Author Topic: L200 circuit problem and safe limit ?  (Read 370 times)

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Offline MathWizard

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L200 circuit problem and safe limit ?
« on: April 22, 2021, 05:21:54 am »
I've needed more than 2/3 PSU channels for a while, so I made a L200 circuit from the datasheet. It's w/ a uA741 that should be providing a voltage to the L200, based on the L200 OP current.
https://www.ges.cz/sheets/l/l200.pdf

The 741, if I understand the circuit, should try it's best to make Vn=Vp , and so it's OP will move a few volts, from Vp as needed. And the L200 is looking at the difference between is own OP voltage, and the OP from the 741......and comparing that to a ref of Vsc=0.45V. The math seems to work, and I can get LTspice to do about the same (w/ more op-amps/BJT as a basic model of a L200).

But when it's wired up, the 741 is basically just acting as a buffer, turning the 100k pot, only moves it's OP by 2-3mV, and so the difference voltage to L200 is way off the Vsc=0.45V threshold.

And so the L200 does not cut back it's OP V or I

So I unhooked the 741's OP, and applied my own voltage to the L200's Ilim, and it works. But the 741 still acts like a buffer, just matching to within a few mV of the load voltage, regardless of the 100k pot setting.

I swapped in a new 741 and cleaned it and checked all the rest, same thing.

In the middle picture, I would say the op-amp output should be
Vo = Vp - (Rf/Rn)*Rsc*Ilload
[attachimg=1][attachimg=2][attachimg=3]
 

Online ledtester

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Re: L200 circuit problem and safe limit ?
« Reply #1 on: April 22, 2021, 07:40:54 am »
What supply voltage were you using?

Btw, a datasheet for the L200 with more explanatory text may be found by searching for the SGS application note "A DESIGNER’S GUIDE TO THE L200 VOLTAGE REGULATOR", e.g.:

http://ni-cd.net/nicddownload/ApplicationNote_L200.pdf

Be aware of potential formatting issues and typos, though. Anywhere you see "V5-2" should be interpreted as "V5-V2" and one place I saw the formula
$$
I_o = V_{\text{ref}}\left(1 + \frac{R_2}{R_1}\right)
$$
and it's clear it should the left side should be \$V_o = ...\$
 
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Offline MathWizard

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Re: L200 circuit problem and safe limit ?
« Reply #2 on: April 22, 2021, 09:24:44 am »
Up to 24V, but over 15V. I've burned out poteniometers before, so I better check that, but it did seem to read as expected when I desoldered the 1st 741.

And so far I never really set the voltage off from were it started near 10V.

I used all 1W resistors , for just in case the op-amp goes real low.

I figured copying the datasheet, I wouldn't need to breadboard it.
« Last Edit: April 22, 2021, 09:31:19 am by MathWizard »
 

Offline MathWizard

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Re: L200 circuit problem and safe limit ?
« Reply #3 on: April 23, 2021, 09:44:00 pm »
The 100k pot reads 82k but is fine otherwise. Again with the OP of the 741 disconnected from the Ilim. So I tried my resistor sub-box, and that changed nothing, the 741 OP barely moves. I unhooked the 1k pull-up resistor, so it's not in parallel with the pot anymore, and that changed nothing, for DC on paper it shouldn't matter anyways right?

So I rewired it for an LM358, and again, with the OP of it NOT connected to the Ilim of the L200, the OP actually went up to 10.1V from 10V.

I've rechecked all the wiring and cleaned around all the solder joints So what am  missing here?
« Last Edit: April 23, 2021, 09:45:32 pm by MathWizard »
 
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