Author Topic: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)  (Read 16904 times)

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Offline jaycee

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #25 on: November 15, 2017, 12:21:32 pm »

I made a rough model of a 30V 150VA transformer

Would you be willing to share that model ? So far in my own power supply simulations, ive just used AC voltage sources, and added some Rser in a crude attempt to model the voltage drop under load. I'd very much like to see a real transformer model :)
 

Offline robert67Topic starter

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #26 on: November 15, 2017, 08:01:05 pm »
Hi not1xor1, Hi Blackdog,

Concerning the use of a buck converter:

1. It's not vintage ;)
2. A tracking one is also a dozen parts or more
3. Otherwise it is a highly efficient solution (but efficiency is not my design goal)

Concerning my vintage transformer:

1. It was good for a 30V/5A purely linear regulated lab power supply
2. Besides the output voltages without load I know nothing else about it
3. It's from a time when we had 220V instead of 230V AC in Europe

Concerning my new approach ;)

1. 13 parts (without rectifier and filter cap)
2. Soft on and soft off (low noise, no spikes)
3. Of course wasting power in the series transistor  ;)
4. Relatively nice to the transformer

Reasoning:

1. Why is a zero point detection needed?
2. Look at it as a linear regulator with a very ripply input voltage (Vripple = Vp)

Design:

1. The power part with constant current source stays the same
2. Instead of the thyristor a BJT can gracefully "steal" current from the series transistor's base
3. The voltage feed back stays almost the same, just introducing some resistance before the base of the PNP BJT to "soften" it's on/off
4. The zener plus resistor is just a stop-gag measure, because the regulation is in fact quite crappy/soft



Questions:

@Blackdog: Do I have your approval noise wise?
@not1xor1: Do I have your approval transformer stress wise?
Who knows this song: "This is my threaaa-aaad, and I cry if I want to, cry if I want to. You would cry too if this happens to you!"  :-DD
« Last Edit: November 16, 2017, 05:29:06 pm by robert67 »
 

Offline not1xor1

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #27 on: November 16, 2017, 10:24:48 am »
Hi,
See attachment Pre-Regulator 4.1.jpg (I really have to learn how to include attachments as images in the HTML)

you first have to post the message, then click on the Modify link opening a new tab window (with firefox I use the middle mouse button)

as soon as the new tab opens, go back to the previous one, where you have your original message, and copy to the clipboard the address of the image you want to embed

then return to the edit tab and click on the small Gioconda icon (first from the left on the second raw) to embed an image and paste the address between the tags

once you have finished just save your modified message

The soft switch-off (D5, R8, R3) actually works as long as there is a load, that is current is drawn from the filter cap C1. And if there's no current drawn from C1 there's also no switch-off spike. In addition it doesn't interfere with the regulation. The soft switch-off can only supply base current when the voltage of C1 is decreasing, so no overshoot. However, if there is a load transient at switch off time from, e.g. 5A to 0A, the soft switch-off can not supply any current to the base and there is a spike  :(

The "active" RC snubber (D4, R6, R7, C2) works as follows: D4 just prevents the base of getting any current from the snubber. R7 discharges C2 while D3 is off, so it's nice and empty to absorb the switch-off spike. R6 is just a security measurement (that thyristor can take up to 16A pulses). Please ignore the values of the components, I had not the time to calculate them properly.

according to LTspice the effect of the soft switch is very small as the base voltage drops too quickly
a darlington transconductance (deltaIc/deltaVbe) is just too high

the snubber takes most of the burden, but at the expense of sinking almost 200mA RMS at low output voltage

I also doodled together a simple linear pre-regulator (you suggested in a previous post it would be the best for the transformer - don't try to simulate that  ;) ):

See attachment: Pre-regulator 6.jpg

I didn't mean that
I just stated that a preregulator increases the transformer losses compared to any linear regulator so you just have to reduce a bit the maximum allowable current of the PSU
 
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Offline not1xor1

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #28 on: November 16, 2017, 10:37:16 am »

I made a rough model of a 30V 150VA transformer

Would you be willing to share that model ? So far in my own power supply simulations, ive just used AC voltage sources, and added some Rser in a crude attempt to model the voltage drop under load. I'd very much like to see a real transformer model :)

I do not have neither the proper instrumentation, nor a real 30V transformer, so it is just a gamble  :)

I based the old model on the measurements (just inductance and resistance) of another 150VA 24V transformer.
Now I measured those data again with a better instrument, but had anyway to modify them a bit to get a reasonable behaviour...

The new model is just  a couple of inductances with the K spice directive:

* 230-30VAC-5A-transf
.subckt 230-30VAC-5A-transf 1 2 3 4
L1 1 2 .975 Rser=21
L2 3 4 22.3m Rser=0.25
R1 4 2 1G
K1 L1 L2 0.99
.end


Now I'm going to run a few tests to check if it may approximate fairly well a real transformer
Later will post an attachment with all the asc asy and sub files
 

Offline robert67Topic starter

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #29 on: November 16, 2017, 05:24:10 pm »
Hi not1xor1,

you first have to post the message, then click on the Modify link opening a new tab window (with firefox I use the middle mouse button)

as soon as the new tab opens, go back to the previous one, where you have your original message, and copy to the clipboard the address of the image you want to embed

then return to the edit tab and click on the small Gioconda icon (first from the left on the second raw) to embed an image and paste the address between the tags

once you have finished just save your modified message

Thank you for that! The wiki help for the forum had nothing on that. I'll try that right away with my last post.

The soft switch-off (D5, R8, R3) actually works as long as there is a load, that is current is drawn from the filter cap C1. And if there's no current drawn from C1 there's also no switch-off spike. In addition it doesn't interfere with the regulation. The soft switch-off can only supply base current when the voltage of C1 is decreasing, so no overshoot. However, if there is a load transient at switch off time from, e.g. 5A to 0A, the soft switch-off can not supply any current to the base and there is a spike  :(

The "active" RC snubber (D4, R6, R7, C2) works as follows: D4 just prevents the base of getting any current from the snubber. R7 discharges C2 while D3 is off, so it's nice and empty to absorb the switch-off spike. R6 is just a security measurement (that thyristor can take up to 16A pulses). Please ignore the values of the components, I had not the time to calculate them properly.

according to LTspice the effect of the soft switch is very small as the base voltage drops too quickly
a darlington transconductance (deltaIc/deltaVbe) is just too high

the snubber takes most of the burden, but at the expense of sinking almost 200mA RMS at low output voltage

200mA RMS at 40Veff is just about 8W, I could live with that (burning more than 30W in my series BJT at high loads).

I also doodled together a simple linear pre-regulator (you suggested in a previous post it would be the best for the transformer - don't try to simulate that  ;) ):

See attachment: Pre-regulator 6.jpg

I didn't mean that
I just stated that a preregulator increases the transformer losses compared to any linear regulator so you just have to reduce a bit the maximum allowable current of the PSU

Got it. But I wanted to explore that avenue anyway, just to see if it is in feasible any way (which it is not).

Any thoughts on my latest brain aneurysm (Reply #26 - now with picture in the text :) )?
« Last Edit: November 16, 2017, 05:34:53 pm by robert67 »
 

Offline not1xor1

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #30 on: November 17, 2017, 06:38:38 am »
Any thoughts on my latest brain aneurysm (Reply #26 - now with picture in the text :) )?

First thing, not related to your question, is that I always forgot to write you can just use a depletion mosfet and a resistor to get a current source withstanding 100s of Vs, rather than an LM317.

Now returning to your circuit... I've not simulated it but got the feeling that, with enough load current, it would be a great noise generator, a sort of low frequency inductor-less buck converter, but with unpredictable behaviour due to the current leakage through the zener diode which might be enough to switch on Q4 depending on the temperature... a resistor between BE would solve that problem, but not the other ones

You might reduce some of the spikes and get some more hysteresis through a capacitive divider, i.e. connecting for a short while a 100-470µF capacitor from the transformer to the 12.000µF one, it would need a series resistor to limit the peak current, a Pmosfet to open/close the circuit, a reverse polarized schottky diode to dissipate the capacitor charge when you switch off the Pmosfet and  some sort of timer or another higher threshold Vpre-Vpost comparator to keep it switched on just a while... so it would just became much more complicated with little benefit.

If a vintage solution is your main goal, if I were you, I would explore an SCR solution...

The clean way is to charge a capacitor through a resistor discharging it on each zero-cross to get a 100Hz sawtooth sync-ed with the transformer secondary.
Then you compare the difference of Vout - Vprereg with the sawtooth value and as soon as the sawtooth goes above the delta you switch the SCRs on.

A several mH inductor (like those used in passive PFC PSU) would avoid a sudden rise of current through the transformer and when the voltage gets down so that there is no current through the SCRs and they switch-off, a schottky diode would let to discharge the rest of the energy stored in the inductor to the levelling capacitor.

As far as I can remember (I ran the simulations about one year ago), at least in the magic world of LTspice  :), that circuit can provide high efficiency and low noise if you use mosfets rather than SCRs and update constantly the Vout-Vprereg threshold according to the set output voltage, the output current and the transformer voltage variations (not instantaneous, but the RMS ones, i.e. those due the AC line) and put a large non polarized capacitor in parallel to the transformer secondary.

But you might just keep it simple, still saving some power, by using a high enough Vout-Vprereg threshold and skipping all those sophistication reducing so the required components.

That is:
- skip the sawtooth comparison circuit which charge the levelling capacitor as suitable each 10 milliseconds
- just use a larger threshold and switch on the SCRs as soon as the voltage gets below that

Sometimes it would get 2 cycles to return quite above the threshold or you might skip many cycles when you have low load and so would have some very low frequency noise depending on the deltaVout/deltaVin capability of the linear regulator, but it should work.
 
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Offline not1xor1

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #31 on: November 17, 2017, 08:19:42 am »
But you might just keep it simple, still saving some power, by using a high enough Vout-Vprereg threshold and skipping all those sophistication reducing so the required components.

That is:
- skip the sawtooth comparison circuit which charge the levelling capacitor as suitable each 10 milliseconds
- just use a larger threshold and switch on the SCRs as soon as the voltage gets below that

Sometimes it would get 2 cycles to return quite above the threshold or you might skip many cycles when you have low load and so would have some very low frequency noise depending on the deltaVout/deltaVin capability of the linear regulator, but it should work.

OK that seems to work... just 3 BJTs and one PNP darlington and 8 resistors... and much lower ripple than I expected.
But I still have to check for peak currents and power losses, transient load, low load, etc...


Epic fail... I made a big mistake... too good to be true  ;D

BTW, Jaycee, I didn't forget that transformer model... later I'll post everything
« Last Edit: November 17, 2017, 10:04:20 am by not1xor1 »
 
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Offline not1xor1

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #32 on: November 17, 2017, 05:08:25 pm »
I'm attaching a zip file with the transformer model and 3 different test circuits as ltspice .asc files.
.meas statement are included in the .asc files.

have fun

BTW tomorrow I'll post a few screenshots and the simulation files of a preregulator which may suite Robert67 needs. It is SCR-style but uses a PNP darlington as switch. Ripple is high (less than 10mV) but there are no spikes.
 

Offline not1xor1

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #33 on: November 18, 2017, 09:28:50 am »
Concerning my new approach ;)

1. 13 parts (without rectifier and filter cap)
2. Soft on and soft off (low noise, no spikes)
3. Of course wasting power in the series transistor  ;)
4. Relatively nice to the transformer

Reasoning:

1. Why is a zero point detection needed?
2. Look at it as a linear regulator with a very ripply input voltage (Vripple = Vp)

Design:

1. The power part with constant current source stays the same
2. Instead of the thyristor a BJT can gracefully "steal" current from the series transistor's base
3. The voltage feed back stays almost the same, just introducing some resistance before the base of the PNP BJT to "soften" it's on/off
4. The zener plus resistor is just a stop-gag measure, because the regulation is in fact quite crappy/soft



I tested that kind of circuit years ago, but I was using MOSFETs then, while BJTs are much slower and your circuit, under some circumstances (regarding set voltage and load) behaves perfectly.
Unfortunately in some instances it shows the defects I verified with MOSFETs, although to a lesser extent. In the screenshot below you will see:
- V(base,N001): darlington Q1 is switched twice per half wave (i.e. at 200Hz)
  with mosfets that might be even 4-500Hz or more with huge spikes
- the red trace V(bridgeout) shows 80V spikes and some distortion the second time Q1 is switched on
- the sky-blue trace shows the current through D2
- output ripple with 3A load is anyway quite good at less than 1mV


And you cannot make it slower by just reducing the base current of Q4 since BJT parameters are highly dependent on temperature, besides that components age and resistor values drift and trimmer resistors age and drift even more (unless you want to invest 10ths of € in metal foil metrology grade trimmer  :) ).

The circuit may be anyway quite valuable if we manage to make it reliably (i.e. independently of component tolerances and value drift) slower.

I'm attaching the current .asc file in case somebody else wants to play with it.

BTW I suspect that sort of crowbar made by D3, R5, Q4 and Q1 being too slow to protect other circuits from damage.

P.S. it looks like files with .lib extensions can't be uploaded so I changed the extension of the darlington model to .txt. Now to make the simulation work either modify the include statement or change back the extension to .lib.
 
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Offline oldway

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #34 on: November 18, 2017, 09:45:40 am »
There is something we have not talked about for phase controlled pre regulators: the characteristics of the transformer.

Indeed, many make mistakes in choosing the transformer: they choose a toroidal transformer, which is the worst choice.

Why ?
Because a toroidal transformer has a very small short circuit impedance, which causes a very bad operation of the pre regulator with high peak current pulses, high rms current, strong heating of the transformer and diodes / scr's, high noise, low range of control angles, and so on....

A pre-regulator requires a high short-circuit impedance transformer (of the order of 20%) .... This is achieved by winding primary and secondary side by side, such as transformers for battery charger or MMA welders, or transformers for microwave ovens (MOT).
However, they have a magnetic shunt, which is not necessary for pre regulators.
 
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Offline not1xor1

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #35 on: November 18, 2017, 09:58:56 am »
The circuit may be anyway quite valuable if we manage to make it reliably (i.e. independently of component tolerances and value drift) slower.

Reducing by 10 times the current from the current source (not feasible with LM317 that's specified at 10mA minimum load) and placing a 2.2nF cap between C-B of Q4 seems to cure all problems.

It looks quite promising.
 

Offline oldway

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #36 on: November 18, 2017, 10:02:39 am »
As for the original schematics, it is not a phase controlled pre regulator and it is a real disaster in terms of efficiency. 
Indeed, the drops in voltages are very high: 1.2V on the bridge rectifier, 1.2V on Q1, 0.4V on D2, 1.25V on LM317, ie in total 4.05V.

EDIT: you did not use a LM317 but current source by 2 transistors instead....then, 0.7V instead of 1.25V, that's 3.5V .... :palm:
EDIT2: using a thyristor as in original schematic, Q1 only will be full on or full off....this mean reduced power dissipation.
Without thyristor, Q1 will work in the linear caracteristics, it will limit current and dissipate a lot.
I already tried such a schematic for battery chargers, it is useless because it has too much loss.
« Last Edit: November 18, 2017, 11:19:27 am by oldway »
 
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Offline robert67Topic starter

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #37 on: November 18, 2017, 01:07:08 pm »
Any thoughts on my latest brain aneurysm (Reply #26 - now with picture in the text :) )?

First thing, not related to your question, is that I always forgot to write you can just use a depletion mosfet and a resistor to get a current source withstanding 100s of Vs, rather than an LM317.

I'll keep that in mind for future designs.

Now returning to your circuit... I've not simulated it but got the feeling that, with enough load current, it would be a great noise generator, a sort of low frequency inductor-less buck converter, but with unpredictable behaviour due to the current leakage through the zener diode which might be enough to switch on Q4 depending on the temperature... a resistor between BE would solve that problem, but not the other ones

I did know the regulation is crappy (as I wrote), but I didn't thought about the zener leakage. Thanks!

You might reduce some of the spikes and get some more hysteresis through a capacitive divider, i.e. connecting for a short while a 100-470µF capacitor from the transformer to the 12.000µF one, it would need a series resistor to limit the peak current, a Pmosfet to open/close the circuit, a reverse polarized schottky diode to dissipate the capacitor charge when you switch off the Pmosfet and  some sort of timer or another higher threshold Vpre-Vpost comparator to keep it switched on just a while... so it would just became much more complicated with little benefit.

That seems to be the theme of this thread: Simple, but to get it actually working you have to make it much more complicated  ;)

If a vintage solution is your main goal, if I were you, I would explore an SCR solution...

The clean way is to charge a capacitor through a resistor discharging it on each zero-cross to get a 100Hz sawtooth sync-ed with the transformer secondary.
Then you compare the difference of Vout - Vprereg with the sawtooth value and as soon as the sawtooth goes above the delta you switch the SCRs on.

A several mH inductor (like those used in passive PFC PSU) would avoid a sudden rise of current through the transformer and when the voltage gets down so that there is no current through the SCRs and they switch-off, a schottky diode would let to discharge the rest of the energy stored in the inductor to the levelling capacitor.

As far as I can remember (I ran the simulations about one year ago), at least in the magic world of LTspice  :), that circuit can provide high efficiency and low noise if you use mosfets rather than SCRs and update constantly the Vout-Vprereg threshold according to the set output voltage, the output current and the transformer voltage variations (not instantaneous, but the RMS ones, i.e. those due the AC line) and put a large non polarized capacitor in parallel to the transformer secondary.

But you might just keep it simple, still saving some power, by using a high enough Vout-Vprereg threshold and skipping all those sophistication reducing so the required components.

That is:
- skip the sawtooth comparison circuit which charge the levelling capacitor as suitable each 10 milliseconds
- just use a larger threshold and switch on the SCRs as soon as the voltage gets below that

Sometimes it would get 2 cycles to return quite above the threshold or you might skip many cycles when you have low load and so would have some very low frequency noise depending on the deltaVout/deltaVin capability of the linear regulator, but it should work.

I tried to comprehend what you're writing. And it seems to work ... but I'm not sure. Guess I need to draw another circuit diagram  ;)
 

Offline robert67Topic starter

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #38 on: November 18, 2017, 01:17:04 pm »
A pre-regulator requires a high short-circuit impedance transformer (of the order of 20%) .... This is achieved by winding primary and secondary side by side, such as transformers for battery charger or MMA welders, or transformers for microwave ovens (MOT).
However, they have a magnetic shunt, which is not necessary for pre regulators.

Well, my vintage transformer is neither toroidal nor does it use side-by-side windings. Its a nice square shape hulk of laminated metal with under-over windings. So I guess its impedance is somewhere in the middle. 
 

Offline robert67Topic starter

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #39 on: November 18, 2017, 01:23:44 pm »
But you might just keep it simple, still saving some power, by using a high enough Vout-Vprereg threshold and skipping all those sophistication reducing so the required components.

That is:
- skip the sawtooth comparison circuit which charge the levelling capacitor as suitable each 10 milliseconds
- just use a larger threshold and switch on the SCRs as soon as the voltage gets below that

Sometimes it would get 2 cycles to return quite above the threshold or you might skip many cycles when you have low load and so would have some very low frequency noise depending on the deltaVout/deltaVin capability of the linear regulator, but it should work.

OK that seems to work... just 3 BJTs and one PNP darlington and 8 resistors... and much lower ripple than I expected.
But I still have to check for peak currents and power losses, transient load, low load, etc...


Epic fail... I made a big mistake... too good to be true  ;D

"If it's to good to be true, it's probably not true."  :-DD

I also struggled controlling those SCRs (with a "reasonable" number of parts >:D ) but I completely failed  |O
 

Offline oldway

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #40 on: November 18, 2017, 01:42:51 pm »

...
Well, my vintage transformer is neither toroidal nor does it use side-by-side windings. Its a nice square shape hulk of laminated metal with under-over windings. So I guess its impedance is somewhere in the middle.
The transformer of your 1993 bench power supply use side by side windings, that's ok.
 
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Offline robert67Topic starter

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #41 on: November 18, 2017, 01:43:49 pm »
I tested that kind of circuit years ago, but I was using MOSFETs then, while BJTs are much slower and your circuit, under some circumstances (regarding set voltage and load) behaves perfectly.
Unfortunately in some instances it shows the defects I verified with MOSFETs, although to a lesser extent. In the screenshot below you will see:
- V(base,N001): darlington Q1 is switched twice per half wave (i.e. at 200Hz)
  with mosfets that might be even 4-500Hz or more with huge spikes
- the red trace V(bridgeout) shows 80V spikes and some distortion the second time Q1 is switched on
- the sky-blue trace shows the current through D2
- output ripple with 3A load is anyway quite good at less than 1mV


I was actually thinking about changing to a MOSFET as series transistor. There seems to be some old stock of TO-3 types (IRF140, IRF250, BUZ24, IRF150, 2N6764) available on eBay (European sellers - I wouldn't trust an "IRF150" for €3.99 from China).

The circuit may be anyway quite valuable if we manage to make it reliably (i.e. independently of component tolerances and value drift) slower.

Yes. I still like the basic idea of it.

However, making the regulation more reliable/precise with the current amplifications of three BJTs basically multiplied (one of them being an darlington) might be a mission impossible (and I'm not the Tom Cruise of electronics  ;) ).

Anyway, I thought of using op amps for the regulation, getting rid of all the ever changing h(FE) and V(BE). Maybe that will work out somehow. But I'm skeptical if I can keep the parts count "reasonable"  ;)
 

Offline robert67Topic starter

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #42 on: November 18, 2017, 01:52:49 pm »
The circuit may be anyway quite valuable if we manage to make it reliably (i.e. independently of component tolerances and value drift) slower.

Reducing by 10 times the current from the current source (not feasible with LM317 that's specified at 10mA minimum load) and placing a 2.2nF cap between C-B of Q4 seems to cure all problems.

It looks quite promising.

I'll change the current source to 2mA with a depletion mode FET plus resistor and add the cap in my circuit diagram.
 

Offline robert67Topic starter

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #43 on: November 18, 2017, 05:22:55 pm »
As for the original schematics, it is not a phase controlled pre regulator and it is a real disaster in terms of efficiency. 
Indeed, the drops in voltages are very high: 1.2V on the bridge rectifier, 1.2V on Q1, 0.4V on D2, 1.25V on LM317, ie in total 4.05V.

Yes it is ;D The numbers are in my circuit diagram (over 30W are burned worst case in the series BJT). But then, efficiency was never one of my design goals. Simplicity (low parts count) and a the use of a TO-3 transistor are. And I will quite happily burn up to, say 70-80W, to achieve those  >:D

EDIT: you did not use a LM317 but current source by 2 transistors instead....then, 0.7V instead of 1.25V, that's 3.5V .... :palm:

But that's two parts more  ;) (see above)

EDIT2: using a thyristor as in original schematic, Q1 only will be full on or full off....this mean reduced power dissipation.
Without thyristor, Q1 will work in the linear caracteristics, it will limit current and dissipate a lot.
I already tried such a schematic for battery chargers, it is useless because it has too much loss.

The Vulcan science council (see the whole thread) has determined that a hard switch off via the thyristor causes (A) destructive voltage spikes due to the transformers inductivity [not1xor1] and (B) unacceptable noise [Blackdog]  ;)

Basically you're buying "nice and quite" for selling out efficiency. If I wanted real high efficiency I could simply go for a buck converter (which has already been suggested [not1xor1]). But that would neither be "vintage" (so a no-go for me) nor low noise (for which that suggestion has already be damned  [Blackdog]).
 

Offline not1xor1

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #44 on: November 19, 2017, 06:00:42 am »
The circuit may be anyway quite valuable if we manage to make it reliably (i.e. independently of component tolerances and value drift) slower.

Yes. I still like the basic idea of it.

However, making the regulation more reliable/precise with the current amplifications of three BJTs basically multiplied (one of them being an darlington) might be a mission impossible (and I'm not the Tom Cruise of electronics  ;) ).

Anyway, I thought of using op amps for the regulation, getting rid of all the ever changing h(FE) and V(BE). Maybe that will work out somehow. But I'm skeptical if I can keep the parts count "reasonable"  ;)

After many simulations I think that there are really no shortcuts.
You have to take into account many different parameters, i.e. power dissipation, spikes, output noise, etc...
Some circuit may work at a given output voltage, but may cause huge spikes at another one, or may respond quite badly to a transient load, or may burn your transformer, etc...

If your goals are save just a little of power, use not too many components and use a vintage circuit then it may be better to just make use of the 2 transformer taps and use a plain linear regulator like in this old oltronix circuit where a darlington and a couple of diodes are used to switch between the 2 different DC voltage levels:

« Last Edit: November 19, 2017, 06:05:37 am by not1xor1 »
 
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Offline robert67Topic starter

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #45 on: November 19, 2017, 10:41:24 am »
The circuit may be anyway quite valuable if we manage to make it reliably (i.e. independently of component tolerances and value drift) slower.

Yes. I still like the basic idea of it.

However, making the regulation more reliable/precise with the current amplifications of three BJTs basically multiplied (one of them being an darlington) might be a mission impossible (and I'm not the Tom Cruise of electronics  ;) ).

Anyway, I thought of using op amps for the regulation, getting rid of all the ever changing h(FE) and V(BE). Maybe that will work out somehow. But I'm skeptical if I can keep the parts count "reasonable"  ;)

After many simulations I think that there are really no shortcuts.
You have to take into account many different parameters, i.e. power dissipation, spikes, output noise, etc...

Maybe you're right :( Anyway, I've added the additions you suggested (lower constant current, cap between collector and base) to the circuit diagram. However, now that the constant current is lower every thing else needs to be re-dimensioned too. And that leads to more problems ...



But I've mentioned in my last post that I thought about using opamps for the regulation ... So I'm given it one more try ;) Instead of using two BJTs to do the regulation I went for a single opamp used as inverting summing amplifier and a MOSFET as series transistor. The opamp "ground" is the voltage of the main filter cap C1. It is summing the output voltage of the linear regulator (negative to the opamp "ground", that is V at C1) and a reference voltage (positive to the "opamp" ground, that is V at C1). If the output voltage of the linear regulator is lower than the reference voltage, the output of the opamp is positive, driving the MOSFET open, and vice versa. The gain of the opamp controls how "soft" the switching is. E.g. at a gain of 10 a voltage difference of 1V is needed to switch on the MOSFET fully. Every voltage difference below that will drive the MOSFET into its linear region.



That's my last try at a "simplistic" pre-regulator. If that doesn't work I will go either for an evil buck pre-regulator, or completely forget about pre-regulation and do as you suggested (using the two secondary windings - Incidentally I currently do exactly that while breadboarding my linear regulator :-DD ).
« Last Edit: November 19, 2017, 10:44:05 am by robert67 »
 

Offline not1xor1

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #46 on: November 20, 2017, 12:06:56 pm »
But I've mentioned in my last post that I thought about using opamps for the regulation ... So I'm given it one more try ;) Instead of using two BJTs to do the regulation I went for a single opamp used as inverting summing amplifier and a MOSFET as series transistor. The opamp "ground" is the voltage of the main filter cap C1. It is summing the output voltage of the linear regulator (negative to the opamp "ground", that is V at C1) and a reference voltage (positive to the "opamp" ground, that is V at C1). If the output voltage of the linear regulator is lower than the reference voltage, the output of the opamp is positive, driving the MOSFET open, and vice versa. The gain of the opamp controls how "soft" the switching is. E.g. at a gain of 10 a voltage difference of 1V is needed to switch on the MOSFET fully. Every voltage difference below that will drive the MOSFET into its linear region.



That's my last try at a "simplistic" pre-regulator. If that doesn't work I will go either for an evil buck pre-regulator, or completely forget about pre-regulation and do as you suggested (using the two secondary windings - Incidentally I currently do exactly that while breadboarding my linear regulator :-DD ).

I think I already mentioned in a previous post an old schematic I devised years ago that uses a schmitt trigger as sort of SR flip flop.
Here it is:



Most of the spikes are eaten by the capacitor (3 100uF low ESR in parallel). The discharge resistor and the fill resistor are used to cut the input spikes when the mosfet is switched on.

The switch speed is easily tuned by changing C2 while the preregulator delta is almost independent of the output voltage and can be changed by adjusting the R3-R5 voltage divider.

Rfill dissipates quite a lot of power on high load and low output voltage, but the overall power loss is still low and both the mosfet and the linear regulator do not dissipate much power.

This is appropriate especially when you use a linear regulator bootstrapped by the positive voltage.
 
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Offline robert67Topic starter

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #47 on: November 21, 2017, 06:17:04 pm »
... The gain of the opamp controls how "soft" the switching is. E.g. at a gain of 10 a voltage difference of 1V is needed to switch on the MOSFET fully. Every voltage difference below that will drive the MOSFET into its linear region.



I think I already mentioned in a previous post an old schematic I devised years ago that uses a schmitt trigger as sort of SR flip flop.
Here it is:



Most of the spikes are eaten by the capacitor (3 100uF low ESR in parallel). The discharge resistor and the fill resistor are used to cut the input spikes when the mosfet is switched on.

Thanks for reminding me about that solution of yours. Going through all the alternatives I forgot about that one.

Anyway, the idea of my last design was to avoid switch-off spikes completely (by wasting power in the series transistor of course). The opamp works as an error amplifier or proportional regulator (voltage of C1 being the "process variable" and the desired voltage being the "setpoint"), not as a Schmitt trigger or comparator. It doesn't need a capacity at the gate of the MOSFET.

  • As the voltage of C1 approaches the target voltage the MOSFET enters its linear region
  • The current to C1 is decreased, the voltage of C1 rises slower
  • As the voltage of C1 rises further the MOSFET is driven further into its linear region
  • The current to C1 is further decreased, the voltage of C1 rises even slower

Anyway, after counting parts ;) I decided it will be easier to switch my secondary transformer windings to keep the total power losses at bay, and to redesign my linear regulator to take 60V (switching from a L200 to a LM317 high voltage variant at the core).

Thank you so much for your help and your patients with my "brain farts".
 

Offline not1xor1

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #48 on: November 23, 2017, 09:46:18 am »
Anyway, the idea of my last design was to avoid switch-off spikes completely (by wasting power in the series transistor of course). The opamp works as an error amplifier or proportional regulator (voltage of C1 being the "process variable" and the desired voltage being the "setpoint"), not as a Schmitt trigger or comparator. It doesn't need a capacity at the gate of the MOSFET.

  • As the voltage of C1 approaches the target voltage the MOSFET enters its linear region
  • The current to C1 is decreased, the voltage of C1 rises slower
  • As the voltage of C1 rises further the MOSFET is driven further into its linear region
  • The current to C1 is further decreased, the voltage of C1 rises even slower

I'll have a look at it... I'll see how it behaves in LTspice

Anyway, after counting parts ;) I decided it will be easier to switch my secondary transformer windings to keep the total power losses at bay, and to redesign my linear regulator to take 60V (switching from a L200 to a LM317 high voltage variant at the core).

Thank you so much for your help and your patients with my "brain farts".

no problem
I think that using a multi-tap transformer, in most cases, may be even more efficient than using a pre-regulator.
Actually I'm quite interested in that subject and in my spare time I'm simulating many different kinds of preregulators.

In future I will probably start another thread to discuss about that.
 
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Offline robert67Topic starter

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Re: Simplistic Pre-Regulator (Tracking, Mains Zero Crossing Detecting)
« Reply #49 on: November 23, 2017, 05:23:44 pm »
Anyway, the idea of my last design was to avoid switch-off spikes completely (by wasting power in the series transistor of course). The opamp works as an error amplifier or proportional regulator (voltage of C1 being the "process variable" and the desired voltage being the "setpoint"), not as a Schmitt trigger or comparator. It doesn't need a capacity at the gate of the MOSFET.

  • As the voltage of C1 approaches the target voltage the MOSFET enters its linear region
  • The current to C1 is decreased, the voltage of C1 rises slower
  • As the voltage of C1 rises further the MOSFET is driven further into its linear region
  • The current to C1 is further decreased, the voltage of C1 rises even slower

I'll have a look at it... I'll see how it behaves in LTspice

That would be most appreciated  :)

Anyway, after counting parts ;) I decided it will be easier to switch my secondary transformer windings to keep the total power losses at bay, and to redesign my linear regulator to take 60V (switching from a L200 to a LM317 high voltage variant at the core).

Thank you so much for your help and your patients with my "brain farts".

no problem
I think that using a multi-tap transformer, in most cases, may be even more efficient than using a pre-regulator.
Actually I'm quite interested in that subject and in my spare time I'm simulating many different kinds of preregulators.

In future I will probably start another thread to discuss about that.

Depending on the results of your simulation, I might even go back to the pre-regulator approach.

Turns out switching between the secondary windings is not that easy. The transformer has (now, after Europe went from 220V to 230V) some very "odd" output voltages (considering mains AC is +/-10%):

  • 14.3Veff to 17.4Veff, 20.2Vp to 24.6Vp
  • 20.5Veff to 25.0Veff, 29.0Vp to 35.4Vp

To keep worst case losses in the linear regulator at a reasonable level (below 150W - I have two TO-3 BJT on big heat sinks on the backside) I'll have to set the switch point depending on the current AC voltage. After trying to come up with a design using transistors I went very fast for an opamp in schmitt trigger configuration :D Unfortunately that opamp needs some input protection on the feedback from the linear regulator (max. 30V) when I just use the 29.0Vp (below 30V) to 35Vp winding to supply the switcher. That's two more parts and it messes with the nice high input impedance |O Or I run the opamp from the combined windings. But then I'll need 60V capable opamp and relay as well as a second rectifier/filter to measure the AC level on my "lower" winding |O

Anyway, maybe I attach my current design later to this message ...
 


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