Switch a 24 volt relay with a 1.5-2.0V signal 2

[tona2008]

Hi,

I'm working on a project where I will need to switch a 24 volt coil voltage relay with a 1.5-2.0V signal. I've read on some threads that you can use a transistor to do this.
Does anyone know the details of this?

Any help will be appreciated. Thanks,

 

[MiketheEngineer]

Not an EE and I have no clue if you can find a transistor of this capacity. But maybe you could find one that would pull down a 6V+/- relay which could surely pull down the 24V relay??

 

[DRWeig]

Put this in Google:

transistor switch circuit 24v

You'll get 7.8 million results, and lots of the top 50 have example circuits.

Best to you,

Goober Dave

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[OperaHouse]

Do you want to swith at a precise voltage?
How much current can that source provide?
How much current does that relay coil need?

 

[Sparweb]

A MOSFET might be more flexible than a transistor, and more tolerant of design deficiencies (I'm speaking from PERSONAL experience!)
One resistor on the gate to the 2V signal, plus a resistor to tie the gate to ground when the signal is off, is pretty much all you need to switch the relay coil on/off.
Assuming, of course, that the FET is correclty selected for these voltages and questions such as those posed by OperaHouse are adressed correctly.

STF

 

[IRstuff]

a 2V signal may be too low for many FETs. There's not much noise margin to be had...

Note that "transistor" is a more general term, since a MOSFET is also a transistor --> metal oxide semiconductor field effect transistor. I think the expectation is that the OP should hire or borrow someone that can help him out.

TTFN
faq731-376

 

[VEBill]

NPN transistor with the load (relay coil) wired from +V to the collector. Emitter to common ("ground"). Base with a series resistor (probably about 1k) to your control signal.

There's still a design step required to match the transistor to the requirements, and to choose the optimum resistor value. If this is all small signal work (low power relay), then perhaps a common 2N2222A transistor and 1k resistor would work. If the relay is large and requires moderate current, then you may need help. Post details.

This assumes that your 2v signal can provide enough current to match the current gain of the transistor and the current required by the relay coil.

 

[IRstuff]

And, the coil generally needs a snubbing diode to snuff the back EMF generated by the coil when the transistor is switched off.

TTFN
faq731-376

 

[Noway2]

I agree with VE1BLL, for a 2V control signal I would go with an NPN transistor. The 2V will provide enough voltage to forward bias the BE junction and turn it on but the big question as he said is do you have enough current gain for your application. In theory, the CE current is the base current multiplied by the transistor's BETA. While many text books like to base design on BETA it is a poor choice practically speaking. As a rule of thumb, you should assume the minimum beta and then size the base resistor to give you a 10x overdrive for switching capability. To calculate the base resistance, take the applied voltage minus about .6 to .7V and divide by the needed current ((load current / minimum beta)*10). Then you need to also determine if your 1.5 to 2V source (which sounds like a gate off a PLD or other latch) can even provide this. If not, you may need to provide a secondary switching level with something like your IO supply voltage as the source instead of trying to directly drive the output from your logic.

The above assumes your using the transistor as a common emitter and providing the ground side of the relay coil. If you don't drive the transistor hard enough, it will turn on but be in the active rather than saturation region which means that your ground voltage will be high and the transistor will be dissipating a lot of power, but probably not for very long.

 

[OperaHouse]

I'm kinda fond of using a LM431 as a small relay driver when a select voltage is needed to turn on a relay or LED. A three lead package that is basically an op amp and voltage reference capable of driving close to 200 mils. Adjustable down to 2.5V or down to 1.2V in the LMV431 configuration. Often can be found in an old PC power supply.

 

[mikekilroy]

or two 2n2222 or 2n3904 wired as darlington to increase beta from 100 to 1000+. or fet