How does the ConstantCurrent settings work in LM317?

[dz99]

Hello!

I want to build a pwr.suply by LM317. The circuit is linked above.

My questions:
1 - How does the ConstantCurrent seting net works in this circuit? OPamp makes virtually resistance by biasing D1?

2 - Can I change the old LM301 with TL081? How can I manage biasing? TL081 doesn't use pos/neg supply.

Thank you!
Zoltan.

http://dz99.fw.hu/cvcc.pdf

 

[IanP]

cc power supply

In the Constant-Current mode the output of the opamp takes control over the ADJ pin of the LM317 voltage regulator by pulling the voltage down through the LED diode and D1 ..
In the voltage control mode the volatage at the ADJ pin is around 1.25V (Vref) .. So if you take into account that the voltage drop across LED ≈2V and across D1 ≈0.75V the output of the opamp has to go below -1.5V, and that is the reason the designer used that -6V negative voltage ..

You can easily replace 301 opamp with any other opamp including the TL081 (C4 and D2 are for frequency compensation and will not be required with other opamps) ..
Just connect -Vcc to -6 and and +Vcc to max 30Vdc (TL081 is rated for +/-18V =36V) ..
If you replace D1 with a Schottky diode (Vf≈0.3V) and connect it directly to the opamp's output, increase the value of R4 to >3kΩ and insted of connecting it to the LM317 output wire it directly to your +Vcc supply, you will be able to supply the whole circuit without any need for negative (in this case -6V) voltages, so keep this option in mind ..

Regards,
IanP

 

[raybo]

ccpower forum

an LM317 is a 1.25 v regulator this design is defenetly an overkill of a good regulator it has a power input foldback and there is no need for -6 volts at all except to bias the op-amp. but you can biuld it if you want i would not use 082 amp that is too fast and if you cross the [0] DB slope of any power supply too fast it will oscillate at some higher frequency use an lm124 something like that.

 

[dz99]

lm317 + voltage control

an LM317 is a 1.25 v regulator this design is defenetly an overkill of a good regulator it has a power input foldback and there is no need for -6 volts at all except to bias the op-amp. but you can biuld it if you want i would not use 082 amp that is too fast and if you cross the [0] DB slope of any power supply too fast it will oscillate at some higher frequency use an lm124 something like that.

Hi!

Why is the LM317 a 1.25v regulator? from the dsheet: "The LM117 series of adjustable 3-terminal positive voltage regulators is capable of supplying in excess of 1.5A over a 1.2V to 37V output range."

OK, maybe 082 a wrong idea. Then what's your idea for the LM124?
IanP said: neg. voltage needed because of the working mode of the LM317.
Then what? IanP's way of "using a schotky" is workable?

Thanks!
Z.

 

[drabos]

Ｌï¼*３１７　cvcc

Na, asszem ezért 1.25 V-os Ugye, segítettem

Output voltage is calculated from the equation:
\[V_{o}=~V_{ref}(1+\frac{R_{2}}{R_{1}}) \]
where: Vref equals the difference between OUTPUT and
ADJUSTMENT voltages (≈1.25 V).

 

[dz99]

difference between cv and cv power supplies

Definitely

 

[drabos]

lm317 lm301 adjustable power supply

Hello,

I want to design a 0-5A, -40 - 40V DC power supply with CC/CV.
Which regulator do you advise? (ua723, LM317, 78, etc.)

Thanks

 

[raybo]

connect dual supply to lm124

you are going to need a bypass transistor to boost I to 5amps like 2n3055 beta is lousy but the price is right. an LM317 can only have an input of 37v dif. so 40v would only work once. for 40 v maybe PWM is a good choise. then again why 40v it is kind of hi for normal chips and normal electronics.