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Old 29th April 2009, 09:36 AM   #1
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Default Help on power supply

Hi,
I have a spare toroidal tranny lying around with 280V secondary Winding, thinking of making something out of it without rewinding anything, I hope to obtain
1) B+ 290~310V @70~90ma
2) B- -60~-80V @ 20~50ma
3) Vbias -15~-25V

After some frantic R&D (Reap and Develope), I came up with the attached circuit. Wonder if it will work safely?

Cheers

Ken
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File Type: jpg free -ve power rail.jpg (19.7 KB, 414 views)
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Old 29th April 2009, 09:48 AM   #2
billr is offline billr  New Zealand
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hello

It's more normal to put the choke in the +ve line, but not to worry.

the bias voltage system you have won't work as you think it will, you have no earth reference to anything, so the vbias is essentially floating.

further more if you did connect the right hand side of the choke to earth, the only voltage you would have for the bias suppy would be that dropped across the choke, which in turn is dependent upon the current draw. Not good. it could only be a few volts, thing is, it wouldn't be negative either.

get a copy of duncanamps.com PSUD design tool to design a psu.

If you do need a bias supply, use a pair of caps to connect the ac to a second rectifier, you will be able to create a bias supply from that.

hope this helps


bill
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Old 30th April 2009, 02:23 AM   #3
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Hi Bill,

Thanks for the reply.

How about this.
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Old 30th April 2009, 02:37 AM   #4
billr is offline billr  New Zealand
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sorry no.

the voltage divider you have for deriving your bias voltage is actually across the choke, so you will only be getting that much based on the current times the dc resitance of the choke.

As I said a much easier way is to drop all of the bias stuff.

use a pair of caps from the ac secondary to feed a second bridge, then create a separate bias supply, the current draw should be next to nothing, so it's not complicated.

the caps allow you to isolate the two halves.

putting an effective 80v drop in the ht line is a waste.

hope that this helps.

bill

ps. simpler is nearly always better, but not too simple!
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Old 30th April 2009, 04:03 AM   #5
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If I were to use another bridge to the tranny via a cap, I'll get another 400V (or -400V depending on how I arrange the diodes), the I'll have to drop to 300V for the rail, that means even more heat generation.

Having an exact winding would be ideal but in this scenario, it would be a great opportunity to learn more about PS and surely we will run into similar situation.

I know of back biasing but all the examples are based on CT windings, I hope we can do it with winding x/o CT.

http://www.aikenamps.com/BackBiasing.html

Cheers

Ken
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Old 30th April 2009, 06:49 AM   #6
billr is offline billr  New Zealand
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for the bias, as you don't need much current, and guessing here, if you want 30v for bias, convienently 10% of the available HT, then get 180k and 20k resistors, the junction will give you about 30v.

remember to connect the +ve side of the bias supply to the -ve of the main HT.

by using such a high resistance, then the current take is small, approx 15ma, similarly the equivalent dissipation.

taking it to 270k and 30k drops the current take to 10ma. total dissipation next to nothing.

you could even replace the smaller resistor with a zener of your choice, your call

hope this helps.
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Old 1st May 2009, 01:35 AM   #7
billr is offline billr  New Zealand
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sorry it's not 10ma, it's 1ma, see what happens when you do things on the hoof!

the dissipation in the two resistors would be approx 0.3 of a watt, next to nothing.

bear in mind that it's a high impedance source now, so the current delviery is next to nothing, but as it's for setting bias it really does not matter.

hope this helps
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Old 1st May 2009, 02:37 AM   #8
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Bill-
Interesting idea for a bias supply-thanks.

The caps on the AC secondary- these need to be non-polarized ? ie film caps or NP electrolytics- and fairly big - a few uF?

Cheers
John

PS- With a toroid PT, I think I'd just wind on enough turns to do the job, but.....
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Old 1st May 2009, 04:33 AM   #9
billr is offline billr  New Zealand
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hi, in fact turning a few windings round the outside of the toroid isn't a bad idea, i would do that, put on about 20 then see what you get, then pro-rate to get what you want.

bill

as the current draw is virtually zilch, it shouldn't make a big difference t the overall va rating of the transformer.

as for caps, somehing like 1uF non polarised, would be good.
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Old 1st May 2009, 07:49 AM   #10
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no values - I'll leave that to you.

At the top the circuit is essentially as you have set up.

THe -ve rail generates from the half-wave setup with your high current -80v at the junction of R2, R3 and R4, and your bias voltage controlled by the zener. Set R4 into the 100k plus region to limit current, R3 bleeds the cap C2 down at off time as well as providing a convenient voltage divider. R2 sets the voltage for the -80V tap.

Put the choke in the +ve HT line - I have a feeling it won't enjoy the work put on it by straightening up both full wave and half wave circuits. In any case, you are not going to have a lot of ripple at the -80V tap and the zener will effectively lock the bias tap.
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