# Re-winding a 24v solenoid to 12v

Discussion in 'General Electronics Discussion' started by quatfro, Feb 28, 2012.

1. ### quatfro

4
0
Feb 28, 2012
Hi all,

Hopefully someone can help me.

I have a set of 24v air horns and they work reasonably on 12v up until the point the supply pressure reaches about 60lb and then they don't quite have the grunt to defeat the pressure and open the valve. This is slightly irritating as I'd also like to use the supply tank at a higher pressure for other uses in and around my van.

As they stand, the solenoids are 11.5 ohms. At 12v they draw 1.08 Amps and are effectively 13watts. At 24v therefore, they should draw 2amps and be nearer 48 watts, much more grunty affairs.

So, do I simply need to up the wire gauge to lower the resistance to then get the wattage out of the solenoid with as many turns as I can get away with in the casing? Or is it a bit more complicated than that?

There are many calculators out there on-line but despite my electronics background, I've forgotten more than I ever learnt and don't quite understand the bigger picture...so any help or suggestions would be very gratefully received!

Cheers,

Tom.

2. ### jackorocko

1,284
1
Apr 4, 2010
Not knowing a whole lot myself, I would assume that inductance will have a say since it is a coil of wire. I know it doesn't help you out. Hopefully someone else will be able to clarify what needs to happen. I will be reading with interest as well.

3. ### quatfro

4
0
Feb 28, 2012
This is the thing. I have most of the pieces, but not the guide picture to help finish the puzzle!

4. ### Harald KappModeratorModerator

9,405
1,921
Nov 17, 2011
The magnetic force is generated by the current flowing through the coil. The important number are the ampere-turns (I*No. of turns).
I assume the valves are operated by DC, so the inductance plays a negligible role (only during turn-on and turn-off).
If you want to rewire the coil, mae sure that the number of turns times the current is the same as for 24V operation. You could:
1) double the number of windings. Use thicker wire so the DC resistance stays the same as for teh 24 V coil. Same resistance measn 1/2 current (at 12 V). 1/2 current times 2*No of windings means same ampere-turns.
OR
2) keep the number of windings halving the DC resistance by using thicker wire. The math comes out the same.

Harald

5. ### duke37

5,256
726
Jan 9, 2011
I am not sure how to work this out, but to get the same current density with double the current, you will need to go up by 2 SWG. To fill the spool to the same level you will need half the number of turns.

Is this correct?

6. ### quatfro

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Feb 28, 2012
I see. So I was thinking / barking roughly up the right tree. As you say, it's all about the current as this is effectively the unit of strength in this instance.

So whether I go for option 1 or 2, I'll need thicker wire as the resistivity is proportional to cross sectional area. Ok.

Just to double check.

I could keep the resistance at 11.5ohms but by using double the windings and thicker wire I would end up with the same strength as if I would be using 24volts.

Or

I double the cross sectional area of the wire so that I have 5.75ohms and keep the same number of turns, correct?

Both solutions are deceptively similar sounding, though I can't quite imagine whether one would be dramatically different to the other!!

The option which would give me the most compact coil is the one I really need, there's some spare room inside the casing as it stands, and it would be nice to keep it within the original covers. My guess would be option 2, but it's a guess at best.

7. ### duke37

5,256
726
Jan 9, 2011
I think you should double the cross section and half the number of turns.

Alternatively you could make or buy a 12V to 24V invertor.

8. ### JMW

90
3
Jan 30, 2012
Many diesel powered cars use 24 volts to start., but 12 volts to run. You can use two 12 volt ups batteries and two dpdt 12 volt relays. You might even be able to use a couple large stereo 1ufd caps. When the horn button is depressed the relays place the batteries (capacitors) in series. If you use capacitors you will probably need resistors on the 12 volt side to reduce the surge current.

9. ### Harald KappModeratorModerator

9,405
1,921
Nov 17, 2011
I think option 2 (same number of turns, thicker wire -> 1/2 resistance) is mire suitable. Doubling the number of turns will probably use too much room for the windings.

Harald

10. ### quatfro

4
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Feb 28, 2012
Yeah, space is tight.

I'll get on to doing it later in the week and report back!

Thanks everyone.