I'm trying to use my little AVR to control up to 12 120VAC loads, up to 5
amps. Does anyone know of a cheap (< $5 US) relay (SPST-NO) that only
needs 40 mA or less to activate? My Tyco T77 relays need 90 mA at 5VDC to
pull in the contacts, it turns out, so the processor dies. The lowest I've
found on-line is 60 mA. I don't have a harsh environment, or rapid
switching requirements, just need a simple on-off relay. I'm switching
banks of lights, should look cool if I can get it to work. Thanks for any
advice.
Mike Ross
--
Hi, Mike. In order to get that kind of current rating, you need some
distance between the contacts, thus more power to pull them in. I
don't believe the beastie you're looking for exists.
The best solution might be to go with some solid state relays. They'll
require only a few mA to drive the internal opto-triac LED, and many
can be driven directly from the AVR pin.
If you gotta have relays for some reason, and you've got some AVR pins
to spare, you might want to look into magnetically latching relays.
The magnetically latching relay has two coils -- one to set, and one to
reset. Although they'll require more current than you can afford, each
coil only has to be pulsed (100ms. or so) to change the relay state.
You can use a resistor to slowly charge a local cap, which can be used
for the coil power to turn on or off the relay, like this (view in
fixed font or M$ Notepad):
|
|VCC
| +
| | ___ More relays
| '-|___|-o------o--o-----o--o----->
| R | | |RY1A | |RY1B
| +| - C| - C|
| C --- ^ C| ^ C|
| --- | C| | C|
| | | | | |
| | '--o '--o
| === | |
| GND |/ |/
| -| -|
| | |> | |>
| .-. | .-. |
| | | | | | |
| | | === | | ===
| From AVR '-' GND '-' GND
| | |
| >-------------' |
| >----------------------'
|
(created by AACircuit v1.28.5 beta 02/06/05
www.tech-chat.de)
Latching relays are sensitive to vibration, and obviously don't turn
off by themselves at power down, but your description indicates that
might not be too much of a problem.
But still, SSRs seem to be your best bet.
Good luck
Chris