Hi,
I've been trying to build a circuit using 2 555 timers to drive a 120VAC motor. One controls freq and the other controls pulse width. Everything works fine until I put an actual AC load on the relay then it doesn't always want to swith back to its off position and frequency control 555 starts to fire when it's not supposed to.
What I can't figure out is how the AC current through the relay is causing problems with my first 555 timer (the freq control) because its not even connected to the relay in anyway except through ground (I did use protect diodes btw). This is an inductive load I'm driving so do I need some kind of filtering to keep the relay from being affected by EMI? Is it just a piece of crap relay?

thanks

 

Hi,
I've been trying to build a circuit using 2 555 timers to drive a 120VAC motor. One controls freq and the other controls pulse width. Everything works fine until I put an actual AC load on the relay then it doesn't always want to swith back to its off position and frequency control 555 starts to fire when it's not supposed to.
What I can't figure out is how the AC current through the relay is causing problems with my first 555 timer (the freq control) because its not even connected to the relay in anyway except through ground (I did use protect diodes btw). This is an inductive load I'm driving so do I need some kind of filtering to keep the relay from being affected by EMI? Is it just a piece of crap relay?

thanks

please post a schematic so we can see what is going on

 

Hi,
I've been trying to build a circuit using 2 555 timers to drive a 120VAC motor. One controls freq and the other controls pulse width. Everything works fine until I put an actual AC load on the relay then it doesn't always want to swith back to its off position and frequency control 555 starts to fire when it's not supposed to.

What do you mean by "fire"? Surely not an acctual fire with flames and magic blue smoke!

(I did use protect diodes btw).

Did you use a diode in series with the relay coil aswell as a reveresed one in paralel? That's what my 555 timer needed in order to work properly with a relay.

 

No not that kind of fire, JJ. What would be more accurate term? Trigger?

Yes, I have a diode in series from output of the 555 to the relay coil and another in parallel across the coil. The relay will switch positions correctly until I connect a motor and then it sticks. I can understand if it is just a sticky relay but when I disconnect the motor it still sticks until I remove power.

I do have the (off-state pin?) of the relay just floating. Should this maybe be grounded through a big resistor? Doesn't seem to me like it should matter but I'm at a loss here.

Sorry I don't have a schematic posted yet. I'll try and get one drawn up soon.

Thanks

 

schematic is attached