How can I use a 5 vdc source to turn on a 3 vdc relay?

Current Boosting

How can I use a 5 vdc source to turn on a 3 vdc relay when the source can't handle the current?

Re: Current Boosting

One method is to use a power MOSFET in series with the relay on the ground side. Use the 5V to turn the MOSFET on. You will need to also put a diode across the relay coil to prevent turn off transients from damaging the MOSFET.

Re: Current Boosting

Jeff Schaub said:

How can I use a 5 vdc source to turn on a 3 vdc relay when the source can't handle the current?

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The primary coil is connected to 3Vdc? Could the relay be operated at 1.5V?

Re: Current Boosting

relay need curent not voltage, i hove this will help you

Re: Current Boosting

Jeff

If I were you I’d use an npn bipolar junction transistor (bipolar transistors are really current amplifiers).

This can be a little tricky because of varying HFE and junction V changes due to temperature. But by varying the resistor sizes you can taker care of that.

I’ve done this before, so I can assure you it will work.

I had a quick look and found this http://www.kpsec.freeuk.com/trancirc.htm

Have a look at the section called “Connecting a transistor to the output from a chip”

The circuit diagram in the above link is almost exactly what you need except you may have to place a resistor in-between the transistor collector and relay. (because the relay is 3V the supply is 5v Vce across the transistor will be aprox 1.2v you will apply about 0.8v too much to the relay. This may not matter depending on the relay)


Just a quick point you can usually sink more current than you can source from most chips, have you checked to see if you can sink the current (just a shot in the dark)

Hope that’s of some help, if you’re still struggling post the details of the Chip and Relay and I’ll help out next time I’m here.


Regards Engineer_bob