# Will this application work??

Discussion in 'The Projects Forum' started by EEDude, Nov 18, 2008.

1. ### EEDude Thread Starter Active Member

Nov 18, 2008
40
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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.

2. ### mik3 Senior Member

Feb 4, 2008
4,846
70
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.

3. ### EEDude Thread Starter Active Member

Nov 18, 2008
40
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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

4. ### scubasteve_911 AAC Fanatic!

Dec 27, 2007
1,202
1
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

5. ### mik3 Senior Member

Feb 4, 2008
4,846
70
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.

6. ### EEDude Thread Starter Active Member

Nov 18, 2008
40
0
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.