Looking for a relay with 120v trigger but 12v circuit

[Hennesseystealth]

I can find a lot of 12v triggered relays that close a circuit for a 120v line but not the opposite. I have a situation where a charging system for the battery has a small reverse flow (bad design by the manufacturer). It would be a pain to remove the charger and replace it, so I want to just add a relay on the positive leg between the charger and the battery so that when the charger is plugged in to 120v, the relay closes and the battery gets charged but when the 120v is removed, the relay opens and prevents the charger from drawing down the battery.

Anyone know where I can look for this type of relay? Putting a diode on that line won't work as the charger is 3 stage and needs to sense the voltage on the battery and the diode blocks that operation.

[H.A.]

I say you are not looking very hard...
There are dozens of on-line suppliers,
Amazon, Digikey, Newark, et.al. 'Brick & Mortar' suppliers to the professional trade as well as the local Radio Shack.

A socket base general purpose relay for your application could be had for less than 20$

[GuitarPhotog]

You want a relay with a 120VAC coil and spdt contacts rated at 12VDC 5A. They are available as open frame or enclosed.

Here's a whole assortment from digikey
http://www.digikey.com/product-search/e ... dt%20relay

<Chas>

[Hennesseystealth]

I say you are not looking very hard...
There are dozens of on-line suppliers,
Amazon, Digikey, Newark, et.al. 'Brick & Mortar' suppliers to the professional trade as well as the local Radio Shack.

A socket base general purpose relay for your application could be had for less than 20$

The issue is you have to have the search terms right. I looked at hundreds of items from dozens of sites, including DigiKey. Nothing was 120v trigger and 12v circuit. Again, if you don't have the name they have the item listed as, your search will be pretty fruitless.

[Hennesseystealth]

You want a relay with a 120VAC coil and spdt contacts rated at 12VDC 5A. They are available as open frame or enclosed.

Here's a whole assortment from digikey
http://www.digikey.com/product-search/e ... dt%20relay

<Chas>

Thanks, but not sure what I am missing. I went through those tables yesterday and couldn't find any listing for the 12v contacts. So, as an example, I took your link and ran a single filter for 120VAC coil voltage. The resulting list only has a few items with a contact rating of 5amps and nothing on the page says 12VDC. I'm not an electrician or electrical engineer, so I have to guess a bit at which column should say 12VDC (switching voltage, turn on voltage).

[Hennesseystealth]

Digikey part PB837-NB for the relay and PB177-ND for the socket. Maybe more flexibility than I need but it will work.

[H.A.]

The issue is you have to have the search terms right. I looked at hundreds of items from dozens of sites, including DigiKey. Nothing was 120v trigger and 12v circuit. Again, if you don't have the name they have the item listed as, your search will be pretty fruitless.

Ok, Fair enough. DigiKey has an elaborate searchfilter to match their equally elaborate inventory, Wow...
Anyway here is an example after wading thru DigiKey. Its Potter Brumfield nr. T92P11A22-120.

If you Google that part nr. will reveal several vendors, Pricing varies but not by much.

A couple points about this specific relay. (There are plenty of other relays what can do the job)
Sounds like you have a custom build thing going, I made a couple search choices based on your descriptions.
As a flange mount it may prove easier mountings & its wiring connects with familiar spade terminals.
Its two sets of 20A N.O. contacts, Whats likely adequate (& probably way overkill) on your multi step 'smart charger'.

[Hennesseystealth]

Thanks. About half the price of the one suggested by DigiKey and won't need a socket.

[bdosborn]

Head over to eBay and look for one. I gave up on Digikey years ago, their search engine is the worst, unless you know exactly what you want.

Just search on "120V, 10 amp relay with base".

http://www.ebay.com/itm/POTTER-BRUMFIELD-KRPA-11AG-120-120V-12A-12-AMP-A-RELAY-WITH-SQUARE-D-BASE-NR52-/291279322489?pt=LH_DefaultDomain_0&hash=item43d199bd79


Bruce

[Hennesseystealth]

Thanks. I found a "trick" that seems to have worked. I have some contacts in HVAC and they use 120v to 24v transformers and relays all the time. They said I could have this one for $8. Not sure the make or model but they said it will work just fine and has a flange on the back to secure to my power skid.

[Dale M.]

How about a simple diode to prevent reverse current flow....

Dale

[Hennesseystealth]

How about a simple diode to prevent reverse current flow....

Dale

The diode messes up the sensing circuit and the charger turns itself off. That was the first thing that was thought of. This issue apparently isn't isolated and several other people tried the diode approach and all had the same results.

[kludge]

You don't need a 120VAC trigger.

Any old 12VDC coil relay will work.

Put the coil on the output of the Converter but before the contacts of the relay.

The Converter will turn on when AC is applied and give you 12V on the output. The output from the Converter will power the relay coil, the contacts will close connecting the battery, and the battery will charge.

Let me know if you need a picture.

[H.A.]

You don't need a 120VAC trigger.

Any old 12VDC coil relay will work.

Put the coil on the output of the Converter but before the contacts of the relay.

The Converter will turn on when AC is applied and give you 12V on the output. The output from the Converter will power the relay coil, the contacts will close connecting the battery, and the battery will charge.

Let me know if you need a picture.

A picture might be nice,
As I envision by your description, What happens when 120vac is removed from the charger ?
Will the battery back feed thru the closed contacts to keep the relay coil energised ?

Further, Many of these 'smart chargers' wont switch on its charging output until it first measures a correct polarity voltage from the battery.
Its a safety feature what prevents its output from becoming energised if for example its output leads are shorted to each other or connected in reverse polarity.
I expect the small constant battery drain previously mentioned is related to this voltage measuring circuit.

[Hennesseystealth]

You don't need a 120VAC trigger.

Any old 12VDC coil relay will work.

Put the coil on the output of the Converter but before the contacts of the relay.

The Converter will turn on when AC is applied and give you 12V on the output. The output from the Converter will power the relay coil, the contacts will close connecting the battery, and the battery will charge.

Let me know if you need a picture.

A picture might be nice,
As I envision by your description, What happens when 120vac is removed from the charger ?
Will the battery back feed thru the closed contacts to keep the relay coil energised ?

Further, Many of these 'smart chargers' wont switch on its charging output until it first measures a correct polarity voltage from the battery.
Its a safety feature what prevents its output from becoming energised if for example its output leads are shorted to each other or connected in reverse polarity.
I expect the small constant battery drain previously mentioned is related to this voltage measuring circuit.

Battery --- Charger/Maintainer --- 120VAC source

The original circuit is that simple. The issue is when you disconnect the 120VAC source, the charger pulls power from the battery. Not for a short time, but until the battery is, for all practical purposes, drained (I haven't allowed this to happen but I have watched it for over 24 hours). I know this from research but also from the fact that the green power LED never goes out. The relay is just a way to stop the continuous back flow from the battery. Others have measured the drain at about 40mA. Not much, but enough to drain a camper battery that is only used a weekend a month or so. I could put a disconnect on the batter ground, but was looking for an "automatic" switch so I don't forget to reconnect the battery and end up camping and find I have to run the generator. I have a relay I can pick up this weekend and will just add it to the circuit and see what happens.

I think your question on not needing a 120VAC coil is interesting. If the coil operates with a lower voltage, could the battery keep the circuit closed and end up with the same back draw issue. Good question.

Thanks for all the ideas!