# Attempting to keep current constant in a High Current DC supply circuit with variable voltage.

#### mrkek1232

Joined Feb 9, 2018
11
I've been at this for a while now and have gotten somewhat close to getting my desired results. I want the voltage to be variable between 0-12 volts ideally zero with a constant 1 amp output. I'll upload the schematic I'm using. Any help will be greatly appreciated.

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#### DickCappels

Joined Aug 21, 2008
7,248
I guess this depends on how much voltage can be dropped across R2, but in some cases you would not be able to get down to 1 amp because the LM317 current limits at 1.5 amps.

#### crutschow

Joined Mar 14, 2008
26,968
The LM317 has a minimum output voltage of 1.25V nominal so will require a negative voltage to be adjusted to 0V.

The LM3081 has a built in current-limit adjustment and can go to zero volts with just two pots, one for current and one for voltage.

#### mrkek1232

Joined Feb 9, 2018
11
I guess this depends on how much voltage can be dropped across R2, but in some cases you would not be able to get down to 1 amp because the LM317 current limits at 1.5 amps.
thanks for the reply, how'd I go about setting them independent of each other? 1.5 amps should be acceptable for the 12V Dc power supply I intend to make. If it isn't too complicated I'll further add sub circuits for fixed voltages is why I'm using this circuit specifically for its high current capability

#### mrkek1232

Joined Feb 9, 2018
11
The LM317 has a minimum output voltage of 1.25V nominal so will require a negative voltage to be adjusted to 0V.

The LM3081 has a built in current-limit adjustment and can go to zero volts with just two pots, one for current and one for voltage.
Thank you for the reply. If I'm to make the voltage variable and current at a constant output where would I place the pots for that?

Joined Feb 20, 2016
3,810
Are you actually after a variable voltage supply with a max current limited to 1A, or a 1A constant current supply?
You cannot have a constant current and a manually variable voltage supply at the same time. A constant current supply will automatically adjust the voltage to keep the current constant with varying loads.

#### mrkek1232

Joined Feb 9, 2018
11
Are you actually after a variable voltage supply with a max current limited to 1A, or a 1A constant current supply?
You cannot have a constant current and a manually variable voltage supply at the same time. A constant current supply will automatically adjust the voltage to keep the current constant with varying loads.
yea that's what I was after and probably explains why I haven't been able to get it. Right now though I have an op amp with 2N3020 transistors connected and i can vary the voltage approx 4 volts before the current drops out. If i have the pot below 5% or above 95% for slightly different set up. Not sure whats going on.

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#### DickCappels

Joined Aug 21, 2008
7,248
Taking into account what dendad said in post #6, you might want to consider something I once made -an adjustable LM317 voltage regulator with an adjustable current limit by putting an LM317 current source ahead of the adjustable regulator. No need for the external transistors in your circuit in post #1.

#### mrkek1232

Joined Feb 9, 2018
11
Taking into account what dendad said in post #6, you might want to consider something I once made -an adjustable LM317 voltage regulator with an adjustable current limit by putting an LM317 current source ahead of the adjustable regulator. No need for the external transistors in your circuit in post #1.

View attachment 145601
Just to clarify you mean two LMH317 regs? one ahead of the one I have already?
edit: nvm think i understand you mean to just have the current output fixed? not with variable voltage.?

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#### AnalogKid

Joined Aug 1, 2013
9,157
The circuit in post #1 is not a constant-current regulator. It is a constant voltage regulator with a current booster to increase the maximum output current above what is possible with the LM317 regulator by itself. The circuit in post #8 is a constant current regulator, but it has two drawbacks. First, it not adjustable down to zero output current. Second, 100% of the output current goes through the adjustment pot; you need a pot that can carry over 1 A at every point in its adjustment range.

Note that you cannot adjust the output voltage of a constant current regulator. The regulator attempts to source 1 A (or whatever the desired output current is) into the load, and automatically adjusts the output voltage according to Ohm's Law to make this happen. The voltage range the circuit can cover is called its compliance. In your case you want a constant current regulator adjustable up to 1 A with 12 V compliance.

ak

#### mrkek1232

Joined Feb 9, 2018
11
The circuit in post #1 is not a constant-current regulator. It is a constant voltage regulator with a current booster to increase the maximum output current above what is possible with the LM317 regulator by itself. The circuit in post #8 is a constant current regulator, but it has two drawbacks. First, it not adjustable down to zero output current. Second, 100% of the output current goes through the adjustment pot; you need a pot that can carry over 1 A at every point in its adjustment range.

Note that you cannot adjust the output voltage of a constant current regulator. The regulator attempts to source 1 A (or whatever the desired output current is) into the load, and automatically adjusts the output voltage according to Ohm's Law to make this happen. The voltage range the circuit can cover is called its compliance. In your case you want a constant current regulator adjustable up to 1 A with 12 V compliance.

ak
thanks , so basically with this circuit it'd impossible to have adjustable voltage and constant current output? If so would you have any circuit recommendations to achieve this and have enough current for two sub circuits for two fixed voltages?

#### AnalogKid

Joined Aug 1, 2013
9,157
thanks , so basically with this circuit it'd impossible to have adjustable voltage and constant current output?
With *any* circuit.

In a CV/CC lab or bench supply, both are adjustable. BUT, only one control loop is running things at any given moment. If the supply is adjusted to 12 V and 0.5 A, then the output remains regulated at 12 V for any load current below 0.5 A. Constant voltage, output current varies depending on the load according to Ohm's Law.

The crossover point is a load resistance of 24 ohms (R = E / I, 12 ? 0.5 = 24). When the load resistance decreases below 24 ohms, the load current current exceeds 0.5 A. The output voltage decreases to whatever voltage maintains 0.5 A through the load. For example, if the load resistance is 12 ohms, the output voltage will be 6 V. Constant current, output voltage varies depending on the load according to Ohm's Law.

ak

#### mrkek1232

Joined Feb 9, 2018
11
With *any* circuit.

In a CV/CC lab or bench supply, both are adjustable. BUT, only one control loop is running things at any given moment. If the supply is adjusted to 12 V and 0.5 A, then the output remains regulated at 12 V for any load current below 0.5 A. Constant voltage, output current varies depending on the load according to Ohm's Law.

The crossover point is a load resistance of 24 ohms (R = E / I, 12 ? 0.5 = 24). When the load resistance decreases below 24 ohms, the load current current exceeds 0.5 A. The output voltage decreases to whatever voltage maintains 0.5 A through the load. For example, if the load resistance is 12 ohms, the output voltage will be 6 V. Constant current, output voltage varies depending on the load according to Ohm's Law.

ak
thank you. you probably just saved me some hours of trying to sim circuits for that. Looks like I'm gonna need to re-propose a circuit for this class. I'm probably gonna try going for fixed current and fixed voltage since I already basically have that in the schematic I posted in #7. Now for sub circuits of other fixed voltages at one amp would that be possible for the circuit since it seems to be able to supply enough power?

#### mrkek1232

Joined Feb 9, 2018
11
or nvm I guess I wouldn't need a current booster since the LM317 can put out 1 amp anyway. so probably will just use switches to use other fixed voltages

#### AnalogKid

Joined Aug 1, 2013
9,157
so probably will just use switches to use other fixed voltages
That is the standard way.

ak

#### DickCappels

Joined Aug 21, 2008
7,248
Just to clarify you mean two LMH317 regs? one ahead of the one I have already?
edit: nvm think i understand you mean to just have the current output fixed? not with variable voltage.?
I put an adjustable current regulator between the AC rectifier/filter circuit and an adjustable voltage regulator. This makes an adjustable voltage power supply with adjustable current limit.

Same scheme as this:

Joined Feb 20, 2016
3,810
or nvm I guess I wouldn't need a current booster since the LM317 can put out 1 amp anyway. so probably will just use switches to use other fixed voltages
Be careful using switches to adjust the voltage. For instance, if you use a normal rotary switch to select a string of resistors, you will get max volts pulses as the switch is moved from one selection to the next.
Just use a correctly rated pot, like shown in DickCappels circuit above.

#### Ylli

Joined Nov 13, 2015
1,036
I'm probably gonna try going for fixed current and fixed voltage
I don't think you really understand Ohm's Law. Voltage, current and resistance (load in this case) are related by the formula I = E/R, or variants E = IR and R = E/I.

If you have a power source with a fixed output voltage of 12 volts, then it will supply 1 amp only when it is loaded with 12 ohms. If it is loaded with 24 ohms, it will supply 0.5 amps. If it is loaded with 6 ohms, it will supply 2 amps.

If you have a power source with a fixed output current of 1 amp, then it will produce and output voltage of 12 volts when driving a 12 ohm load. When driving a 6 ohm load, it will output 6 volts, and when driving a 24 ohm load will develop 24 volts.

You can't have both at the same time. However, you can have a power supply that crosses over between constant voltage and constant current at set points. See AnalogKid's post #12.

#### mrkek1232

Joined Feb 9, 2018
11
I put an adjustable current regulator between the AC rectifier/filter circuit and an adjustable voltage regulator. This makes an adjustable voltage power supply with adjustable current limit.

Same scheme as this:
View attachment 145621
Fell asleep earlier. I'm going to try making this right now.