On Sunday, April 19, 2015 at 4:26:23 PM UTC-4, email@example.com 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.
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?
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.
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..
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.
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.
Something like that would work, 12 VDC coil, 120 VAC rated contacts.
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
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
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
I don't have the foggiest notion if NAPA or O'Reilly would have
I unintentionally learned something long ago. Starter solenoids are
rated for intermittent or continuous use. Intermittent really means
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
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.
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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