I have an application that requires a relay. I am using an input signal to the coil that once was used to control a fluorescent light so it has a high frequency, around 55Khz (also has a peak voltage of around 240). I have tried using a 120VAC relay with a resistor in series with the relay coil to step the voltage down, but it is not working because of the frequency I think (since I measured the voltage at the relay coil input on the scope as 175, and 118V with a DVM seems like this should be right). So I want to know if this might work instead...Use a full wave rectifier circuit to create a DC signal then use a series resistor with the relay coil to make a voltage divider to step the voltage down to around 24VDC, and then use a relay with a 24VDC coil input. Will this work?? Anyone have any other ideas or better ways to accomplish this? Thanks.

 

If the relay coil is rated 120VAC and your mains voltage is 120VAC then you don't need a resistor in series with it, just connect it directly to the main. However, if the relay you are talking about is not working (maybe broken) buy another one with a coil rating of 120VAC so you wont need to use a rectifier.

 

No the main signal is 240Vpeak so I stepped it down to 170Vpeak but the swich isn't closing. I am thinking that it has to do with the fact that the frequency is so high. All of the relays I see are for 50/60Hz and this signal is around 55KHz

 

Did you say you are thinking of making a rectifier at 55KHz? The recovery time is too slow, you'd have to use fast-recovery diodes for that.

I don't really understand the problem after reading it once, but that's something I thought I would mention.

Steve

 

Now i see, the coil of the relay is actually an inductor. A 50/60 Hz coil relay cant work on a supply of 55 KHz because at such a high frequency its reactance (impedance) becomes very high and the current through it is much less that the current needed to pull it in. A simple solution is to use a fast recovery diode. as scubasteve suggested, and create a half wave rectifier and smooth its output with a capacitor filter. Then use a DC relay and limit the current through it with a resistor.

 

Good stuff thanks for your guys help, I am going to try experimenting with that. If anyone else though has any ideas or suggestions please post.