What type of relay do I need?

[cstradling24]

I am currently a student at ITT Technical Institute and I am building a lab for my final project this quarter. I have a traffic light and I have made a DC (9V) counter circuit. Everything there works fine. I originally made the circuit with simple LEDs so I could see it work. The traffic light I have is a 120V AC power supply. Now here comes the problem.

I need to light three lights in the traffic light (R, Y, G) which are 120V, 25W AC. My counter is a DC circuit which has three LEDS substituting the traffic light right now. What is the easiest way I could power the AC traffic light using my DC counter circuit? I figured I would need some simple relays but I cant seem to find a relay that seems like it would work. As you can imagine the current in my DC counter circuit is quite low. Any ideas of an easier way or what specific type of relay I need?

Thank you in advanced!

 

[Sceadwian]

That's not a lot of power to switch, a 1 amp 120 volt relay will work fine, pick a transistor that will saturate at whatever current your counter is able to produce and use the transistor to drive the relay coil, you could also use a logic level Mosfet instead which won't require any current from the counter circuit itself at all. Since the current is pretty low solid state relays are an option too. How much current is your counter circuit able to source?

 

[Gaston]

if you use a mechanical relay, don't forget the flyback diode. leaving that off has caused me great misery.

 

[eblc1388]

I originally made the circuit with simple LEDs so I could see it work. The traffic light I have is a 120V AC power supply. Now here comes the problem.

Using solid state relay(SSR) is the easiest as no additional drive circuit is required. The SSR inputs work just like a LED.

SSR is a four-terminal device, with two for inputs and two for outputs that control AC load like lamps.

You can still keep your existing LEDs for indication purposes. Places the input of each SSR in series with existing LEDs and your are done.

 

[RadioRon]

I've used this kind of relay before and found it quite easy:

http://pewa.panasonic.com/pcsd/product/ssr/pdf_cat/aqr.pdf

although these are expensive at around $22 each. The 5V input model takes about as much current as an LED (input resistance on the control side is 260 ohms) and the output is isolated from the input, so it might fit easily into your circuit.

There are lots of conventional relays out there, like this one, but it will require more coil current:


http://pewa.panasonic.com/pcsd/product/pwr/pdf_cat/hn.pdf

 

[Sceadwian]

RadioRon, those are 10-20 amps SSR's... He only needs to switch 200ma's at 120 volts, an equivilent 1amp SSR should cost less than 5 dollars and that gives a decent amount of headroom. They're driven via opto issolators so it's not different than driveing an LED.

 

[RadioRon]

RadioRon, those are 10-20 amps SSR's... He only needs to switch 200ma's at 120 volts, an equivilent 1amp SSR should cost less than 5 dollars and that gives a decent amount of headroom. They're driven via opto issolators so it's not different than driveing an LED.

It seems my link is gross overkill indeed, thanks for the suggestion. I didn't see any 1 Amp SSRs on Digikey's site. Can you recommend one?

 

[k7elp60]

Another solution would be to use an opto coupler/triac driver such as MOC3021 and then a triac for each lamp.