I have a working relay drive circuit using a low-side N-Channel MOSFET driven by a microcontroller. The relay coil is 6V, and everything there works. However, I want the same coil to work with a 12-volt supply as well, without burning the coil. My thought was to have a resistor in series with the coil to drop the voltage, and another MOSFET in parallel with that resistor. If the voltage is below 8, the MOSFET is driven to bypass the resistor. If it's above 8V, the MOSFET is off so that the resistor can drop the voltage. It's the part where I drive the MOSTFET based on voltage levels that I'm struggling with.

 

Not exactly the solution you are looking for but something like this?

 

You could put a current limiter on the low side switch, that way it would work well at any voltage of 6 or more. I think it would need only a BJT and resistor. How much current does the relay pull at 6V?

 

Not exactly the solution you are looking for but something like this?
View attachment 304812

Note that this circuit clamps the supply to 10.6 Volts via the B-E junction and Zener, add a resistor in series with the base.

 

Using chip AL8861 you can feed coil of relay
by constant current up to 1.5 A in voltage range 4.5-40V
with high efficiency.
Relay coil L1 should be connected without LEDs.

 

What is the Current requirement for the Relay-Coil at 6-Volts ?
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The relay coil is 1W, so 167mA. I forgot to mention that, when the MOSFET is off, the circuit cannot draw any current. When I look at what Jerry has drawn up, that was more or less what I had in mind except I wasn't sure where to put the zener. In that drawing, the bottom end of the Zener is connected to ground. Could I connect it to the drain of the MOSFET instead for the same result? I assume the transistor is a PNP?

Bob, can you explain a bit more about your idea of a current limiter?

 

How about a constant current regulator LM317, set to 166 mA using a 7.5ohms resistor.

What is the resistance of the relay coil has it might pull in with a lower current?

 

what is the reason to use relay with 6V coil?
if the device can be powered from different sources why not step voltage down for the entire circuit, rather than messing with relays individually?

 

Here is the idea:


V2 represents the micro output.
R2, Q1 and R3 are the current limiter. Adjust R3 to adjust the target current.

As you can see, it does not exactly regulate the current, but it might be sufficient to keep it within the operating range. It loses about a volt with the 6V input

 

Why not use a 5V relay? You obviously already have a regulated 5V supply.

 

Why not eliminate the Relay altogether ??

This endeavor is looking at only one small part of the Circuit.
There are probably ~50 different ways to accomplish the same end result.
Some are much more efficient than others.
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Unless the supply voltage varies very widely, a single series resistor will not waste any more power than any of the much more complex circuits shown. So to drop siz volts at 167mA, R=V/I=6V/.167A=39.2 ohms, so use a 33 ohm resistor or a 47 ohm resistor in series.

 

Unless the supply voltage varies very widely, a single series resistor will not waste any more power than any of the much more complex circuits shown. So to drop siz volts at 167mA, R=V/I=6V/.167A=39.2 ohms, so use a 33 ohm resistor or a 47 ohm resistor in series.

That woks fine as long as the 6V relay pulls in at 3V.

 

When I look at what Jerry has drawn up, that was more or less what I had in mind except I wasn't sure where to put the zener. In that drawing, the bottom end of the Zener is connected to ground. Could I connect it to the drain of the MOSFET instead for the same result? I assume the transistor is a PNP?

Thanks for the mention, but I think I got it wrong! The PNP transistor needs to be turned on when the supply is 6V in order to short out the resistor and then I think you need another PNP transistor to turn it off when the voltage is 12V. Zener to the drain of the MOSFET? Probably, can't immediately see why not.

 

Using chip AL8861 you can feed coil of relay
by constant current up to 1.5 A in voltage range 4.5-40V
with high efficiency.
Relay coil L1 should be connected without LEDs.
View attachment 304820

The Vset pin takes and logic input to enable/disable the output. So no additional transistor is required.

 

OK, my math may have been off, I make no claim there, and I was suggesting standard resistor values, not the exact calculated value.
But if the circuit must work with both 6 and 12 volt supplies, then the first thought is indeed to power the relay from the 5 volt source.
AND, if this is for a standard product to be built by the 100's, use a 5 volt relay and use the 5 volt regulated supply. But then add that diode across the relay coil because there might actually be a transient pulse generated if it switches off fast.

 

But if the circuit must work with both 6 and 12 volt supplies…

That is the entire point of question. Read the OP again.

 

The only regulated supply I have in this circuit is 3.3V for the MCU (that drives the MOSFET to control the relay). The relay coil itself is connected to the unregulated side. So if my 6V coil is 40 Ohms, then another 40 Ohms resistor in series should do the trick for operating it at 12 Volts. It's the part where I "short-circuit" the resistor when the supply is at 6 Volts that's stumping me. What Jerry drew up uses 7 components. In my head, I'm thinking this should be possible with maybe 3? Like a MOSFET, a Zener, and a resistor (or two). But I'll be damned if I can put it together.

 

The only regulated supply I have in this circuit is 3.3V for the MCU (that drives the MOSFET to control the relay). The relay coil itself is connected to the unregulated side. So if my 6V coil is 40 Ohms, then another 40 Ohms resistor in series should do the trick for operating it at 12 Volts. It's the part where I "short-circuit" the resistor when the supply is at 6 Volts that's stumping me. What Jerry drew up uses 7 components. In my head, I'm thinking this should be possible with maybe 3? Like a MOSFET, a Zener, and a resistor (or two). But I'll be damned if I can put it together.

AL8861, Diode, Resistor is three (and it eliminates the MOSFET that drives the relay) - see @Danko's circuit.