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Old 06-17-2012, 12:59 AM   #21
0verdrive
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Originally Posted by carlisle_bob View Post
Hi

All of your loads except for one are running with relays. You already have low voltage power supplies in the design. You would do *much* better to spend another $5 on a relay to drive the pump and then do all low voltage switching. Simply route switching to the relays rather than switch main power. It's much safer, much cheaper, and more reliable.

Bob
Another follow-up question: If I were to take your advice and switch low-voltage DC to relays, I'd need different mechanical relays, which could be switched on/off with DC, rather than 120V AC. If so, would this relay work? http://www.ebay.com/itm/8-pins-Plug-in-Type-12V-DC-Coil-30A-250V-AC-Power-Relay-JQX-30F-/230799920781?pt=LH_DefaultDomain_0&hash=item35bcbf 4e8d#ht_2118wt_1163

Do you see any other areas where I'd need to use different parts? (Would I need different SSRs? Anything else?)

Thanks!
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Old 06-19-2012, 09:20 PM   #22
carlisle_bob
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So you're saying to use the same hardware (switches, etc) but to route low DC voltage across them, triggering relays to power the devices?
Yes, or use even cheaper switches.

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1) Am I correct in assuming that if a switch is capable of handling 120V AC that it can also be used for low voltage DC? (It's just completing a connection, so dropping the voltage should be fine, correct?)
Generaly a good bet.

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2) I assume this is safer since if there's a short, it's a lower voltage that's shorting out?
If you have water in the area, getting a leakage path to 12 volts is a lot less likely to kill you than a path to 220v

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3) Out of curiosity, why is it more reliable?
1) Simpiler switches, and fewer of them. They usually are what fails (often by getting hit with something).

2) The switches are coded to the task. "flip this to sparge". Fewer switches to throw each time = less chance to mess up.

3) Less high voltage running around = less chance of a *real* disaster.

4) Errors at low voltage are generaly recoverable, errors at 220V usualy damage stuff.

A stupid example:

You have five "states" a system will operate in (for instance). There are ten things in the system (valves, pumps, heaters). For a direct controll box ten things need ten independant switches. For a low voltage relay / diode system you need one 5 position rotary switch.

No it's never that cut and dried in reality, but that's the idea.

Bob
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