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VC Buck Converter, will it work?

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ronsimpson

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Please tell me what you are doing?
I do not know, why Q1 & R2.
If "Con" is grounded the PWM will regulate at 1.3 volts.
If "Con" is connected to "Vout" then Vout will be just below Vin. (not regulating)
 

Gasboss775

Member
The idea is to use the mc34063 circuit as a preregulator for an lm317 based voltage regulator, the control voltage shall be the output voltage ( from the lm317 ) plus 3 or 4 volts to ensure that the lm317 will regulate properly.
 

ronsimpson

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Crude picture!
PWM is simple MC34063. Vin, Vout, ground, and FB shown. (some not shown)
The PWM will work hard of FB<1.25 volts.
The PWM will stop working if FB>1.25V.
To get 1.25V across the 1K resistor current must flow from the transistor. This same current will cause 2.50V across the 2K resistor.
To get the transistor turned on, the voltage across the LM317 must be about (2.50+0.65=3.15).
If you don't like the 3.15V number change either resistor.

upload_2016-11-9_17-51-46.png
The 100 ohms on the base is not critical. Could be 1k.
 

Gasboss775

Member
Thanks Ron, was lazy when drawing the schematic, hence no lm317! In the circuit that inspired this a P MOSFET was used where its gate threshold voltage determined the voltage across the regulator ( was LT3080 ) I was unsure if there'd be sufficient voltage difference for the lm317 to regulate properly using that method.

Spec: Thanks for the data sheet. You've been busy here I see with over 4K posts since joining last year, shortly before me. In contrast I have a paltry 226 posts ( well 227 now! )
 

ronsimpson

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I was unsure if there'd be sufficient voltage difference for the lm317 to regulate properly using that method.
I don't want to dig up the P-MOSFET data sheet. Look at gate turn on voltage. It varies from part to part and temperature and current level. I chose to use a method that used the "reference voltage" in the PWM. There is a 0.6V base-emitter in the formula but most of the voltage is from the 1.25V that we know is stable. The voltage across the LM317 is not critical. About any thing will work.
 

spec

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Spec: Thanks for the data sheet. You've been busy here I see with over 4K posts since joining last year, shortly before me. In contrast I have a paltry 226 posts ( well 227 now! )
You have been very quiet recently, but nice to see you are still around GB- missed the cat.:)

Yes, it is surprising how the posts mount up- but then I talk a lot too.:D

Interesting circuit function: pre-regulator>precision regulator. Is your objective to limit the dissipation of the LM317? I only ask out of interest because I have been looking at a phase controlled rectifier to do the pre regulation and I wondered if you had considered that approach yourself?

spec
 

Gasboss775

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Interesting circuit function: pre-regulator>precision regulator. Is your objective to limit the dissipation of the LM317? I only ask out of interest because I have been looking at a phase controlled rectifier to do the pre regulation and I wondered if you had considered that approach yourself?

spec
I have thought of this idea on a number of occasions and will no doubt experiment with said circuits at some point. They were popular in early colour TVs I recall.

Yes, my intention was to minimise dissipation in the lm317 as my input will be 32 V ( from an old computer printer PSU ) and the output will be variable over 1V25 to around 25 Volts.
 

Nigel Goodwin

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They were popular in early colour TVs I recall.
Not very popular as I recall, and I repaired thousands of colour TV's :D

The Thorn 3000 series used a switch-mode supply, the first domestic product to do so, and the RBM A823 used a thyristor PSU (as did a few others) - but no particular 'pre-regulators'.

It's a valid choice for a PSU though, although you could also simply switch the input voltage based on what the output is set to (using a tapped transformer).

Presumably though your reasoning is to use an existing (unsuitable :D) supply.
 

Gasboss775

Member
Not very popular as I recall, and I repaired thousands of colour TV's :D

The Thorn 3000 series used a switch-mode supply, the first domestic product to do so, and the RBM A823 used a thyristor PSU (as did a few others) - but no particular 'pre-regulators'.
I probably should have explained myself better as it was those thyristor PSU s that I was thinking of NOT as preregulators but as the actual regulator. I guess they weren't as widely utilised as I'd assumed. I seem to recall the Thorn 3000 PSU being referred to as a "chopper" PSU, perhaps the term "Switch mode" hadn't been adopted at that point.
 

spec

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I have thought of this idea on a number of occasions and will no doubt experiment with said circuits at some point. They were popular in early colour TVs I recall.

Yes, my intention was to minimise dissipation in the lm317 as my input will be 32 V ( from an old computer printer PSU ) and the output will be variable over 1V25 to around 25 Volts.
Right, got you. So a phase controlled pre regulator would not be possible with your set up.

spec
 

Nigel Goodwin

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I probably should have explained myself better as it was those thyristor PSU s that I was thinking of NOT as preregulators but as the actual regulator. I guess they weren't as widely utilised as I'd assumed.
As I said, the main user was RBM in the A823, but there were a few others as well.

The A823 was a 'nice' set in that it had a decent little mains transformer (uncommon in TV's) that you could take from scrap sets and use for making things :D (I still have a couple here).

I seem to recall the Thorn 3000 PSU being referred to as a "chopper" PSU, perhaps the term "Switch mode" hadn't been adopted at that point.
Probably so - 'a rose by any other name smells as sweet'.
 

dr pepper

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Like the little circuit there by ron, some decoupling caps would be a good idea, and the 1k is also a good idea if the circuit were to be very lightly loaded having a higher resistance from the i/p to the o/p of the lm317 would give noise less of a chance to bleed through.
 

Nigel Goodwin

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Going back many years, I seem to recall very simple pre-regulators in the magazines using a thyristor between bridge and reservoir, with the gate triggered by a zener diode.
 

dr pepper

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I've seen that before nige on old industrial stuff, often with chunky thyristors, one time a thyratron, h&s wouldnt like those these days.
 
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