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Old 01-06-2011, 05:05 PM   #1
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Default Mixing ac & dc on a mechanical dpdt relay

Would it be a bad idea to use a mechanical dpdt relay with it's coil relayed with 120V AC, and one switch DC, and the other switch AC? Does the dc leg present a problem?

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Old 01-06-2011, 05:22 PM   #2
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In general, you should be able to do this. Remember that you're only switching 1 line of the AC, which is fine if your dealing with 120V, not so fine with 240V (or more) as the higher voltages of AC have 2 hot wires, and you can only break one.

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Old 01-06-2011, 06:29 PM   #3
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Shouldnt have a problem at all.

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Old 01-06-2011, 07:18 PM   #4
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Golden thank yous

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Old 01-07-2011, 01:23 PM   #5
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CHECK THE SPECS!

The current rating of those contacts is usually significantly different for AC vs DC...

Not likely an issue (Not sure why you would have any high-current DC in your system) But something you should be aware of. EDIT: AC and DC are usually interchangeable when it comes to mechanical, dry-contact switches relays and other components, but their current ratings are not the same. (Solid state devices are entirely different)

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What does the primary pressure gauge on the tank tell us? That's right, the temperature. Put it on a scale if you want to know how much is in it...
Put some duct tape over the gauge - Or better yet - Replace the high pressure gauge with a plug - High pressure gauges are useless!
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Old 01-07-2011, 02:18 PM   #6
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Quote:
Originally Posted by SweetSounds View Post
CHECK THE SPECS!

The current rating of those contacts is usually significantly different for AC vs DC...

Not likely an issue (Not sure why you would have any high-current DC in your system) But something you should be aware of. AC and DC are not interchangeable when it comes to switches relays and other components.
Typically the contacts are rated for a resistive DC load. Their AC resistive load rating is less and the inductive load rating is significantly less.

Short of current ratings, mechanical switches and relay contacts are universally interchangeable between AC and DC.

The coil on the relay is a different matter but thats not the question that was asked. Unless I am missing something?
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Old 01-07-2011, 02:58 PM   #7
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Quote:
Originally Posted by CodeRage View Post
Typically the contacts are rated for a resistive DC load. Their AC resistive load rating is less and the inductive load rating is significantly less.

Short of current ratings, mechanical switches and relay contacts are universally interchangeable between AC and DC.

The coil on the relay is a different matter but thats not the question that was asked. Unless I am missing something?
You're absolutely right as far as AC and DC being interchangeable in a mechanical contact. I just remember seeing different current ratings for AC and DC on switches and relays...

This is from Automation Direct for a 22mm switch:
(Control of AC electromagnetic loads)
24 volts AC at 10 amps
130 volts AC at 6.5 amps
(Control of DC electromagnetic loads)
24 volts DC at 1.5 amps
110 volts DC at 0.5 amps

It's a pretty significant difference on paper, but it shouldn't be a big deal as I can't imagine high current DC in a brewing system. But I thought it was worth a mention...
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What does the primary pressure gauge on the tank tell us? That's right, the temperature. Put it on a scale if you want to know how much is in it...
Put some duct tape over the gauge - Or better yet - Replace the high pressure gauge with a plug - High pressure gauges are useless!
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Old 01-07-2011, 03:01 PM   #8
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Gotchya. The last statement in your previous post alluded, at least to me, that there may be other reasons why DC and AC weren't compatible for dry contacts. All clear now

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Old 01-07-2011, 03:11 PM   #9
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Originally Posted by CodeRage View Post
Gotchya. The last statement in your previous post alluded, at least to me, that there may be other reasons why DC and AC weren't compatible for dry contacts. All clear now
Fixed the original post... Good catch
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What does the primary pressure gauge on the tank tell us? That's right, the temperature. Put it on a scale if you want to know how much is in it...
Put some duct tape over the gauge - Or better yet - Replace the high pressure gauge with a plug - High pressure gauges are useless!
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Old 01-07-2011, 09:38 PM   #10
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Thanks guys, you did answer my question.

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