I'd like to use the +12 and -12 leads from an old PC power supply to power 24v irrigation solenoid valves. The values are rated 0.22A inrush current and 0.1A holding current.
I only plan on 1 or 2 solenoids being energized at a time. I can arrange for them not to be initially energized at the same time.
I see from some wikedpedia info on PC power supplies that the -12 is only rated for "minimal" current. But what is a typical -12 max current value for some random old PC supply?
Q: Would a 1A max load be a problem?
Bonus Qs: Any problem using the +12 and -12 leads for 24 V potential? I'll be controlling the solenoids through a relay. For the solenoid loads, should I use diodes across the solenoids?
Best Answer
Here is a random 350W power supply (http://intrl.startech.com/Computer-Parts/PSUs/Dell/350-Watt-ATX12V-201-Replacement-Power-Supply-for-Dell-PC~ATXPW350DELL). It can provide 0.8A on the -12V rail. The answer for your specific power supply will depend on the power rating - try to find the datasheet or look for stickers on the back. My guess is it will struggle with 1A. Why do you need the 1A? 2 solenoids x 0.22A inrush makes me think you need about 0.5A at worst case scenario?
Bonus Qs: There is no problem using a -12 and +12 to get a 24V potential. Is there anything else on that line (especially semiconductors?) It is good practice to put catch diodes on the relays and solenoids to avoid large voltage swings but if there is nothing sensitive on the line you can do without them.