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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?

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  • \$\begingroup\$ Have you looked thru the EPS12V and ATX12V specs referenced from the Wikipedia article you linked. They have figures for -12V max current. \$\endgroup\$ – Andy aka May 8 '13 at 10:31
  • \$\begingroup\$ I'm working on something similar - but using disk drive supplies (12 volt only), which I'm not happy about. I'd rather use a 24-volt solenoid, but have settled with a 12v. You may want to do the same. Please post however if you happen to settle on a clever solution. \$\endgroup\$ – Brad May 8 '13 at 16:30
  • \$\begingroup\$ @Brad, I'm at the early stages with the project. The answer that supplies seem to be rated anywhere from 0.3 to 0.8 means I'll try it and see. AFAIK, the common irrigation valves are all 24v. While they may work at 12v, I don't think it would be reliable. But experimentation would be best. \$\endgroup\$ – Larry K May 8 '13 at 19:58
  • \$\begingroup\$ @LarryK I'm 99% positive Brad meant he used a 12v solenoid, not a 24v one with 12v. But yes, test it, and you will see that they pretty much all take 12v without issue. \$\endgroup\$ – Passerby May 9 '13 at 1:54
  • \$\begingroup\$ I am using a 12v - because that's the power that I have available - but I'm afraid it wont give me enough kick. So I may have to go to 24v - but that would totally change my whole system, so - seeking alternatives. \$\endgroup\$ – Brad May 9 '13 at 13:35
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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.

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  • \$\begingroup\$ Thanks. I was just using 1A as a semirandom number. I'd want a bit of margin. I agree that 0.8A sounds fine. \$\endgroup\$ – Larry K May 8 '13 at 19:56
  • \$\begingroup\$ I believe the -12v rail was intended for stupid things, like the negative side of an RS232 signal. Rarely used, and not a lot of umph. So - beware! ;-) \$\endgroup\$ – Brad May 9 '13 at 13:36
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The maximum load for -12V on a typical ATX power supply (regardless of output rail) is only 300mA. That won't help you much.

You'd be better off boosting the the +12V rail to 24VDC with a switching converter and using it to power the solenoids.

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Have you tried triggering the Solenoid with 12v only? I think you will find they are quite liberal with their input voltage range, and with a standard ATX supply, the +12v rail can handle a couple amps no sweat.

As to your direct question, most if not all ATX or similar power supplies have their voltage and current ratings listed on a sticker on them. Since you are combining rails to produce a 24v differential, which ever has the lower current rating would be the one that decides how much power you can source/sink through it. Undoubtedly, the -12v Rail will be the weaker one.

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In my experience the 24 volts is for AC not DC. Check the markings carefully. I am thinking of using a 24 vac valve ( solenoid ) on dc to make it easier to power with a battery. I expect a greatly reduced voltage. I do worry that I may polarize the magnetic material, but will have to try the experiment.

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