Controlling small AC loads with no relay.

Chicago Mike

Im trying to control neon from an SX without using a mechanical relay.... I can't have the 'click'. The neon will need to cycle every second on and off continuosly. Solid state relays don't work because the load is too small for them to function. the neon is 120VAC @ about .0015A. Straight transistor switch circuit? I'm never controlled AC with a transistor, is this the 'right' way? What is the best may of handling control of a very low current AC load?

Any pointers would be great!

Thanks

kwinn

You would need more than one transistor to control an AC signal and it is very hazardous to connect them to a micro without an optoisolator. I would suggest using an optoisolated triac such as an MOC 3011 which is rated for 10mA, 250V with 7500V isolation. The MOC3031 with zero crossover turn on may be a better choice since it switches on when the AC voltage crosses zero and thus produces less electrical noise. Jameco has a good selection of surface mount and thru hole packages.

Chicago Mike

Thanks for the pointer. I've been looking at these Triac opto drivers, and they seem to do what I want. Specificly the MOC3031. One question though, all of these are drivers for larger triacs. Is it common practice to use just the driver (since it has a triac inside of it, just small), and leave out the external triac, on very small loads such as mine? The total load is 3.65ma @ 120VAC. The ratings seem to support this, I just want to make sure I'm not missing something.

Thanks again,

PJAllen

Neon lamps typically draw less than 1ma.

As a matter of brevity, the drawing doesn't include a fuse, but you ought to.

kwinn

PJ Allen is correct and his schematic is typical for driving a neon lamp. As to your question of it being typical to use the opto driver without a larger triac or scr's to drive small loads,
yes it is. Opto isolators are often used to directly drive relay coils, solenoids, lamps, etc. The only limitation is that the load current and voltage are within the device rating.

Chicago Mike

Great Stuff! Thanks so much!