LM723 based PSU with min voltage of 0,00V

spec · Feb 24, 2017

earckens said:
Hi spec, I can't find R29 even though I just had a Uigeadail (you know, the one from the north of you ).
And C9 is 22nF. Three in parallel still make only 66nF

Sorry Earckens- I have had a hard day mixing concrete.

The cathode of the 1N4148 should connect to the LM723, pin 4.

Oh dear, another error.

The 200uF capacitor needs to be non-polarized to protect against reverse voltage.

The trouble is that a non-polarized capacitor of that value would be big/expensive.

I think that the minimum output capacitor that you could get away with would be around 100uF, so reducing the value of the output capacitor may help to find a reasonable component- I will have a look.

spec

tcmtech · Feb 24, 2017

spec said:
About zero volts.

It is the holy grail in lab power supplies to be able to adjust the output voltage down to 0V.

The current limit at 0V is the same as any other output voltage.

spec

Okay?
How do you get current out of power supply that's supplying 0.00 volts?

Isn't the load working as a source and the power supply is working as a current sink at that point in order to get any current flow?

earckens · Feb 24, 2017

spec said:
Sorry Earckens- I have had a hard day mixing concrete.

The cathode of the 1N4148 should connect to the LM723, pin 4.

spec

Cathode to pin 4? And anode facing the rest behind it??
Are you sure those 0.7V won't upset this sensitive pin 4?

spec · Feb 24, 2017

tcmtech said:
Okay?
How do you get current out of power supply that's supplying 0.00 volts?

Isn't the load working as a source and the power supply is working as a current sink at that point in order to get any current flow?

No, a power supply with current sinking is the holy holy grail and if you over volt normal power supplies they can get upset.

Of course, you are right- you cant get a normal flow of electrons without a potential difference.

But if you connect the power supply to the virtual earth input of a current to voltage converter, for example, you would be pretty close.

One application for a 'zero' volt output is fuse and wire testing.

But it is also handy to be able to smoothly increase the output voltage from zero with certain low voltage devices: point contact diodes for example and some microwave semiconductors.

Zero volt is also handy for checking shorts- hot spot testing (infrared) with shorts on multi-layer PCBs.

It is also handy when you have a complex board that is taking too much current.

spec

spec · Feb 24, 2017

earckens said:
Cathode to pin 4? And anode facing the rest behind it??
Are you sure those 0.7V won't upset this sensitive pin 4?

Cathode to pin 4- yes
Anode to the power supply negative output (another error by me). All other connections of the circuit in post #7 remain as shown.

The idea is that when the power supply is operating normally, the diode is reverse biased, so has no effect.

spec

spec · Feb 24, 2017

Here is a link to some suitable 22uF, 50V, X7R ceramic capacitors- bit pricey though.
https://www.digikey.co.uk/products/...=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25

Perhaps try two 22uF capacitors in parrallel to make 44uF- should be OK.

The existing 220nF ceramic capacitor can be removed if you fit the 22uF ceramic capacitors.

spec

PS: You can get lower prices on eBay and AliExpress

earckens · Feb 24, 2017

spec said:
Cathode to pin 4- yes
Anode to the power supply negative output (another error by me). All other connections of the circuit in post #7 remain as shown.

The idea is that when the power supply is operating normally, the diode is reverse biased, so has no effect.

spec

Hi spec, sorry I do not understand how this would protect the 723? If a reverse voltage is applied it goes straight through the diode to pin 4?

earckens · Feb 24, 2017

spec said:
Here is a link to some suitable 22uF, 50V, X7R ceramic capacitors- bit pricey though.
https://www.digikey.co.uk/products/en/capacitors/ceramic-capacitors/60?k=capacitor&k=&pkeyword=capacitor&pv17=5&FV=fff40002,fff8000b,1c0002,340082,380020,ffe0003c&mnonly=0&newproducts=0&ColumnSort=1000011&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25

Perhaps try two 22uF capacitors in parrallel to make 44uF- should be OK.

The existing 220nF ceramic capacitor can be removed if you fit the 22uF ceramic capacitors.

spec

PS: You can get lower prices on eBay and AliExpress

Hi spec, what if I leave the elco, and let it pop when reverse voltage is applied? What else can get harmed?
And I would get an auditive warning

spec · Feb 24, 2017

earckens said:
Hi spec, what if I leave the elco, and let it pop when reverse voltage is applied? What else can get harmed?
And I would get an auditive warning

You just don't care- do you.

Quite right, nothing else should be damaged electrically, but you may have a caustic liquid splattered over everything!

spec

spec · Feb 24, 2017

earckens said:
Hi spec, sorry I do not understand how this would protect the 723? If a reverse voltage is applied it goes straight through the diode to pin 4?

The diode is in series with a resistor (or two depending where the voltage output photometer is set) so the maximum negative voltage that the LM723 would see would be the forward drop of the diode. It would really be better to fit a small-signal Schottky diode, like a BAT43, which has a forward voltage of around 0.25V at the low current involved. But a 1N4148 will be adequate in practice.

spec

tcmtech · Feb 24, 2017

spec said:
One application for a 'zero' volt output is fuse and wire testing.

But it is also handy to be able to smoothly increase the output voltage from zero with certain low voltage devices: point contact diodes for example and some microwave semiconductors.

Zero volt is also handy for checking shorts- hot spot testing (infrared) with shorts on multi-layer PCBs.

It is also handy when you have a complex board that is taking too much current.

I just use simple low current constant current sources (continuity tester) for such things.

earckens · Feb 24, 2017

spec said:
The diode is in series with a resistor (or two depending where the voltage output photometer is set) so the maximum negative voltage that the LM723 would see would be the forward drop of the diode. It would really be better to fit a small-signal Schottky diode, like a BAT43, which has a forward voltage of around 0.25V at the low current involved. But a 1N4148 will be adequate in practice.

spec

spec, below modified schematic: is the diode correctly placed? I use a 1N5817 because on hand (Vf=0.6V If=1A) but I might buy 1N60 (Vf=0.3, If=0.05A): acceptable?

spec · Feb 24, 2017

Hi Earckens,

sorry to say, diode not in right position.

Goes to show how difficult it is to describe circuit modifications- I will post a sketch showing the correct position.

It is very important that the diode you use has a very low reverse leakage, or the power supply voltage stabilization will be compromised.

When the LM723 protection diode is fitted it will have no effect on the power supply at all. The diode would only conduct if a reverse voltage were applied to the power supply input.

spec

earckens · Feb 25, 2017

spec said:
... I will post a sketch showing the correct position.

Thks!

spec said:
It is very important that the diode you use has a very low reverse leakage, or the power supply voltage stabilization will be compromised.

When the LM723 protection diode is fitted it will have no effect on the power supply at all. The diode would only conduct if a reverse voltage were applied to the power supply input.

spec

Then I would opt for the 1N60: less then 0.1uA leakage **broken link removed**

spec · Feb 25, 2017

Like below.

spec

earckens · Feb 25, 2017

spec said:
Like below.

spec

View attachment 104554

spec · Feb 25, 2017

earckens said:
Then I would opt for the 1N60: less then 0.1uA leakage **broken link removed**

0.1uA is quite high. That corresponds to an error of around 5mV at the LM723 difference amplifier inputs. I would go for a 1N4148, which has a maximum leakage current of 0.025uA.

spec

spec · Feb 25, 2017

earckens said:
View attachment 104556

That's fine.

spec

dr pepper · Feb 25, 2017

I used a little trick of generating a negative rail just for the '723, using just a diode and a cap, so the supply will adjust all the way down to zero volts.
Actually I adjusted it to 100mV to guarantee it wouldnt try and go negative, and seeing as the pass transistor has 0.6v drop the o/p does go all the way to zero v.

earckens · Feb 25, 2017

spec, I

spec said:
0.1uA is quite high. That corresponds to an error of around 5mV at the LM723 difference amplifier inputs. I would go for a 1N4148, which has a maximum leakage current of 0.025uA.

spec

spec I am puzzled by your comment: 1N4148 Irev=50uA, 1N60 Irev=0,5uA. 1N4148 is worse, or not?? https://www.vishay.com/docs/81857/1n4148.pdf

LM723 based PSU with min voltage of 0,00V

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