# Need simple way to detect 120V vs 240V to shut off some devices



## Sunking (Aug 10, 2009)

Boulder Hybrids said:


> While plugged into 240V, I think I can tap into 1 leg of the 240V to get the 120V for the 120V accessories. That should be easy and total watts won't be a problem.


Only if you carry a neutral, and there is noting to tap. 240 VAC is only 120 VAC to ground. With 240 you are connected between L1 and L2. For 120 you connect from either L1 or L2 to Neutral. So to have both 240 and 120 you will have to have carry four conductors. L1, L2, N, and G.

So the answer to your question is simple RELAY LOGIC. Connect 2 relays. 1 between L1 to N, and another to L2 to N. Then use the contacts on the relay to the LOGIC configuration you seek of OR, NOR, Exclusive OR, AND, or NAND operation.


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## piotrsko (Dec 9, 2007)

Simplified: use positions on the plugs on a homemade splitter. Left position is L1, right is L2, ground is case ground if you run it, neutral goes to the ground prong. For this I use a nema odd prong European style male rated 20 amps @ 220v. so you can t plug it into normal plugs. I use home despot 220 20 amp extension cords with this configuration: 
_| |
.. 0

Nema 5-20? And EVERY THING in my car is double isolated, boxed, rubber mounts, mounts to phenolic or plywood.

In America, neutral and ground go to the same place in your distribution box

If you want 120, left position and "ground" pin. 220 is normal left pin, right pin, no neutral. 

Do NOT use both L1 and L2 with the same neutral for 2 sets of 120 unless your neutral wire has a fuse system and is the next size larger wire


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## Moltenmetal (Mar 20, 2014)

Be careful, as in North America the ONLY place that the neutral is connected to ground is at the panel entrance. It is separate in all sub panels and all distribution wiring, as currents normally flow through the neutral but should not be flowing through the ground lead unless there's a failure somewhere.

From what I heard from the OP, the only thing they need is a 240v coil relay which has NO contacts in series with the auxiliary loads. Only when there is power at both L1 and L2 will these loads be powered- when L2 is floating (when vehicle is connected to 120V) the relay will not energize. 

The 120/ 240V switching and how to protect against someone plugging BOTH in, potentially with L1 and L2 reversed, is another matter for design.


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## dougingraham (Jul 26, 2011)

Boulder Hybrids said:


> While plugged into 240V, I think I can tap into 1 leg of the 240V to get the 120V for the 120V accessories. That should be easy and total watts won't be a problem.


This won't work with 240V on a J1772 EVSE because the EVSE looks for ground faults between the ground pin and the hot legs and will shut down when it sees the ground current. It would work if the EVSE was not doing GFCI.

A 240 to 120 volt isolation transformer would let you do this as there will be no ground current to trip the EVSE.


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## major (Apr 4, 2008)

Boulder Hybrids said:


> Looking for a simple way to detect 120V then open a relay or similar and disconnect the 120V "accessories" from the overall load when plugged into 120V (but only disconnect when connected to 120V).


How about a relay with a 240VAC coil and a pull-in above 120VAC? It won't turn on the circuit at 120, but will at 240.


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## tomofreno (Mar 3, 2009)

dougingraham said:


> This won't work with 240V on a J1772 EVSE because the EVSE looks for ground faults between the ground pin and the hot legs and will shut down when it sees the ground current. It would work if the EVSE was not doing GFCI.
> 
> A 240 to 120 volt isolation transformer would let you do this as there will be no ground current to trip the EVSE.


 Yes, because there is no neutral on J1772, only L1, L2, G, Proximity, and Pilot. OP was likely thinking of the more typical wiring situation where you have a neutral. Seems the suggestion of a DP 240VAC relay with 240VAC coil in combination with the isolation transformer would work. L1 and L2 would be connected to the transformer when plugged into 240VAC, but not when plugged into 120VAC.


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## Moltenmetal (Mar 20, 2014)

If the J1772 doesn't have a neutral connection, the transformer is the only way to do this. Would be a shame to have to carry around a chunk of iron just for this feature.


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## Moltenmetal (Mar 20, 2014)

Missed the obvious solution since the loads are all resistive: just put a diode on the load side of the relay so it runs half wave rectified DC to the loads when running on 240. Only for resistive loads, mind you! No cross connections to anything that won't like the DC...


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## Boulder Hybrids (Jan 30, 2013)

Moltenmetal said:


> Missed the obvious solution since the loads are all resistive: just put a diode on the load side of the relay so it runs half wave rectified DC to the loads when running on 240. Only for resistive loads, mind you! No cross connections to anything that won't like the DC...


I like this idea, if I isolate the block heater and battery warming pads and put them downstream from a 240V coil relay and big diode it might just work.

Anyone have a link for a suitable power diode that can handle up to 1000W of resistive loads? (or two 500W diodes).

Other option was to install a transformer like this in the car but seems like a kludgy/bulky solution. http://amzn.com/B0038OF5R8


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## Tesseract (Sep 27, 2008)

Moltenmetal said:


> Missed the obvious solution since the loads are all resistive: just put a diode on the load side of the relay so it runs half wave rectified DC to the loads when running on 240. Only for resistive loads, mind you! No cross connections to anything that won't like the DC...


This is absolutely verboten by most major electric utilities because it sets up a DC bias in the upstream distribution transformer which can then cause it to saturate and blow the expulsion fuse on the primary side. And lest you think that your little 10A load couldn't possible upset a 10kVA or larger "pole pig", note that it takes surprisingly little DC bias to cause even a very large distribution transformer to saturate.


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## Moltenmetal (Mar 20, 2014)

It's a trick we use with heaters and heat trace all the time. Just took me a moment to remember.

Any 10A diode good for 250 V should do, or separate smaller diodes for each load.


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## Sunking (Aug 10, 2009)

piotrsko said:


> Simplified: use positions on the plugs on a homemade splitter. Left position is L1, right is L2, ground is case ground if you run it, neutral goes to the ground prong.


Sorry but that does not comply with any electrical codes even Europe. Equipment ground conductors are forbidden to carry normal load currents. Doing so can be extremely dangerous, thus why it is not allowed. If you are going to carry a grounded circuit conductor (aka neutral) in a 240 circuit is going to require 4-wires.


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## dcb (Dec 5, 2009)

you can put like loads in series pairs to run them on 240.


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## Dougnutz (Aug 22, 2011)

I'm running into pretty much the exact same issue. I have a couple of 120v items that I want to run along with some things like the charger that will run on 120 or 240v. My understanding of the J1772 spec is that it can be either 120v or 220v. Meaning L1 and a Neutral or L1 and L2.

This is confirmed by the leafs EVSE, which plugs into a 120v outlet for L1 charging or the standard L2 240v EVSE's commonly available.

Either way does anyone have a good solution + diagram for this problem?


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## nimblemotors (Oct 1, 2010)

May I ask a related question, how many public charging stations are only L1 ?

To address the question, while there is probably a simple EE analog way,
I would be inclined to just rectifiy the input and then read the voltage with a micro and activate some relays. Seems like an easy thing to me.


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## Dougnutz (Aug 22, 2011)

I am sure very few public chargers are L1 only. But I rarely use public charging, I generally charge with 120V at home. I have taken some down time over Christmas to change over to a j1772 connector. I have a leaf so it would be nice to use the same EVSE. But I need to be able to charge from both L1 an L2

I am in the middle of adding a computer (adruino style) to control the pack heater and pre-heat the cab in the morning. So using the micro controller to sense voltage seems like it would work fine. I'm just not sure about the sense circuit.


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