Instructables

Current limiting?

Hi 

For my digital power supply I'm building, I require a way to control the output current. How would I control the current coming from the PSU in the simplest way? The current limiter would need to be microcontrolled (using PWM?) . 



Thanks

vreinkymov1 year ago
You've got a voltage control loop, so you have a bulk capacitor with some type of feedback signal. To do a current limit, you'll need a current feedback. You do this by inserting a low value resistor (1 ohm or less, I like to use 0.1 ohm). The supply current is found via the resistor's drop: I = V/R

If you insert the resistor on the high side (before bulk capacitor) it will not interfere with your voltage feedback, but you would need to use a low offset voltage op amp in differential mode with precise resistors.

If you're willing to live with some offset error, put the resistor between ground and the bulk capacitor. Take the high side of this sense resistor and connect it to the inverting end of an open-collector/drain comparator (say LM393). Connect an RC filter from your PWM signal to the LM393 non-inverting terminal and you can use the comparator's output to sink your switching mosfet until the overcurrent condition disappears.
kellyclark349 months ago
You should ask an engineer. They might know.
rickharris1 year ago
DO you use google much? There is wealth of circuits you can search for. Including current limiting transistor circuits.

https://www.google.co.uk/search?hl=en&site=imghp&tbm=isch&source=hp&biw=1920&bih=919&q=curent+limiting+cct&oq=curent+limiting+cct&gs_l=img.12...1955.13421.0.15462.19.13.0.6.0.0.88.1002.13.13.0....0...1ac.1.19.img.vM4_yGiDXqs
You keep asking the same question, we keep suggesting answers. The simplest method is to use the LT3080.
Yes, I realized my questions are a bit repetitive :) Except for this one I wasn't really looking for a chip like an LT3080, I was looking for a circuit using transistors ect. I wanted the limiter to be standalone from the voltage regulation which I'm not sure if it is possible.

Thanks
Its possible to separate the functions. There are LM317 circuits out there that do just that. The current limit can be handled with an LM334.