Variable Voltage and Current Regulator configuration Help

variable current regulator

Hello,

I designed a 0-30V, 1A variable power supply.
Now I would like to add variable current adjustment instead of fixed 1A.
How could I do that? By simply adding another regultor in variable current configuration to the output of the present ckt.
Are there any other alternatives....

Thanks

l200 high current configuration

Use a current sense resistor in series with the output of your power supply, but before the output capacitor and voltage sense divider.
Have an opamp compare the voltage across this shunt with a variable reference. By adjusting this voltage you adjust the current limit.
At that point this opamp takes over the regulation, so it will maintain constant the current instead of the voltage.
Generally this is done by connecting the output of this opamp and the output of the error amplifier through two diodes to the control element.

If you post the schematic, I can suggest something.

variable current limiter

L200 Adjustable voltage and current regulator IC ....

l200 regulator parallel

WE don't know what power supply you designed but here is nice example how can you implement current regulation into linear power supply:

**broken link removed**

lm338 current limiter configuration

Here is my schematic, A bit simpler one,but lot more 2 add
digital controlling ckt,short ckt protection,7segment display.
But I want to start gradually
Thanks

lm338 + external pnp

Hi. Look LM317T from national semiconductor.
*www.national.com

Best Regards.

bd677 resistor

The main regulator is the LM338.
Data sheet: **broken link removed**
However, I don't think you connected the BD677 transistor in the right way.
What is its role? To increase the current?
From the data sheet you can see that to increase the current you may connect these regulators in parallel.
You can increase current by the use of an external PNP transistor with its emiter connected to +43V, base to Vi and a resistor between +43V and Vi, collector to Vo.
Even in this case BD677 doesn't seem to be the right choice (Darlington, 60V, 4A).

add on variable current adj

I think I used the wrong transistor in the schematic.
It should be a PNP, not an npn.

So How could I add output current adjustment to my existing circuit ??

Also I had a doubt from experimentation of the circuit. Why does the load regulation is very poor if the R1 is high (>1K). In the data sheet its been said typical value is 240ohm. I thought if the R1 is too low, regulator quiscent current goes high to maintain 1.25 voltas across output and adjust. so we have poor regulation. but it worked exactly opposite. can any one explain me why was that?

Thanks

l200 current regulator

Take a look at this circuit. To adjust the output from zero volts you need a -1.25V (or more). referred to TP4. The external transistor you added is not necessary if the current will always be 1A max.

To limit the current you use the opamp, with a 0~0.250V reference created by R6-R7 from the +5V power. The +5V source is also referenced to TP4. The current sense resistor is R3. You can simply connect four 1Ω resistors in parallel. So you should now see how the current limit works: you set R6 and the opamp now adjusts the output to mainain the voltage across R3 equal to the voltage across R6. Since the voltage across R3 is proportional to the output current, the latter is now kept constant. R6 is used to adjust the current limit from 0-1A.
Note that if you do not have the -1.25V the power supply will only work from 1.25V to 30V and the current limit will not work properly under short circuit conditions.

As for the 240Ω resistor, the LM338 develops a 1.25V reference between the OUT and the ADJ pins. Since the current that flows into the ADJ pin causes an additional voltage drop across R1, to minimize it you select a fairly high current (5mA) through the output divider R1-R2.
So, the output voltage is in good approximation: Vout=(R1+R2)/R1*1.25-1.25, that is from 0V (R2=0) to 31.25V (R2=5K).

Good luck!

nt

nt