Dual polarity power supply.

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pre65
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#1 Dual polarity power supply.

Post by pre65 »

I've got my thinking cap on. :shock:

My thoughts are for a +/- 22v dual polarity power supply for a JLH class A sand amp.

Would I be best to use 24v 78xx and 79xx regulators and then "lose" 2v with resistance ?

If not, is there a better way ?
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#2

Post by simon »

Good question, but I don't think it's that simple.

The regulator will need to drop a minimum voltage across it; and if you use cap input then the voltage on the caps immediately after the bridge will be ~1.4x what comes out of the bridge. Then you'll drop 2 x 0.7V for the diodes in the bridge (assuming ordinary 1N types rather than Schottkies).
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#3

Post by pre65 »

I was thinking of losing the 2v after the regulators.
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#4

Post by simon »

If you want a 22V supply then let's assume the regulator needs to drop a minimum of 1.25V across it (don't know, haven't looked) so you need 23.25V before the reg. Assuming you use cap input the voltage "before" the caps will be 23.25V / 1.4 = 16.6V. And if you use a plain vanilla bridge you need an extra 2 x 0.7 = 1.4V. So you need a secondary winding of at least 18V.

But you probably want a little more to make sure the regulator doesn't drop out. So maybe a 20V secondary and let the regulator do its job and "consume" the extra volts across it (in heat through a heatsink). If you feed the reg with bang on the volts it requires there's a very good chance it will drop out of regulation.

But I've never designed a SS power supply so there could be all sorts of hidden pitfalls. Hopefully someone who knows what they're doing pipes up ;-)
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#5

Post by pre65 »

The 7824 and 7924 regulators are fixed at 24v. They don't do a 22v version.

For cheapness and convenience I was thinking of using two of these.(monoblocks).

http://www.ebay.co.uk/itm/291443700146? ... EBIDX%3AIT
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#6

Post by simon »

Ah, okay didn't realise they were fixed regs. I can certainly see the attraction of £4.09 ready made from t'other side of the world. How are we ever to compete?

Anyway, can the device handle 24V? If so job done, if not, I suspect you might need another regulator - unless the current draw is constant, which it may be, otherwise the voltage drop will change.

Have you ever thought of using valves ;-) :lol:
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#7

Post by pre65 »

I, like most of us probably, am suspicious of the electrical components quality on things that cheap.

Alternative is a pair of these, and populate with UK sourced parts.

http://www.ebay.co.uk/itm/261874553096? ... EBIDX%3AIT
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#8

Post by simon »

Or possibly hang on for Andrew's latest board?
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#9

Post by pre65 »

simon wrote:Or possibly hang on for Andrew's latest board?
I'd forgotten about that. :oops:

PS. I don't think it is suitable for a +/- voltage. :(
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#10

Post by pre65 »

Another question.

If I used schotty diodes I might be able to use the 18v toroids that were on the GK-71 heaters.

Are there better schotty diodes than others ?
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#11

Post by pre65 »

Looks like I will be using two separate regulators linked together now, because the current limit of my previous suggestion is not quite enough.

Ray P is helping me by supplying the PCBs I need.

Thanks Ray.:)
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#12

Post by Nick »

pre65 wrote:Looks like I will be using two separate regulators linked together now, because the current limit of my previous suggestion is not quite enough.

Ray P is helping me by supplying the PCBs I need.

Thanks Ray.:)
If its for a power amp, I would use a couple of mosfet followers and a lot of caps. Have a look at some of the pass labs power supplies.

Is there a reason it has to be exactly 22v?
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#13

Post by pre65 »

I'll know more when I've received the PCBs and done some homework on the circuit details. :)
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#14

Post by Ray P »

No the voltage doesn't have to be exactly 22V, the amps will happily work on rails from +/-15-25V, obviously affecting the output of the amp.

Assuming LT1084 regulator chips installed in the power regulator boards that I've sent you, and making some basic assumptions about rectifier loss, 18V AC should be OK for around 22V DC out, but if you want to use a CR pre-filter you'll need a little more AC in.

The amp is Class A and you'll need the regulators to be delivering a constant 1.5 - 2.0A so in my build I'll be using LT1084 regs, which are 5A devices.

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#15

Post by Ray P »

BTW, a capacitance multiplier power supply is often recommended for the JLH Class A amp.
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