Question about 12 volt auto relays

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On 4/19/2015 12:03 PM, trader_4 wrote:

|| || [christmas presents]
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On Sun, 19 Apr 2015 07:55:31 -0700 (PDT), bob_villa

Just plonk Trader so I don't need to read his drivel second hand.
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On Sunday, April 19, 2015 at 4:26:23 PM UTC-4, snipped-for-privacy@snyder.on.ca wrote:

Just go plonk yourself. If you don't read what I post, that could explain why you enter threads and make an ass of yourself. You're like the idiots taking cheap shots at Fox News, yet they don't even watch it.
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On 4/19/2015 4:25 PM, snipped-for-privacy@snyder.on.ca wrote:

> [christmas presents]
I tend to plonk people who leave excess trailing text.
- . Christopher A. Young learn more about Jesus . www.lds.org . .
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To the op Consider the failure modes and the consequences. The contacts may weld close. Consider the insulation between the contacts and between the contacts and the relay mounting frame and the relay coil circuit. What are the consequences of an insulation breakdown between the 120v circuit at the contacts and the low voltage relay coil circuit?
Mark
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On 04/19/2015 07:58 AM, trader_4 wrote:
[snip]

Are you sure about that for the CONTACTS, or is it just that the COIL has to have DC?
--
Mark Lloyd
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On Sunday, April 19, 2015 at 1:11:27 PM UTC-4, Mark Lloyd wrote:

See the posts where that was discussed/clarified. I did initially think he was talking about AC for coil and contacts. There are issues on the coil side, a DC relay isn't designed to work with AC. And there are issues on the contact side too. The contacts can typically handle AC or DC, but are derated for less voltage and/or less current with DC. I provided a datasheet that shows a typical 10A rated relay contacts can be used with 250V AC, or 30V DC.
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wrote:

I suspect most of it really has to do with "listing". A lot of automotive stuff is not U/L listed or evaluated. You don't really know what it will do. The "insulation" thing may be overstated if the relay has all of the contacts potted in the plastic base and there are no wires involved at all. You still have no way of knowing what happens if it does burn up tho, without that U/L (or other NRTL) evaluation. Of course you have the same issue with a lot of offshore stuff, no matter how it is labeled. Just because some asian manufacturer says 250v, does not mean it was not actually tested at that voltage by anyone but the customer..
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wrote:

Any "switch", whether a manually operated or relay, will handle significantly higher current in AC than in DC, at the same voltage. Particularly as voltage increases. Breaking a high current (particularly high voltage) circuit requires a very fast opening to a wide gap to extinguish the arc, or a pneumatic or magnetic quench mechanism. AC breaks the arc quite effectively even with a slower opening contact set. Not unheard of th see a 15 amp AC switch rated for 5 amps or less DC.
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On 04/19/2015 06:58 AM, trader_4 wrote:

I assumed the OP wanted to switch 120 VAC with a relay with a 12 VDC coil, possibly for some sort of home automation project. If the intent was to try 120 VAC on the coil and not just the contacts, good luck.
http://www.allelectronics.com/make-a-store/item/rly-557/12-vdc-spst-n.o.-pc-relay/1.html
Something like that would work, 12 VDC coil, 120 VAC rated contacts.
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On Sun, 19 Apr 2015 05:34:22 -0700 (PDT), bob_villa

I would also say most of TODAY's 12 volt relats would handle the 115 vac 1 amp without a problem, as they are using thermoplastic injection moulded components instrad of the phenolic crap they used years ago. I woulsn't use a metal cased (crimped to phenolic board base), for sure, but the all plastic versions would LIKELY work and be safe - although they are not approved for that use - so I would not "recommend" you use them. The thermoplastic material used to mold the 12 volt relay and the 120 volt relay are very similar if not exactly the same.
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bob_villa wrote:

Not the contacts, the coil....
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snipped-for-privacy@invalid.com wrote:

This doesn't answer your question but you can buy some that do. There's one here: http://tinyurl.com/m9gsp82 This link is to Zorro.com. This http://tinyurl.com/m7frmuj will take you to one of the Allied Electronics pages with relays filtered for 12VDC coil voltage. Allied doesn't sell to individuals but this might give you an idea what's available. I don't have the foggiest notion if NAPA or O'Reilly would have something. I unintentionally learned something long ago. Starter solenoids are rated for intermittent or continuous use. Intermittent really means intermittent.
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On 04/19/2015 08:24 AM, Dean Hoffman wrote:
[snip]

They did when I tried it (around 1980). However, there was a minimum order so it wasn't worthwhile for just one small part.
[snip]
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Mark Lloyd
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On 04/19/2015 11:16 AM, Mark Lloyd wrote:

http://www.allelectronics.com/
They're consumer oriented and have enough odd stuff it isn't hard to spend $75 and get the free shipping if you do any electronic stuff.
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On Sun, 19 Apr 2015 08:24:09 -0500, Dean Hoffman

Can you explain this last line. Last time I had need to crank for more than a few seconds was 10, 20, or more years ago, but I recall that one could crank the car for 2 minutes, and the books said that was okay.
So by intermittent, you must mean 2 minutes with a 2? minute pause to cool off.
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micky wrote:

Sure. We have 12vdc to 120vac inverters permanently mounted in our service pickups and on lift equipment to run impact wrenches and drills. We have them wired so we can switch them off and on plus they can't run unless the engine is running. That's where the solenoid comes in. Some won't take that kind of continuous use.
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wrote:

There are "starter solenoids" and there are "Accessory relays" or "load relays" that are built in the same can. Starter solenoids generally have a lower resistance than the load relays, and some 3 terminal starter relays have the coil "grounded" through the Motor side of the solenoid, while load relays would ground the coil directly - never through the load, Generally starter solenoids are under 16 ohms (12-16) resistance, while load or accessory relays run roughly 21 ohms and up.(some as low as 16). Some continuous units have silver plated or tungsten contacts, while most starter solenoids are pure copper.
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