if it's a big relay).
http://www.farnell.com/datasheets/1720547.pdf
is just around his price point.
Delete the LEDs and L is the relay coil inductance.
The device in question won't manage 4V, but there are plenty of others that work the same way.

MrChips

Joined Oct 2, 2009
29,245
But that would make the 6V relay pull-in even more marginal at the 4V minimum supply the TS mentioned.
TS did not state the current required to activate the relay.
Therefore I did not specify the resistance and power of the resistor and the power of the zener diode.

BobTPH

Joined Jun 5, 2013
7,557
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
2,594
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.

crutschow

Joined Mar 14, 2008
32,926
TS did not state the current required to activate the relay.
Any resistance will reduce the voltage to the relay.

BobTPH

Joined Jun 5, 2013
7,557
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
32,926
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..

Ian0

Joined Aug 7, 2020
8,406