Inexpensive 120VAC relays?

[Peepsalot]

I'm looking for a solid state relay to be used in a general purpose wall outlet controller. So basically the specs need to be for 120VAC and preferably be able to handle as many amps as the outlet provides (I think most breakers in houses are 30A?).

I have heard of one model (Crydom D1225) that runs about $30 on Mouser. I was just wondering if anyone knows of something that would be less expensive than this?

I'm willing to consider mechanical relays if they can handle the task for better price. I think the higher power mechanical ones are just as expensive though?

Or maybe there is another method entirely that can be used to switch high loads on and off electronically. The plan is to have this ultimately controlled by a PIC.

 

[dknguyen]

I thought electro-mechanical relays (EMR) were much cheaper than solid state relays (SSR) for higher power since it's just more metal instead of more silicon. Yeah...silicon is expensive.

If your application is "fine-switching", or many switches per second or something like that you must use a SSR. If it is just continuous conduction without "mid-conduction switching" like PWM, go with a EMR. EMRs are worn down by the switching but conduct much more efficiently than SSRs (just a bar of metal, not silicon). EMRs will last longer in situations where conduction is continuous for long periods of time and switching is infrequenct (ie. either 100% duty cycle or 0% duty cycle, nothing in between like PWM). SSRs switch very effortlessly but are worn down by the actual conduction. They conduct less efficiently than EMRs and need heatsinks which can be larger than the mechanical relay.

Can't you just use a TRIAC or alternistor and a TRIAC opto/isolator driver (if needed)? I forget how much mine was, but it wasn't $30. Cheaper than a solid state relay with everything bundled into one. An alternistor is basically two one-way TRIACS (thyristors I think) in reverse parallel with each other to do the job, except of being two discrete components they are in one package. They are better for some reason, I forget why. Switching characteristics or higher power or something.

Maybe one of these:
**broken link removed**

(I like the TO-218X package with the tabs so you don't have to have AC current entering the PCB).

Fairchild also has optical isolating chips, TRIAC/alternistor driver chips in case you want to drive the triac with the proper polarity at the right time for best effectiveness (or you can just drive it directly with another MOSFET or something running "negative" pulses into the TRIAC gate no matter what polarity the power AC signal is). Read the app notes from Fairchild.

http://www.fairchildsemi.com/ds/MO/MOC3031-M.pdf

Or you can try looking at the T9A series relays from Amp/Tyco that do 20-30A or so. You wil need a MOSFET to boost the from a PIC to be sufficient to drive the 1000mW coil. There are various models with 5VDC, 12VDC, 20VDC, and AC voltages which affects the amount of current required to activate the coil. If you can, go relay (ie. just 100% or 0% duty cycle required with no high frequency switching).

EDIT: Looked at prices of the triac and relays I used for 30A-40A
~$3 CAD for a 30A T9A relay from Amp-Tyco
~$8 for 40A,400V littelfuse alternistor and $1 for the isolating optical triac driver 400V

Must better than a $30 solid state relay.

 

[audioguru]

120VAC electrical outlets have 15A breakers. 30A breakers are used for 240VAC stoves, clothes dryers and central air conditioning.

 

[stevez]

Keep in mind that not all loads in a house are resistive - you may want hp rated contacts or SSRs that can handle inductive loads.