Driving a relay from a 3V AC signal

[cckrobinson]

I have a low voltage signal (3VAC) that I'd like to use to the trigger some relays that will be switching 110VAC. I need to isolate the low voltage from the 110, so I'll be using an opto-isolated relay board. I'm curious what would be the simpliest way to generate the TTL level signal that I need to control the relays.
My goal is to use as few components as possible (I'm designing it for a friend and he wants to put it together him self). Here's my plan:<p>1. Run the 3VAC signal through a full wave bridge rectifier. I'll add an RC network to the output of the rectifier to clean it up a bit. The result will be a 3VDC signal.
2. I'll invert the 3VDC twice using a CMOS hex inverter (74VHC04). I'll have a 5VDC supply available, so I should be able to produce an output voltage compatible with the TTL controlled relay.<p>How does this sound?

[Chris Smith]

The 3 VAC will be closer to 5 VDC already so the current and voltage is already there. <p>Then use a fet or a transistor to "Gate" this into a on off signal.

[sofaspud]

I don't fully understand your description, but could you try running the 3VAC through a voltage doubler, then through the rectifier. Regulate it down to 5VDC and use it as input to your TTL controlled relays?

[Robert Reed]

3VAC--RMS? This would be 4.3 Volts Peak .Just clamp the negative portion to ground with a diode thru 10K resistor. This will be enough to drive 5V CMOS logic. Assuming that your 3VAC is from lo freq. source.

[philba]

3VAC is about 4.2V DC once rectified. <p>If the device sourcing the 3VAC can deliver the coil current, you should be able to directly drive a 5V relay with the rectified AC. Most relays have a pull in voltage of 70% the rated V so you should be ok. I doubt you would need to filter it but it wouldn't hurt. However, a large cap will cause some delay on the relay shut off.<p>check the data sheet on the relay you want to use for the pull in (also called "must operate") voltage and the coil resistance to compute current draw.<p>If your source can't handle the current, then use a general purpose tansistor plus a seperate power supply.<p>[ May 10, 2005: Message edited by: philba ]</p>

[sofaspud]

Don't forget that the rectified 3VAC will include a diode drop. With 3VACrms using germanium rectifiers you'll be lucky to get a full 4VDC out.

[Robert Reed]

OOPS--May have made too many assumptions, When you say 3VAC--are you referring to 60 Hz line or other. If you are driving an opto device prior to the relay coil. what is input Z. If hi enough, the before mentioned circuit driving CD4528/38 (retrig. one shot) with period 20 milliseconds would do the trick. These devices accepy slow rise times on their input.

[philba]

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by sofaspud:
Don't forget that the rectified 3VAC will include a diode drop. With 3VACrms using germanium rectifiers you'll be lucky to get a full 4VDC out.<hr></blockquote><p>
doh! maybe use a schottky diode for a smaller drop but it's gonna be close. probably too close to 3.5V (pull in V at 70% spec'd coil V) for relialble operation.<p>It would be helpful to understand what the source is. We can make things up all day long but with out more info, its just guessing.

[cckrobinson]

The 3VAC source is used to switch on a gas fireplace. The purpose of this project is to automatically raise a projector screen when the fireplace is turned on. Also, I need to make sure you can't lower the screen if the fireplace is already running. It's my understanding that the 3V is generated from a transformer inside the firplace junction box. The box contains 110V for a fan and then the 3VAC for the fireplace switch. That would imply that the 3V is 60hz. I don't know how much current we are talking about, but I'm sure it's minimal.<p>I'm planning to use an isolated relay board. <p>http://www.futurlec.com/TrainingBoardAccessories.shtml <p>It's the opto-isolated relay board at the bottom of the page. I can't build one for that price. I still don't know if I'll be able to use this board or not. The electrical code frowns on having low and high voltage sources in the same box. My friend is checking with the electrical inspector to see if he will allow 110V and the 3V to come into the same box as long as the low and high voltage are isolated on the circuit board.<p>Adding a filter after the rectifier shouldn't be a problem because the transition time isn't a concern.<p>[ May 11, 2005: Message edited by: cckrobinson ]</p>