Questions on op-amp based adjustable power supply

Increase r3 or reduce r2/P1 etc. the exact ratios depends on what range of output voltage you want, there will now only be 2.5v acros R2 and P1 and output voltage less 2.5v accross r3. the voltage is in proportion to the resistance.

I wouldnt recomend altering the supply of the 741, I would suggest a transformer of the right voltage, it would mean a smaller transformer and less heat/smoke !

Colin =^.^=

Unfortunately a lower-voltage power supply is not an option right now.

As far as over-powering the op-amp, I am somewhat confused on how they are rated. According to the spec sheet for the 741 that I have, the supply voltage is rated for "+/- 18v". Since I am using it with a single-voltage supply, would I double this rating to find out the effective supply voltage it is rated for? (i.e. if it normally operates from -18v to +18v, would the supply voltage on a single-voltage supply be 18*2 = 36v?)

Thanks Kevin

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To answer your most recent question, a '741 can operate from a 0V/+36V supply, (absolute maximum).

Regarding the transformer, 18VAC will convert to about 24VDC after the bridge rectifier, if the transformer's load approximates it's current rating. e.g. A transformer rated at 12VAC, 5A will usually be within about

10% of 12V if a current of 5A is drawn. The voltage will increase as the load is decreased and vice versa. Using a 26VAC transformer will result in approx. 36VDC, ((26 x 1.414)-1.2), which may be higher if the typical load of that transformer is not met, exceeding the rating of the '741. It would not hurt to use a regulator, (at the minimum, a Zener diode), to keep the '741's supply voltage within specs. ... Donkey

Thanks for the reply. Come to think of it, 36V is even high for a standard voltage regulator. How would I use a zener diode, and what value would I choose? If I do lower the 741's supply voltage, do I need to connect any of the voltage reference components (U1, P1, R2, R3 in schematic) to the same lower supply voltage or would they remain on the

+36V rail as they are in the schematic?

Thanks again, Kevin

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You'll need to apply the lower voltage to the op-amp, the LM336Z/R1 and the R2/P1/R3 voltage divider. You'd need a 1W Zener, something like 33V. Having chosen the Zener, you'd then need to recalculate the resistor values to suit.

... DD

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Correction: You don't need to apply the lower voltage to the R2/P1/R3 voltage divider, if you calculate the correct values for these. ...Donkey

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unfortunatly you cant reduce the positive or negative supply to the op amp, if you reduce the negative supply, the output transistors will not be able to turn off, and the positive supply needs to be higher than the input to the op amp (obviously) however there are op amps with a higher voltage rating wich might be ok, the other alternative is to reduce the inputs to the op amp so that the positive supply can then be regulated, so that its used like a differential amplifier, or a string of big diodes folowing the rectifier ... you only need to lose 4 volts to keep the 741 happy :)

Colin =^.^=

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Actually there is another way, that is to simply put a zener in series with the negative supply of the 741 AND its output, you only need to lose 4 volts, I didnt notice r4/r5 forms a voltage divider before so you can almost get away with it just in the negative supply (maybe just reduce R5), adding a 100R resistor from the base of the output transistors down to the negative rail will ensure the output transistors are off with subsequent higher op amp op voltage.

Colin =^.^=

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Why can't a Zener be used to limit the (positive) supply to the op-amp, the reference AND the feedback voltage divider? (The R4/R5 divider would also need attention.)

... Donkey

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zeners on both the inputs would make the regulation very poor. resistor dividers making a diff amp would be more accurate. its simpler to put zeners on the neg rail and op amp output where their poor regulation doesnt affect the regulation.

Colin =^.^=

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Of course - I'm a bit slow sometimes - must be old age.

... Donkey

snipped-for-privacy@mn.rr.com wrote: ...

...

High-current voltage regulation probably is a dumb idea for your application. Here are two possibly-better approaches:

1. Attach a full-wave-bridge to the secondary of your 26VAC 15A transformer, and a variac on the primary. Connect the bridge DC output to your capacitor bank through a sub-ohm resistor. Set variac to get proper output voltage.
2. Wire one LM338 in constant-current mode* to deliver 5A to your capacitors until they get to target voltage, then open a relay or whatever. 1.4 seconds for 10V charge, 2.8 for 20V.

*See p. 12 of which shows R1 = .24 ohm, 2W

-jiw