12v/24v voltage sensitive relay / circuit

[Juniper]

I am trying to find a way to automatically charge a trailer battery from tow vehicles that may be either 12v or 24v. The trailer battery is 12v and I intend to keep it charged via the tail light circuit. I am trying to avoid the necessity of the driver having to switch to either 12v or 24v before using the trailer, hence avoiding damaging circuitry when they forget. I thought I could do it with a small solar panel regulator which is supposed to handle either 12 or 24v automatically, however it seems to use the input voltage to output either 13/15 v or 26/27v approx, so that doesnt work. Any ideas?

 

[ghostman11]

this may be a realy stupid question but isnt the light circuit 12v on a 24v veichle? i only ask as we used to have a large livestock transport lorry that was 24v but the light circuit was 12v and we used 12v bulbs on it??

 

[Diver300]

A lot of 24 V vehicles will have converters to produce 12 V. I've seen them used to power radios. At least one of the vehicle tracking units I deal with was powered from a 24 V to 12 V converter, in spite of the fact that it will run on either voltage.

The vehicles that Ghostman dealt with were obviously arranged to run a 12 V trailer, but I think that there are 24 V vehicles that are wired for 24 V trailers.

If Juniper is wanting to charge a 12 V battery from a 24 V vehicle, he is going to need a voltage converter, something like this:-

**broken link removed**

However, that won't work with a 12 V input, so it would need to be bypassed for 12 V operation. I would suggest a 12 V relay to bypass it, and a voltage-sensing circuit to turn on the relay when there is a 12 V input, but not a 24 V input. I would also suggest a time delay of a second or so, to prevent the relay operating when the circuit is plugged into 12 V, or if the 24 V supply dips briefly.

 

[Juniper]

Thanks Diver300,
You have the problem in a nutshell. I have actually purchased 24v to 13.8v reducers similar to the one you suggest which will charge the battery fine, but as you say, they are no good with a 12v input. What I am looking for is the voltage sensing circuit. The time delay is a good idea I hadn't thought of. At present a simple on/off/on switch to either use or bypass the 24 to13.8 converter works fine...... until the first time someone leaves it on 12v and plugs in a 24v vehicle. That voltage sensing circuit is what I need.

 

[alec_t]

Something like this?

 

[Juniper]

Many thanks Alec ... I will take this to someone who understands it and hopefully will put it together for me. The thumbnail downloaded fine and hopefully is all I need, ... can't persuade the computer to download the .asc file so will probably have to find out why and fix it.
Cheers, Juniper

 

[alec_t]

The .asc file is for simulating the circuit in LTSpice; so not necessary unless you want to run a sim. It's only a text file, so I can repost it with a .txt extension if your ISP filter rejects .asc files for some reason.

 

[harold777]

I wonder if a simpler cct wouldn't be to use a 24v relay? (or 12v with some diodes to raise the minimum pull-in V). This selects either output depending on the input voltage and hence the charge-rate. The snag being it's not adjusting the current for the state-of-charge..... but to do that you'd need a charger cct attached to the 12v battery - - - maybe that's the way to do this = a wide-input voltage 12v bat, charger.

Fit some fuses, to be sure. And maybe some LED's, so you know what's happening when the 12v battery dies.
I'd like to see some input filtering also, just in case some nasty transients appear on the tow vehicle, or the trailer wiring is detached and suffers static...?

 

[Juniper]

Thanks Harold ... I tried a 24v relay but found that 12v activated it just as happily as 24v so not selective. I wide input 12v battery charger seemed the way to do it but to find one that will charge using anything from 13.8v to say 27v input is a bit tricky. I have not yet tried Alec_t 's circuit, (it is a holiday weekend here) but have high hopes. The small solar panel battery charger should keep the battery happy as long as it has a suitable input voltage. I am currently feeding it with a 24 to 13.8v reducer, but of course the input voltage from the vehicle still has to be switched to 12v or 24v and it is that switch I am trying to make automatic depending on input voltage.... hence the need for a (input) voltage sensitive relay.
Cheers, Juniper

 

[harold777]

The 12v supply may be above 12v (try measurinig it), if it's a car supply with engine running you might read 16v - this might trip a 24v relay, but you have to do your measurements to be sure.
- otherwisae it's an odd 24v relay that pulls in at 12v - try a half-watt resistor equal to about half the coil resistance and this might help. I think you need to try a variable PSU to check the operation with above and below voltages . . . . it might be a good idea to put a high temperature (100 deg c, say) capacitor (maybe 1000uF) across the coil, so if there is a brief dip the relay stays pulled "in".
It's crazy you can't control the relay with a 2:1 voltage change . . . good luck. The realy solution has "Simplicity" - remember that car-based circuits can get hot in the sun (hence the Hi Temp spec mentioned above) . . . this is worth bearing in mind when building the electronic cct, should you be forced down that route.
The simplest is Best, IMHO.

 

[Juniper]

Thanks Harold ... I will play about with some 24v relays and see where I get to. Many thanks. Juniper