Controlling DC Current using PWM, but no pulsed output

Thread Starter

hihihi8

Joined Oct 4, 2017
10
Hi. I'm pretty new to this stuff and theres a good chance something like this already exists but I just don't know the name of it.

I have a big power supply capable of supplying a LOT of 12V DC current, and I'm looking for a way to control the output current using a PWM signal, but the output itself cannot be pulsed so relays or buck-converters wont work.

The output should stay 12V, but the current supplied would have to be adjustable on the fly via a low-power PWM signal.

does something like that already exist? or is there an easy way to make one? Thanks in advance!
 

Thread Starter

hihihi8

Joined Oct 4, 2017
10
If i search "Variable DC power supply" on amazon, i get results that have the function i want (the dial/knob that says "current"), but those all have the switching power supply built in. I want just the control module, if that makes sense.
 

Papabravo

Joined Feb 24, 2006
16,773
Hi. I'm pretty new to this stuff and theres a good chance something like this already exists but I just don't know the name of it.

I have a big power supply capable of supplying a LOT of 12V DC current, and I'm looking for a way to control the output current using a PWM signal, but the output itself cannot be pulsed so relays or buck-converters wont work.

The output should stay 12V, but the current supplied would have to be adjustable on the fly via a low-power PWM signal.

does something like that already exist? or is there an easy way to make one? Thanks in advance!
What you want is not possible. Ohms law tells you this. If you have a load on a DC power supply with a fixed resistance, then you can control the voltage, OR the current, but not both. So fugeddaboudit.
 

Thread Starter

hihihi8

Joined Oct 4, 2017
10
What you want is not possible. Ohms law tells you this. If you have a load on a DC power supply with a fixed resistance, then you can control the voltage, OR the current, but not both. So fugeddaboudit.
Wait so how do these work then, with the current knob?81QwL+N7qwL._SL1500_.jpg
 

Thread Starter

hihihi8

Joined Oct 4, 2017
10
Also, im not regulating resistance. A variable resistor would work, but would it blow up from the heat of pushing higher loads (say 5 amps)?
 

Papabravo

Joined Feb 24, 2006
16,773
I never said you, or anybody else for that matter, was regulating resistance. It is what it is for every conceivable load.

Q: How do you think those power supplies work?
A: In CV (constant voltage) mode you set the voltage and the load draws whatever current is compatible with the resistance it offers. If the voltage is set at 5 volts you cannot force a 1KΩ load to accept 100 milliamps. It won't do that. It will only draw 10 milliamps. No more and no less. In CC (Constant Current) mode you set the current and the voltage is whatever value allows that particular current to flow.

As I said originally: you cannot independently set BOTH the voltage, and the current. Try it and let us know what happens.
 

Thread Starter

hihihi8

Joined Oct 4, 2017
10
I never said you, or anybody else for that matter, was regulating resistance. It is what it is for every conceivable load.

Q: How do you think those power supplies work?
A: In CV (constant voltage) mode you set the voltage and the load draws whatever current is compatible with the resistance it offers. If the voltage is set at 5 volts you cannot force a 1KΩ load to accept 100 milliamps. It won't do that. It will only draw 10 milliamps. No more and no less. In CC (Constant Current) mode you set the current and the voltage is whatever value allows that particular current to flow.

As I said originally: you cannot independently set BOTH the voltage, and the current. Try it and let us know what happens.
I see, makes sense. Looks like it will have to be either PWM or voltage-controlled then. Thanks.
 
There are PWM to voltage converters. Actually duty cycle to voltage.
There are power supplies that can be controlled via say a 0-5V signal (one for voltage, 1 for current)
Many times that signal needs to be isolated.
 

crutschow

Joined Mar 14, 2008
27,696
If you use PWM to control the current, you will need an added series inductor and capacitor to ground to filter out the pulses.
 
As its name suggests, pulse width modulation speed control works by driving the motor with a series of “ON-OFF” pulses and varying the duty cycle, the fraction of time that the output voltage is “ON” compared to when it is “OFF”, of the pulses while keeping the frequency constant.
 

crutschow

Joined Mar 14, 2008
27,696
What do you think you need a current limit?
Most devices just take the current they need without requiring a current limit.
 

Alec_t

Joined Sep 17, 2013
12,197
Note also there is a distinction between a constant current supply and a supply which merely limits the maximum current available.
 

Papabravo

Joined Feb 24, 2006
16,773
Note also there is a distinction between a constant current supply and a supply which merely limits the maximum current available.
I agree, but it is still the case that you can not force a device to take more current than it's resistance will allow.
A current limited supply will adjust the voltage when the current limit is reached.
A 1 amp supply cannot force a 1K resistor to draw 1 ampere, unless it can supply that 1 Ampere at 1000 VDC.
 

MrAl

Joined Jun 17, 2014
8,473
Hi. I'm pretty new to this stuff and theres a good chance something like this already exists but I just don't know the name of it.

I have a big power supply capable of supplying a LOT of 12V DC current, and I'm looking for a way to control the output current using a PWM signal, but the output itself cannot be pulsed so relays or buck-converters wont work.

The output should stay 12V, but the current supplied would have to be adjustable on the fly via a low-power PWM signal.

does something like that already exist? or is there an easy way to make one? Thanks in advance!
Buck circuits normally do not have an output that 'pulses'. The control is PWM, but that normally does not appear in the output because part of the design process is to make sure the output ripple voltage is not too high. So using a buck circuit would control the output voltage without having an output that pulses. You can actually get a very smooth output with proper filtering.
The only real problem with a buck when it comes to the pulsing is the switching noise that radiates out and can interfere with circuits that are sensitive to radio frequency noise. The switching transients generate various radio frequency signals that affect those sensitive circuits, and this means switching supplies are harder to use for some applications. But for most it works fine.
 

DickCappels

Joined Aug 21, 2008
7,689
This is one implamentation of the filter mentioned by crutschow in post #10. The LEDs are driven by a constant current sensed in R3. A capacitor to remove ripple current from the inductor can be placed across the LEDs.

1615199120507.png
 
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