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Old 9th Jun 2019, 12:56 am   #1
crestavega
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Default Modding a 24V PSU to 12V.

I am considering to modify a heavy 24V DC power supply (a Skanti P6001) to be a heavy 12V DC supply.

could it be as simple as simply hobbling the comparator, by adding another 6V2 zener in series with D2?

or would I have to "sympathetically" hobble the drive amplifier as well (by adding another zerner in with D3?
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Old 9th Jun 2019, 3:55 am   #2
joebog1
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Default Re: modding a 24V PSU to 12V

Its not that easy!!! You can reduce the output voltage, but because the transformer and series regulation transistor bank are a "matched pair", you will have to reduce the output current as well, otherwise you will rapidly exceed the dissipation limits of the series regulator bank.
I might add that it seems to be a "monster" power supply. sixteen output transistors!!! although there is no type numbers on your circuit

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Old 9th Jun 2019, 6:54 am   #3
snowman_al
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Default Re: modding a 24V PSU to 12V

Problem I see is that unless you can halve the transformer secondary voltage, you will be wasting so much more heat in the heatsink(s).
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Old 9th Jun 2019, 7:21 am   #4
Radio Wrangler
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Default Re: modding a 24V PSU to 12V

You will be increasing the voltage drop across the power transistors almost by a factor of two. So heat considerations will roughly halve the current rating of the supply.

The result will be a much lower useable output power than you may be expecting, and just as much heat as it would make in its unmodified form when it would have been outputting twice the voltage and twice the current!

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Old 9th Jun 2019, 8:51 am   #5
M0FYA Andy
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Default Re: modding a 24V PSU to 12V

Why spoil a good power supply? Sell the 24v one and buy a 12v one, which are probably more readily available anyway...……..

Andy
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Old 9th Jun 2019, 9:00 am   #6
Herald1360
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Default Re: modding a 24V PSU to 12V

You could feed the unit with 120Vac from a suitable autotransformer to deal with the excess passbank voltage issue. You may need to retap the input for 220V to retain a similar passbank Vce. Whether the control circuits will be happy at around half their normal supply voltage would be a matter for experiment- you might need to incorporate an extra small supply to deal with this.

Alternatively and depending on its physical construction, you may be able to remove secondary turns from the mains transformer to get down to the voltage you need. Same caveat about control supplies.
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Old 9th Jun 2019, 9:49 am   #7
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Default Re: Modding a 24V PSU to 12V.

Perhaps you could modify the rectifier by making half of it
SCR's. A phase firing circuit would reduce the effective output
voltage by 50 % at the capacitor bank.

Then you would have 12 volts but the supply would only deliver
25 % of the transformer load possible.
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Old 9th Jun 2019, 10:03 am   #8
G4_Pete
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Default Re: Modding a 24V PSU to 12V.

Second Herald's comment, Zeners aside another factor will be the drive circuits would have been designed for 24 Volts so biasing and pass transistor drive capability may also need consideration.
Pete

Last edited by G4_Pete; 9th Jun 2019 at 10:08 am. Reason: Just re read Heralds comments
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Old 10th Jun 2019, 8:51 am   #9
crestavega
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Default Re: Modding a 24V PSU to 12V.

Quote:
Originally Posted by radiotechnician View Post
Perhaps you could modify the rectifier by making half of it
SCR's.
interesting.
would that increase harmonic noise significantly?

M0FYA, yes your comment noted. good point. Inherent Scots mean-ness will always out.
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Old 10th Jun 2019, 9:07 am   #10
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Default Re: Modding a 24V PSU to 12V.

Quote:
Originally Posted by crestavega View Post
would that increase harmonic noise significantly?
More than significantly.

You would need to add some very serious filtering to avoid spreading RFI through your fixed wiring.

Also, the PSUs with SCR pre-regulators had specially designed transformers. and they are acoustically rather noisy, too.

David
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Old 10th Jun 2019, 9:34 am   #11
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Default Re: Modding a 24V PSU to 12V.

Agree with David. I once bought a Roband unit that functioned that way.
At small outputs it would noticeably thump and ping as the SCRs fired.
It had to go.
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Old 10th Jun 2019, 10:21 am   #12
Herald1360
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Default Re: Modding a 24V PSU to 12V.

Plus you'd have all the fun of another control loop to stabilise!

Flog it and get an HP6264 or the like

Edit: Just looked up the Skanti- make that two 6264s!
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Old 10th Jun 2019, 1:41 pm   #13
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Default Re: Modding a 24V PSU to 12V.

I'm puzzled by the two bridge rectifier circuits in parallel, would this be done for higher current handling ability?

Seems dodgy to me, surely the one that gives the higher output voltage will end up supplying most of the current, and be over loaded anyway?
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Old 10th Jun 2019, 2:12 pm   #14
Herald1360
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Default Re: Modding a 24V PSU to 12V.

It is dodgy unless some balancing resistance has been engineered into the circuit. At 20A odd per bridge, not many milliohms would be needed to keep things reasonably balanced.
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Old 10th Jun 2019, 5:49 pm   #15
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Default Re: Modding a 24V PSU to 12V.

A 12 volt power supply with that rating needs plenty of gain with a feedback loop around it and as little unregulated voltage above the target output vpltage.
Converting a 24 volt power supply in this way is just not practical.
A float charged battery may well do the job if the overral demand for power will not run it down during usage at full power. It depends on the duty cycle requirement of the load.
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Old 11th Jun 2019, 9:16 am   #16
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Default Re: Modding a 24V PSU to 12V.

A float charged battery, though, will be a 13.8V PSU, not a 12V one. Whether this matters......?
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Old 11th Jun 2019, 5:52 pm   #17
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Default Re: Modding a 24V PSU to 12V.

That will be fine for a two way radio as that is what I assume the power supply is to be used for.
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