### Author Topic: PSU with LM317  (Read 2438 times)

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#### Lyren

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##### PSU with LM317
« on: September 21, 2014, 10:08:09 am »
I have looked at this
https://www.eevblog.com/forum/projects/bench-power-supply-0v-~-15v-cccv/msg264407/#msg264407

But I am a bit confused about the current resistor, is it really only 0.01 ohm? After what I can see its not more than ~ 15 cm AWG24.

Would it be possible to add a bypass NPN to increase the current to 3-5 A?

#### TriodeTiger

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##### Re: PSU with LM317
« Reply #1 on: September 21, 2014, 10:22:24 am »
Your homework: What configuration is the current sense amplifier opamp, and what is the gain?

How many volts per amp of current (V = IR) will come out of the resistor, and the opamp, respectively?

What voltage is being fed in to the current limit opamp to be compared to the current sense amplifier output, and what does it in fact try to regulate the current to since you know the voltage ratios? (random example: non-inverting = 1V,  inverting input = 0.1V, and if at 1V per A, therefor it strives for a 100mA limit.)

Why may a low value resistor be useful when high currents are used? Burden voltage, is a key term.
Quote
Would it be possible to add a bypass NPN to increase the current to 3-5 A?

As long as the return path includes the sense resistor, I do not see why not!

Tiger.
« Last Edit: September 21, 2014, 10:28:14 am by TriodeTiger »
"Yes, I have deliberately traded off robustness for the sake of having knobs." - Dave Jones.

#### Lyren

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##### Re: PSU with LM317
« Reply #2 on: September 21, 2014, 11:42:08 am »
The gain in current sense amp must be ~100 and at one Ampere we have .01x1x100 ~1V but the voltage divider 1K and 10k will give from 0 to 4.5 V so I fail to see how it works.

I think the output voltage of LM317 is regulated by the voltage on adj. If the current and there by the voltage out of current sense amp gets to high it would lower the output voltage. But I fail to see how.
« Last Edit: September 22, 2014, 03:48:05 pm by Lyren »

#### mikerj

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##### Re: PSU with LM317
« Reply #3 on: September 21, 2014, 09:48:08 pm »
The gain in current sense amp must be ~100 and at one Ampere we have .01x1x100 ~1V but the voltage divider 1K and 10k will give from 0 to 4.5 V so I fail to see how it works.

It may work within a small range of the current setting potentiometer, but I agree that the divider is incorrect.  R7 should be 23k to get 0-1.5 Amps range.

#### David Hess

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• DavidH
##### Re: PSU with LM317
« Reply #4 on: September 24, 2014, 10:35:07 am »
Would it be possible to add a bypass NPN to increase the current to 3-5 A?

A PNP bypass is simpler but it could be done by driving an NPN with a PNP.

The current shunt is an usually low value because it directly degrades the load regulation in this design.

#### Lyren

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##### Re: PSU with LM317
« Reply #5 on: September 25, 2014, 12:13:18 pm »
After I build the circuit and it is my impression that it is Current limit, not cc, is that correct.
What I was after was a easy lab supply with cv/cc.

« Last Edit: September 25, 2014, 12:20:12 pm by Lyren »

Smf