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Air core transformer secondary voltage too high

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Kaets

New Member
Hi,

I'm doing a project to design a contactless battery charger. For an underwater vehicle.
So far, I've got two coils - primary powered by a 12V H-bridge circuit, secondary connected to full bridge rectifier and smoothing capacitor.
The secondary circuit needs to supply a maximum of 1A at 8.4V to charge the battery. To make this possible, I've had to make the primary coil 20 turns and the secondary 150 turns, with a series resonant circuit on the secondary.

This setup works fine for the maximum load, but as the battery charges, the current will drop, and with this transformer, when the current drops, the voltage increases quite steeply, to more than 30 or 40V.

Is there any way I can regulate the output voltage to under 12V in the no-load condition? My thoughts were that I could have a circuit that changes the resonance capacitor, or shorts it, when the voltage gets too high, but I can't work out how to do it.

Does anyone have any ideas?
 

ronsimpson

Well-Known Member
Most Helpful Member
Why not change the frequency of operation, move away from resonance?
Air core?????
12 V in 8.4 V out 20:150 turn ratio? What are you dong?
 

Diver300

Well-Known Member
Most Helpful Member
The voltage is generated by the series resonance. It will only take a small current to damp that a lot. Just put a zener diode across the smoothing capacitor.

Are you getting 1 A at 8.4 V?

Can you post a circuit diagram?
 

Kaets

New Member
I realise that ideally I should use a ferrite core, but I've been trying to avoid it since the secondary coil will be mounted inside a sealed plastic sphere (underwater vehicle) and the primary will be in a docking station, so even if I used a core, it'd have a small gap, and a heavy core would affect the ballast.

The loose coupling has meant that slightly silly sounding turns ratio is the only way that I've been able to transfer sufficient power, although it does mean that when the load decreases, it ends up with high voltages.

Changing the frequency sounds like it could work, but I'm not sure how to go about it. The frequency is set with an RC network attached to an H-bridge driver IC. Perhaps I could have something detect the primary current, and somehow change the timing resistor?
 
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