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Variable power supply with range 0 VDC to 24VDC

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kushal nandanwar

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I am looking for Variable power supply with output range 0 VDC to 24VDC with constant 250mA current. Input Voltage 25VDC
 

You want constant current, then the load resistance determines the output voltage. If you want constant voltage that you set, then the load resistance determines the current.
Is that what you want?
 

If the slider of your 10k pot has NO LOAD CURRENT and the 25V supply does not change then the slider will produce 0V to 20.66V. If there is any load current then the voltage will drop because your simple circuit has no voltage regulation.

A voltage regulator circuit usually has an opamp, a voltage reference and some transistors. Look at Voltage Regulator Circuit in Google.
 
A common solution to such a requirement is the LM317 variable regulator.
It has built in over-current and over-temperature limiting so it's very rugged.
It's minimum output voltage is 1.2V but that's usually sufficient for most applications.
 

crutschow said:
A common solution to such a requirement is the LM317 variable regulator.
It has built in over-current and over-temperature limiting so it's very rugged.
It's minimum output voltage is 1.2V but that's usually sufficient for most applications.
Click to expand...

That 1.2V is my problem I do want it upto 0.
 

A simple way to achieve voltages below 1.25v with a LM317 is to provide a small negative bias.
This bias requires to supply a few milliamps (for the adjustment current), so it is not very complex circuit.
 
Audioguru said:
If the slider of your 10k pot has NO LOAD CURRENT and the 25V supply does not change then the slider will produce 0V to 20.66V. If there is any load current then the voltage will drop because your simple circuit has no voltage regulation.

A voltage regulator circuit usually has an opamp, a voltage reference and some transistors. Look at Voltage Regulator Circuit in Google.
Click to expand...


I search on google for regulators, LM317 has internet 1.25 reference voltage, is that a reason why minimum voltage of LM317 is 1.5.

Can I design a regulator with reference zero to get output minimum at zero.

- - - Updated - - -

schmitt trigger said:
A simple way to achieve voltages below 1.25v with a LM317 is to provide a small negative bias.
This bias requires to supply a few milliamps (for the adjustment current), so it is not very complex circuit.
Click to expand...

I have negative supply in my circuit range -5 to -12.
Can you please share circuit diagram of LM317 to get zero output.
 

Check this for the basic idea

http://www.google.com/search?q=lm317&client=tablet-android-asus&hl=en&source=android-launcher-search&v=141278776&prmd=sivn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwj33s-Fm6PJAhWB4SYKHaYfDvsQ_AUICCgC
 

If not the 317 datasheet, the ST 78xx series of regulators datasheet has a schematic that uses an op amp to create a (?) virtual ground, and the output can be from ~0 to (x)V. I think the circuit in the datasheet was 0 - 10V.

Don't want to be a party pooper, and I'm not sure, but it is possible that a constant 250mA over such a wide voltage range is difficult to achieve.

Maybe there is some way that stabilises the current over a wide input range (read the small print and formulas!), this type of circuit will have examples either in regulator datasheets or in current reference IC and voltage reference IC datasheets, maybe have a browse of "application hints" in the datasheets for LM334 or TL431C.
 
Last edited:

In the first post he said, "output range 0 VDC to 24VDC with constant 250mA current. Input Voltage 25VDC". I do not know any voltage regulator or current regulator that has a dropout that is less than 1V.
He did not answer me when I explained "constant current" and I asked if he wants it with the voltage determined by the load resistance, not set by him.
 

Yes, I missed where the op stated he had a 25V supply. **broken link removed****broken link removed**

Here's a low dropout, 1.1A, adjustable regulator that can be set to 0V output with 1 resistor.

Note that for 250mA output current the regulator will be dissipating up to 6.25W worst-case so it will need to be mounted on a heat sink.
 
Last edited:
The input voltage has nothing to do with the OUTPUT current. Do you want the output current always to be REGULATED at 250mA or do you want the output current to be LIMITED to no more than 250mA??
 

Audioguru said:
The input voltage has nothing to do with the OUTPUT current. Do you want the output current always to be REGULATED at 250mA or do you want the output current to be LIMITED to no more than 250mA??
Click to expand...

Minimum output current should be 250mA and voltage could be varies from 0 to 24Vdc.
 

Hi,

I´d go for a SEPIC controller.
You may use a pot in the feedback path to adjust output voltage and a resistor to pull up the feedback voltage to the desired level. Better way is to use an OPAMP for (feedback) voltage regualtion.

For a simple fixed current limitation you could use a shunt with about 2.2 Ohm and a BJT with B and E across the shunt. With a load of abut 250mA the bjt switches ON.

(if you use the current limit with the shunt, then the voltage feedback should be connected after the shunt.9

Klaus
 

KlausST said:
Hi,

I´d go for a SEPIC controller.
You may use a pot in the feedback path to adjust output voltage and a resistor to pull up the feedback voltage to the desired level. Better way is to use an OPAMP for (feedback) voltage regualtion.

For a simple fixed current limitation you could use a shunt with about 2.2 Ohm and a BJT with B and E across the shunt. With a load of abut 250mA the bjt switches ON.

(if you use the current limit with the shunt, then the voltage feedback should be connected after the shunt.9

Klaus
Click to expand...

Problem is power dissipation at 24Vdc/250mA, should decapitate
6W. too much high

Are you talking about switch mode power supply?
 

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