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Old 14th Nov 2018, 1:58 pm   #1
m3vuv51
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Default 723 psu issues.

Hi all,im putting together a psu using a 723,im using this schematic,the issue i have is,i want the current feed back to work from about 100ma upwards,the problem i have is to get 600mv from the current sense resistor,it has to be a high value to produce 600mvfor the current foldback to work on the 723,this lowers the output to much overall,is it possible to amplify the sense voltage to feed the 723 cl and cl pins,therefor use a lower value sense resistor,ive thought of using a sg3532 but they are expensive!,cheers m3vuv.
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Old 15th Nov 2018, 4:07 am   #2
Terry_VK5TM
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Default Re: 723 psu issues.

Look for a 723 circuit that uses an op-amp in the current sense circuit.

I thought I had one here but it is not on this PC, you should be able to find one on the 'net.
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Old 15th Nov 2018, 8:02 am   #3
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Default Re: 723 psu issues.

One word of caution. The 723 is well known for being very sensitive to any RF getting into it, so for use powering an amateur radio station, take care over RF filtering.

David
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Old 15th Nov 2018, 8:58 am   #4
MrBungle
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Default Re: 723 psu issues.

I built something similar on a breadboard a couple of years back. I didn’t use the LM723’s current limit circuit. Instead I used a small shunt resistor and an opamp in differential mode. This pulled the pass transistor driver down when it hit a set limit.

Unfortunately I trashed the idea because when it came to buying a transformer they were more expensive than buying a supply. Schematic went with it also

Edit: I didn’t actually use the LM723 in the end, just an LM358 - one half for voltage feedback and one for current feedback. LM385-2.5 as reference.
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Old 15th Nov 2018, 11:25 am   #5
Terry_VK5TM
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Default Re: 723 psu issues.

Quote:
Originally Posted by Radio Wrangler View Post
One word of caution. The 723 is well known for being very sensitive to any RF getting into it, so for use powering an amateur radio station, take care over RF filtering.

David
Agreed.

Filter both ends - incoming mains side and outgoing supply side and put the input side filter after the ON/OFF switch. It stops switching spikes from killing the chip.

Another problem that makes it susceptible is the capacitor connected to pin 13 - it is often shown as anywhere from .1F to .01F. Many datasheets show it around these values.

A long time back there was a correction published (I think it was by National Semi) and it should be 470pF or lower.

Why this makes a difference, I don't remember.
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Old 15th Nov 2018, 1:01 pm   #6
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Default Re: 723 psu issues.

It's the compensation capacitor for the internal error amp- the 723 might be used with just its own internal pass transistor for a low-current load, or sometimes with an impressive fan-out of external pass transistors. Often, including the internal driver, this amounts to a triple Darlington, sometimes complimentary, more often seemingly an NPN cascade. That application flexibility amounts to a great deal of possible variation in frequency response characteristic, hence the need to choose an appropriate value comp cap.- in most circuits, it seems to range from 100pF to 1nF.
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Old 16th Nov 2018, 2:02 am   #7
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Default Re: 723 psu issues.

I've read several articles that state that the 723 has a very low output current capability and that you need a darlington driver before the series pass transistors to be sure of sufficient regulation.
I know this is not the answer to your current limit query, but it may save a lot of head scratching if you intend to draw a lot of current from your supply.
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Old 16th Nov 2018, 9:20 am   #8
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Default Re: 723 psu issues.

The selection of the loop compensation capacitor affects the response of a PSU to a dynamically changing load. SSB rigs show strong variations in current load at syllabic rate, and Morse transmissions can be worse. Get this wrong and the response of the PSU can exaggerate the droop and overshoot effects.

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Old 16th Nov 2018, 10:51 am   #9
Terry_VK5TM
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Default Re: 723 psu issues.

Thanks for the info on the compensation capacitor guys.

I didn't keep the correction paperwork, just filed the pertinent info about the value in the Grey Cell File.

Andrew, yes, the 723 on it's own is limited to 150mA, so external pass transistors and a driver transistor for them is required for high current outputs.
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Old 16th Nov 2018, 11:26 am   #10
turretslug
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Default Re: 723 psu issues.

As so often in regulator pass transistor situations, it's device dissipation that's far more likely to be the limiting factor than actual current capability (viz. all those myriads of bench PSUs rated at around 2A with a parallel quad of 15A 2N3055s!- admittedly for particularly wide worst-case headroom considerations). Whilst the 723 might be rated at 150mA, that doesn't allow much headroom before the package is pushing towards 1W. That's not good for long term dependability of either device or joints, and it's a shame to choose a "precision voltage regulator" and then make it very hot- whatever the data sheet may say about "temperature-compensated reference". Current limit threshold will also drift unnecessarily. 150mA would be just fine for something like an oscillator or low-level audio pre-amp that needs pampering with a personal low-noise and/or highly stable supply but even then an external assist from the likes of 2N3053 or BD139 gives the device an easy life and will hardly break the bank- headroom considerations aside.

It may now have the status of "superannuated vintage" but I quite like it and often use it- one good feature is the accessible reference and the ability to use its compensation feed resistor as part of an LPF. There are actually very few external components needed, really, and a postage stamp-size "general application block" is easily contrived on matrix board.
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