Convert an unregulated  16Vdc, 35A linear supply to a regulated current source - Page 1
Forum section: electronics / projects

author: Jester
author_rank: Frequent Contributor
timestamp: January 15, 2021, 05:45:31 pm
content: I have an old unregulated linear power supply (X-former bridge and 50,000uF). The transformer is really beefy dual windings with #8 leads, it easily delivers in excess of 35A, however the ripple is substantial at that level.I would like to use this for a constant current source. So I'm thinking about the least expensive way to regulate the output from 0 to 35Adc.I visited Web-bench however that tool seems to only support constant voltage designs.There are some pretty beefy POL modules, however they are typically limited to about 5V, and I would prefer a bit more compliance voltage.Considering the low voltage and assuming a PWM approach, might be able to get away with perhaps just one or two really low RDS FET's to keep it simple.Thoughts, suggestions, sample circuits???

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 15, 2021, 08:00:16 pm
content: If you go linear then your dissipation is >350W so a lot of heatsink & fans.With your existing 50KuF cap your ripple is ~7V at full load, don't know what the trafo regulation is like or the utility range come to that, where I am going to is that if you want to use a buck switcher you need a reasonable voltage headroom, in any case at 35A your inductor is going to be fairly large and expensive so I don't think there is any such thing as a cheap solution of just a couple of fet's. What maximum output voltage are you hoping for anyway ?

author: Jester
author_rank: Frequent Contributor
timestamp: January 15, 2021, 08:40:48 pm
content: For compliance voltage, I will take what I can get.....If I assume 0.15Ohm with leads for 35A worst case, 5.25V should do it.This will be used on the bench only with hefty leads. My high current meter has a burden of 750uOhms. My big DC load can go down to about 10mOhms, so both of them need very little compliance voltage.I don't like the idea of the linear approach (to much power dissipation). I'm thinking keep it under the lowest dip and let the low RDS FET's do the work hard on or off.

author: geggi1
author_rank: Frequent Contributor
timestamp: January 16, 2021, 09:17:47 am
content: Remember that a in constant current source a lot of energy might be burned off in the transistors. In your case it can be 16V*35A=560W. This is when you run a full load (shorted terminals). To burn of this energy you must take in to account the energy in each transistor and the size of heatsink/cooling.https://electronics.stackexchange.com/questions/287087/op-amp-and-mosfet-current-source-not-behaving-as-expectedCheck out the diagram in the link. You probably have to add more transistors to get it working.

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 16, 2021, 10:44:32 am
content: Quote from: Jester on January 15, 2021, 08:40:48 pmWhat is the load ? Is it able to integrate the on/off cycles ?If not you need a hefty output filter, usually L/C and that is what I was pointing out is going to cost you.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 16, 2021, 03:35:51 pm
content: It depends on what you load needs. You might use current sensing and PWM to switch a fat MOSFET(s) or IGBT. Of course you then need a filter which will possibly mess up your cc settling times.I'm going to try it out just for fun.more later.What is your load?

author: Terry Bites
author_rank: Super Contributor
timestamp: January 16, 2021, 05:28:08 pm
content: Some one has always done it before! Allegedly.....https://www.google.co.uk/url?sa=i&url=https%3A%2F%2Fwww.pinterest.com%2Fpin%2F733594226772859732%2F&psig=AOvVaw3DUukItZJdH6UkM1rooubx&ust=1610903693851000&source=images&cd=vfe&ved=0CAIQjRxqFwoTCJiN-4T6oO4CFQAAAAAdAAAAABAeMod this 150A bruiser for your useBe careful.

author: Jester
author_rank: Frequent Contributor
timestamp: January 24, 2021, 03:22:18 am
content: I thought I would try a POL module. Tiny for 30A, this one was inexpensive and seems quite happy regulating up to 30A:https://www.mouser.com/ProductDetail/Murata-Power-Solutions/OKR-T-30-W12-C/?qs=%2Fha2pyFaduhOPBOUTmZ0HhW8R7TP8Z3ukdCPou5%2F%252Blb5ymvpADcn2g%3D%3DThat was a bit easier than I expected, POL module is the little gizmo, just to the right of the shunt. The linear supply can pump out quite a bit more current, the bottom of the ripple was still well above the minimum voltage required for the POL module. I might try a larger module in the future, for now I just needed a stable 30A supply, and this did the trick.