Driving a relay directly from the output of a 555?

[dog80]

I got a kit similar to this one: http://www.velleman.eu/downloads/0/illustrated/illustrated_assembly_manual_k2579.pdf

It is a 555 in monostable mode, with a trim pot to adjust the output ON time. At the output there's a transistor that drives the relay coil.

But the 555 is capable of 200mA output and that specific relay coil draws only 100mA. So why not ditch the transistor and drive the relay coil directly from the 555 output?

[w2aew]

Probably the main reason is that the output can only source that kind of current (can't sink it), and at that output current, the V(OH) will be 2-3V below the supply rail.  Since that's a 12v relay, it may not give enough drive.

[Simon]

I have done this before and it worked but I can't remember the coil current draw. I also got away with no back E.M.F. diode as the sinking transitor on the 555 output presumably took care of that. I'd not recommend it thought for a permanent installation and it will depend on the particular 555 mutant you have.

[SeanB]

Most will do the full 200mA. I drove a piezo tweeter from one at 40kHz as a pest repeller. Good thing I used a TO100 can version, as it did need a clip on heatsink, it was drawing close to 500ma from the supply. I was running hot to the touch without, and just warm with a small clip on heatsink. 2 555IC's with one being the power oscillator and the other being a modulator for it.