Digitally set current limit on PSU - Page 1
Forum section: electronics / projects

author: AlxDroidDev
author_rank: Frequent Contributor
timestamp: December 20, 2015, 07:35:38 pm
content: I am starting a homemade adjustable PSU, based on this project:http://www.electronics-lab.com/project/0-30-vdc-stabilized-power-supply-with-current-control-0-002-3-a/The problem, however, is that I want to set the current limit digitally. The project above uses a potentiometer to limit the current, so there is no way to know what the precise current limit was set to. My intention is to set the current limit on the Atmega328 through an UI and let it do whatever needs to be done to the opamp or BJT to limit the current to the set value.My own PSU won't be exactly as the above, since the transformer I have isn't so large (0-26.5V, 2A), but that doesn't chance much in the project above. That allows me to use an INA219 as a current sense, which I plan to. I know how to sense the current and shut down the power supply once the current goes beyong the set value, but I want to let the PSU limit the current (go into current source mode).I am not dead set on the project above. Feel free to suggest other projects.Thank you.

author: lincoln
author_rank: Regular Contributor
timestamp: December 20, 2015, 08:14:00 pm
content: With out commenting directly on the schematic ...basically, a liner regulator has some sort of pass element that, in the case of bjt's, needs to get some amount of current to the output. That current can come form an op amp and depending on the load the op amp servos the pass element in order to regulate the output voltage.OR (seen here)A current source can provide the current requited to drive the pass element to full on and the op amp regulating voltage can "suck away" enough of that current as to regulate the output. Now, there is no real limit on how may op amps can suck away current, an op amp can be set up to measure current and if it gets to high is can pull current off of the current source and limits the output.Clear as mud?Now or the digital part, the voltage and current settings takes the form of a voltage, and that voltage can be provided by a pot or dac just the same

author: Kleinstein
author_rank: Super Contributor
timestamp: December 20, 2015, 10:00:49 pm
content: The voltage and current at this supply are set via voltages set through pots. Just put a DAC instead.The linked circuit is tricky in that the OPs run at a quite high voltage. So even the 24 V AC suggested in the link is little on the high side is too much for many OPs. With a 26.5 V transformer the OPs may see even more than 44 V worst case (lline voltage a little high).The other problem is, that the current regulation is rather slow and there is no fast backup - in case of a short the current may go very high for a short time. This is specially a problem for relatively high voltage.I would suggest a different type of regulator circuit, one with an floating auxilary supply. The extra supply can also power the controling µC.

author: Cerebus
author_rank: Super Contributor
timestamp: December 21, 2015, 02:07:47 am
content: Quote from: Kleinstein on December 20, 2015, 10:00:49 pmThe op amps used are TL081s, abs. maximum rating for these is 36V so cue the release of magic smoke.I see several other cues that this is a marginal design - e.g. 3A output rating with a 3A rated transformer. If you actually pulled 3A output the peak secondary current could be several times this and it's going to get very warm, possibly even catching fire warm. There's other things too (like the voltage reference design - where's the control of the zener current?) but in the end I think the best advice would be to look for a different design.

author: AlxDroidDev
author_rank: Frequent Contributor
timestamp: December 21, 2015, 10:54:45 am
content: In my project I'll design around the specs of the secondary of the transformer I have: 2.2A. I might even limit it to 2.0A, shutting it down with a relay if it goes beyond it.The "control board" (with the Atmega328, INA219, etc) will be powered by a 9V battery + some buck-boost regulator that will give me 5V.Quote from: Cerebus on December 21, 2015, 02:07:47 amDo you have a suggestion?!

author: dom0
author_rank: Super Contributor
timestamp: December 21, 2015, 11:50:36 am
content: Quote from: AlxDroidDev on December 21, 2015, 10:54:45 amI'll abbreviate this because it has been written so often already by everyone everywhere everytime.Bridge rectifier and filter cap increase RMS current due to the tall, short current spike when recharging the filter caps. Exact  factor (that's what it effectively is) depends on Ri of transformer, rectifier, cap, ripple. Usually around 1.5 - 2, 1.7-1.8 typical. So 1.2 A * 1.8 ~ 2.16 A.I won't comment on the schematic. Rule of thumb for quick rating of "DIY" "lab" "supplies": LM317 or even 78xx as pass element or even regulator? Diff amp, possibly for current sensing? Regulators stacked, like the current regulator changing the set point of the voltage regulator? Transformers rated incorrectly? 1000 µF / A load current?Any of these true, chances are that the schematic is, as usual, poor.

author: Kleinstein
author_rank: Super Contributor
timestamp: December 21, 2015, 12:02:53 pm
content: I would suggest a design similar to many of the old HP supplies. That is having floating regulator and an extra transformer for the regulator and here the µC too.I don't have a good link, only one that shows the rough priciple:http://www.mikrocontroller.net/topic/354091A 2.2 amps AC current from the transformer will give only about 1.2 -1.4 A of DC current. Form the 26 V transformer you might get up to about 33 V at the output, possibly even 35 V at light loads most of the time.Does the transformer have a middel tap ?

author: dom0
author_rank: Super Contributor
timestamp: December 21, 2015, 12:06:09 pm
content: Really no need for a link with all those nice manuals available with full schematics and comprehensive Theory Of Operation sections.