PWM to (Safely) trigger a relay ?

[iso9001]

Hey all,

I have project that requires me to turn a relay depending on an existing pwm output from my micro controller. I know hooking a relay on my PWM line sucks b/c if the pulse width is slow enough I'm going to have a constantly switching relay, if its faster then I'll have made myself an effective EMI generator.

I can't spare anymore outputs as this is a remote bandaid fix for a specific customer,

I was thinking I would put a decoupling cap on the pwm line after a diode and try and smooth the signal out as much as possible, but I don't really like this idea as that cap will wear out and its just not an easily install or replaceable.

Is there better circuit I'm not thinking of ? Or does anyone make relays designed to be driven like this??? (like a latching relay that won't be powering the coil all the time)

Thanks,

 

[Brevor]

I have seen the diode and cap idea used before to drive a relay, it worked without any problems.

 

[iso9001]

Yea,

I am a little concerned about the temps that the cap would be seeing. Its near a pretty large heater so I can assume that 120ºF is realistic for normal operation, maybe more on a hot day so, I would take a look at the cap's 2250 hour rating as a limitation.

I would have though there would be a specific relay type to handle this but I guess not, at least I haven't found one yet,

 

[Sceadwian]

What about using the PWM through a low pass filter into a schmitt trigger? If you use a filter on the PWM it'll act as an analog output and the schmitt buffer will make it act like a switch again.