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Old 15th November 2010, 04:05 PM   #11
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I'd suggest you move the connection of the anode of D2 to the positive output of D3 to make sure C3 won't be reverse-biased a lot during startup. That C3 is so small looks pretty strange too. Using a C3 much smaller than C1 doesn't make much sense unless you are using its reactance to keep the "doubled" voltage down. But an electrolytic capacitor is not going to like that kind of treatment.

Why did you change the negative sensing of the LM317? The way it is now the reference voltage to the voltage regulator decreases with increased output current, making the output impedance even larger than R17 itself at maximum output voltage.

You may also want to add reverse polarity protection diodes across the pass transistor(s) and the output. A diode in series with the base of Q1 might be useful too so the transistors can't be destroyed during downprogramming by current from capacitors or batteries connected to the output. A diode over R17

A base-"discharge" resistor is then needed for the driver and the compensation should prefereably be connected to the output side of the diode to speed up recovery.

Have you made sure voltage regulation isn't lost during turn-on and turn-off? It looks like the pass transistors will go full-on when the main bus voltage drops to about 3-4V if the doubled supply still has charge left. On the other hand most circuits will survive a 3V output...

The current limiter isn't going to have very good regulation, ripple and temperature stability specs but it's not worse than many other DIY PSU circuits. Do you really want the weak foldback characteristic given by R21 or is it perhaps tuned to counteract the early effect in the current limit transistor?
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Old 15th November 2010, 06:05 PM   #12
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Quote:
Originally Posted by megajocke View Post
I'd suggest you move the connection of the anode of D2 to the positive output of D3 to make sure C3 won't be reverse-biased a lot during startup. That C3 is so small looks pretty strange too. Using a C3 much smaller than C1 doesn't make much sense unless you are using its reactance to keep the "doubled" voltage down. But an electrolytic capacitor is not going to like that kind of treatment.
Hi Mega,
The value for C3 was just arbitrary, enough to get the doubler to work, given the low current demand on this part of the supply. I could easily use 100uF there but increasing it (in simulation) didn't have any positive effect so I just went with the 10uF. Although the schematic shows a polarized cap, I'm using a bipolar in the prototype, so not worries of it reverse biasing.

Quote:
Why did you change the negative sensing of the LM317? The way it is now the reference voltage to the voltage regulator decreases with increased output current, making the output impedance even larger than R17 itself at maximum output voltage
I thought it was the other way round - that it would be better to have the LM referenced to the supply side of the sense resistor. "true" 0 volts exists there, on the load side of the sense resistor the "0" volts fluctuates with current demand. ???

Quote:
You may also want to add reverse polarity protection diodes across the pass transistor(s) and the output. A diode in series with the base of Q1 might be useful too so the transistors can't be destroyed during downprogramming by current from capacitors or batteries connected to the output. A diode over R17
My original had a diode across the output but it got lost somewhere during revisions. I'll squeeze it in or mount it across the binding posts.
A diode to protect R17 - will do.

Quote:
The current limiter isn't going to have very good regulation, ripple and temperature stability specs but it's not worse than many other DIY PSU circuits. Do you really want the weak foldback characteristic given by R21 or is it perhaps tuned to counteract the early effect in the current limit transistor?
The current limiter is more of a protection mechanism. I wanted to be able to set the max limit to a reasonable amount for the circuit being tested and I think it does that well enough.
Ripple is very low, even under heavy load. 2 to 3mVpp with a moderate load (400-500mA) is pretty good. Many will look at this and see the doubler and think this is nothing but a 120Hz oscillator - this is not the case.
Temp stability is yet to be determined. A little shift one way or the other is not a big deal for me though.
R21 is supposed to foldback the current output slightly (a bit less than the max current output) during a short circuit on the output. I have not actually tried it out and 3K may not be the most suitable value.

Thank you for the suggestions!
I'll tinker with it a bit more and implement them.
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Old 15th November 2010, 08:11 PM   #13
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Looks like one sentence got cut off there... :P A diode across the output would protect a turned-off supply if you put it in series with another one or if there is a short rail-to-rail when using two as a split supply.

The diode across output transistors would be for protecting the transistors if you connect a battery to the output or put two supplies in parallell and turn off one. I'd add a diode over R17 too because the peak current (in the reverse direction!) could be pretty high when charging the input capacitor from the output and that could be unhealthy for the resistor itself but also the current limit circuit connected to it. Maybe I worry too much.

If it is not a precision current regulator that you need then your current limiter looks fine to me. The voltage regulator is pretty nice, but I'm pretty sure you want the reference voltage referenced to the negative output. You could just try your prototype with a load at high output voltage and see what happens. If I'm not mistaken the LF output impedance will be about 1 ohm.

If you want to lower the ripple it might help to add an R-C filter to the base drive supply, but your 1 mF capacitor there is pretty big considering the current draw so I guess it may not be needed after all.

I'd still suggest you add something to protect the output transistor, driver and "VAS" transistor if a voltage source is connected to the output because the "VAS" transistor would turn full on if the external voltage is higher than the programmed voltage. You'll get reverse breakdown in the driver and maybe the output transistor too and lots of current through the "VAS". A diode in series with the driver base would mitigate that.

Last edited by megajocke; 15th November 2010 at 08:20 PM.
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Old 15th November 2010, 11:16 PM   #14
Bigun is online now Bigun  Canada
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John,

Looking good.

I see my heatsink being put to good use again
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Old 15th November 2010, 11:22 PM   #15
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Mega you are a taskmaster! All good though as I know that you are giving me excellent advice, as you have done many times in the past.
I have implemented the changes:

LAB SUPPLY SCH.png

Hopefully I got everything correct.
In the sim, with the LM317 grounded to the load side of the sense resistor, there is fluctuation in the reference voltage with a load applied. Connection at the supply side and there is no fluctuation. I haven't had a chance to check this out on the prototype yet and I'm not sure which way I grounded it on there.
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Old 15th November 2010, 11:33 PM   #16
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Quote:
Originally Posted by Bigun View Post
John,

Looking good.

I see my heatsink being put to good use again
Gareth! Howdy
That was the first and last project I did with your() heatsink. The majority of the 2 meter length is still untouched. I'm sure I'll find some use(s) for the rest of it.
Board revised to the above scheme:

LAB SUPPLY BRD.png

Not on the board id D8. This will be soldered directly to the legs of Q2.

Last edited by MJL21193; 15th November 2010 at 11:46 PM.
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Old 16th November 2010, 10:28 AM   #17
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Have you looked at dissipation requirements when high current is required from one or both supplies at low output voltage?

A multi-tapped transformer will help a lot in reducing the heat dissipation load.

Even a 115:0,10,20,30+0,10,20,30Vac transformer would massively reduce heat load.
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Old 16th November 2010, 11:23 AM   #18
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A Switching regulator is more likely choice for variable output demands.
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Old 16th November 2010, 12:35 PM   #19
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Default pcb layout

sir MJL21193 im just new to pcb making can i ask what is the software you are using for making the pcb layout...
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Old 16th November 2010, 03:11 PM   #20
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:P Thanks!

I take it you mean the voltage between the output pin of the LM317 and the negative terminal of the bridge changes. That is inconsequential however as what you need constant really is the voltage between the LM317 output pin and the negative output terminal. You want the current sense resistor R17 to be "outside" the voltage regulating circuit.

It may be easier to see if you remove the current limiting circuitry or just define the negative output as ground. The negative terminal of the bridge then has a negative voltage that depends on the current drawn. This unwanted disturbance gets added to your reference voltage if you keep the current connection of the reference voltage regulator negative sensing lead.
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