Driving Relays with 3.3V signal from microprocessor

Thread Starter

donaldm444

Joined Jul 16, 2010
23
Hi,
I am looking to drive electromechanical relays (3 V and 100 mA to turn on) with an Atmel microprocessor. The micro output pins give 3.3 V @ 8 mA, so I looking for a suitable driver to use.

I am considering the ULN2803A. I will be driving 9 relays (not simultaneously) with the micro.

If anyone has any suggestions I would greatly appreciate it.

Thanks,
Don
 

19900110

Joined May 20, 2009
5
If design itself is not that critical, then you can obviously saturate a BJT to get enough current level. However, you need to concern about the beta of the BJT. The current you get from saturation is approximately beta X base current (output of the micro-P).

If you are really serious about the design, I suggest a compatible driver. I am using a Futaba servo and the suitable driver in listed on the servo datasheet. I believe you can find the same thing in your relay datasheet.
 

crutschow

Joined Mar 14, 2008
23,494
If design itself is not that critical, then you can obviously saturate a BJT to get enough current level. However, you need to concern about the beta of the BJT. The current you get from saturation is approximately beta X base current (output of the micro-P).
.............
You do not use the data sheet beta value for determining the base current required for saturation. That beta is only for small signal AC applications. To make sure a transistor is well-saturated you should use a beta value of no more than 10-15.

The ULN2803A should work fine for the OPs application.
 
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