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 (< $5 US) relay (SPST-NO) that only needs 40 mA or less to activate? My Tyco T77 relays need 90 mA at 5VDC to pull in the contacts, it turns out, so the processor dies. The lowest I've found on-line is 60 mA. I don't have a harsh environment, or rapid switching requirements, just need a simple on-off relay. I'm switching banks of lights, should look cool if I can get it to work. Thanks for any advice.
I would get a 1.5 amp wall wort to power the relays and use some NPN transistors as relay drivers. There are even IC circuits if you don't want to use transistors. This may be cheaper than buying new relays. Dave
Hi, Mike. In order to get that kind of current rating, you need some distance between the contacts, thus more power to pull them in. I don't believe the beastie you're looking for exists.
The best solution might be to go with some solid state relays. They'll require only a few mA to drive the internal opto-triac LED, and many can be driven directly from the AVR pin.
If you gotta have relays for some reason, and you've got some AVR pins to spare, you might want to look into magnetically latching relays. The magnetically latching relay has two coils -- one to set, and one to reset. Although they'll require more current than you can afford, each coil only has to be pulsed (100ms. or so) to change the relay state. You can use a resistor to slowly charge a local cap, which can be used for the coil power to turn on or off the relay, like this (view in fixed font or M$ Notepad): | |VCC | + | | ___ More relays | '-|___|-o------o--o-----o--o----->
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.
Mike, at first I believed you were out of luck, but tonight in my Newark Electronics catalog I ran across this interesting little number:
Siemens RY610005, Coil current 44.0 ma., coil resistance 113-ohms, contact rating 8A at 250VAC, Price in quantities from 1-99 is $3.15. Seems perfect for a 5V power relay..
This is likely a 1999 price (Catalog 117), but you can likely check it out on
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if this thing existed years back, think of the lives all all of the 2N2270 relay drives whose lives could have been saved! :-)
Use a higher voltage relay coil, such as 12V, and get one rated for *a tungsten load**at the current you need to operate at*. You will probably need 1W at least per coil to meet your stated requirements.
Using a low coil current relay on a tungsten load at a rating anywhere close to the resistive rating is just asking for very short life at best, and welded contacts at worst.
A typical cheap-ass '30A' (resistive) relay such as the T90 might be derated to 5A with a tungsten load.
You can easily use triacs, but the disadvantages are severe-- the heat loss at 10A will be quite large (so big heat sinks to keep the triacs from failing), and lamps of any size will kill the triac when they burn out unless you significantly overrate the triac and use a good fuse.
The only relatively inexpensive one with sufficient ratings that comes to mind atm is the T92, which has a coil power of around 1.7W for DC coil units, and costs around $5 US in moderate quantity. Check that the life is sufficient for your application. Data sheet values for this item are umm.. 'not necessarily pessimistic'.
Small definite purpose contactors will have longer life, but they are noisy (acoustically and electrically) and are generally nasty beasts.
You didn't mention the voltage. An Omron G6RN-1A-12 will carry up to 8 amps resistive at 250 VAC and needs only 18.3 mA at 12 volts. The 5 volt version of that relay takes 44 mA. Mouser part # 653-G6B-1114P-DC5 is an Omron 5 V coil, 40 mA SPST that will carry 5 amps resistive at 250 VAC
Is there a reason you can't use a driver transistor with your Tyco relays? That's the cheapest solution. Ed
Wow, lots of good replies, thanks for all the fine advice. With help from this group, I found a part at DigiKey, an Omron 10A contactor needing only
40 mA to pull on. This was the easiest solution to implement, not necessarily the best way to go, but I think it will work. I'm hoping that derating by half will be sufficient to drive 5A of HPS lighting without trouble. Thanks to all.
Mike, Siemens as a line of PC board mounted relays that draw 44 ma at 5 Volts, and can switch 8 Amps @120V. They're priced in the $3.00 range. (I found them in the Newark Electronics catalog.
If you would like the model numbers and more specifics, post a followup.
Are you sure about that 40mA spec? That's the limit per pin in the AVR datasheets I've got to hand, but there are other limits - on total current on Vcc and GND and for various combinations of ports. For example, with the ATmega 128, there's total limit of 400mA in or out (you need 480mA for 12 40mA relays), and further limits depending on which ports you planned to use. Check the notes in the Electrical Characteristics section of the datasheet carefully.
I'd go for an IC to drive the relays. You can get your freewheeling diodes built-in and enough drive for whatever relays you fancy, without stressing the AVR's outputs. No more soldering or board space then the diodes alone and it may even pay for itself by allowing you to choose from a wider range of relays.
I don't think you need low power relays... your money would be better spent on a proper interface between the AVR port and the relays. A simple transistor between the AVR output and the relay would let you use your existing relays and would amount to a few cents per relay. You didn't indicate whether the AVR output is high or low when the relay should turn on, so first, I'll assume that it's low. In that case, use a PNP transistor (such as a 2N2907). Connect it as such:
Please view in a fixed-width font such as Courier.
You might want to use a MOSFET (like a 2N7000) to drive the relays. Or maybe a Darlington array, something like the ULN-2803A. This would be easier on the AVR and also make the relay selection less fussy. Brian
You might be better off using transistors, mosfets (like a 2N7000) or a Darlington array (like a ULN-2803A) to drive the relays. It would be easier on the AVR and it would probably be easier to find lower priced relays. Brian
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