Go Down

Topic: How can I replace this relay with a MOSFET? (Read 546 times) previous topic - next topic

LockDots

I'm taking a CFL (flourescent light) and modding it to have it turn on automatically when a door is opened via a trigger from a magnetic reed switch. Ultimately I'm trying to add automatic lighting to a closet so a wall switch doesn't have to be installed or used.

Below is a quick sketch of the overall circuit. I plan to use a cell phone charger as my 5Vdc source to energize a 5V relay to interrupt a 110Vac source that powers my CFL. Quick searches online revealed that the 5Vdc relays I were finding were not capable of handling high current. The light doesn't draw more than 5A but I would like to play it safe and use components that can handle up to a 10A load anywhere the 'high' voltage lines are involved.

I'd like to use MOSFETs (if possible) to replace the relay. Relays are bulky and noisy, but also I'd like to experience using MOSFETS as a replacement for relays.

What would you guys and gals suggest?


jremington

A solid state relay will do what you want, but you can't use a MOSFET.
Here are some ebay examples: http://www.ebay.com/sch/?_nkw=solid%20state%20relay&clk_rvr_id=546963781109
"It seems to run on some form of electricity"

LockDots

I can't believe I completely forgot about solid state relays lol. Thanks jremington!

Why wouldn't I be able to use MOSFETs for something like this?

DVDdoug

#3
Nov 12, 2013, 09:52 pm Last Edit: Nov 12, 2013, 10:17 pm by DVDdoug Reason: 1
MOSFETs don't work with AC, and they don't isolate the high-voltage from the low-voltage, which you need for safety.

Make sure you get an AC solid state relay.  The input/control side of a solid state relay will usually work from 5VDC.  But unlike most mechanical relays, the output/load side of a solid state relay is usually designed for either AC or DC.

Quote
The light doesn't draw more than 5A but I would like to play it safe and use components that can handle up to a 10A load anywhere the 'high' voltage lines are involved.
That's always a good idea.   Plus, most AC loads have a higher "inrush current" when 1st switched-on.

I recently finished a project where I made 4 solid-state relay circuits and built them into a 4-gang (8-outlet) electrical outlet box.   I did it because I could make the circuitry smaller than a high-power AC relay, and it was cheaper.    But, my 1st design has some mechanical-construction issues, so it took 2 attempts to get it right.

It's possible to make your own solid-state AC relay from a TRIAC, a special triac-driver optical isolator, and a couple of resistors.     But it's (relatively) high voltage & high current stuff, so you can end-up blowing parts (or electrocuting yourself) when you are building, testing, and troubleshooting.    Plus, it's a time-wasting distraction that can delay completion of your project.

I recently completed a project where I built 4 solid-state AC relay circuits and installed them in a 4-gang (8-outlet) AC outlet box.  I did it because I could make the circuitry smaller than regualar high-power AC relays, and it was cheaper.  I've made a similar circuit before, but I had some mechanical-construction issues (I broke the leads off of some optoisolators), so I had to order more parts and re-design & re-build it before I got it right.

LockDots


LockDots

Thanks again for the help.

I decided to redraw my schematic to account for a solid state relay. Now the light should only turn on when both the rocker switch is on and the reed switch is on (door open).

What I'm doing is mounting a light and using the relay to interrupt or source power to it while maintaining the rocker switch built into the housing. This will let me cut power to it preventing the light from turning on in case I need to leave the door open for a long period of time, like when I air out the house.

Go Up