Rochey

Isolated Power Supplies
« on: June 27, 2019, 02:01:30 PM »
Gents,

a dumb Q, but this is always a good place to ask both Dumb Q's and smart ones!

I'm building a programmable load to test a DC-DC power supply I'm building. The control circuitry needs to be isolated, so that it can float with the negative rail of the power supply under test. I initially thought using a 9V battery would be safest, but I'm now wondering if I can use an off the shelf 9VDC power supply.

Maybe I've misunderstood, but most of the AC/DC switching supplies all have isolated outputs?
What throws me is there's often a Y-class capacitor that bridges the primary and secondary. doesn't that defeat the isolation?

Cheers

R
Expat Audio Home: http://www.expataudio.com


squarewave

Re: Isolated Power Supplies
« Reply #1 on: June 27, 2019, 05:14:10 PM »
Make a box with a BNC connector on one end and banana jacks on the other. BNC goes to voltage divider and gate of p-channel mosfet. Source and drain of mosfet connect directly to bananas. Put the load on the banana jacks and drive the BNC input with a function generator like one of the old protek ones with a DC control. Make your voltage divider so that the max voltage of the fn generator makes the mosfet 100% on. Now to test you just adjust the DC and if you want to see what the load regulation looks like play with the fn generator frequency / amplitude. No batteries. If the SMPS is isolated (if it's not I wouldn't use it) then you don't need isolation. But if you want further isolation you could add an opto isolator in front of the mosfet. Depending on how long and how much power your test is going to be for, use a die cast aluminum box and mount the mosfet through it to a heatsink. And maybe add a 10 ohm resistor to the source for safety / reduce temp effects / make DC adjust smoother.

Just thinkin' out loud of course.

Rochey

Re: Isolated Power Supplies
« Reply #2 on: June 27, 2019, 05:21:18 PM »
Make a box with a BNC connector on one end and banana jacks on the other. BNC goes to voltage divider and gate of p-channel mosfet. Source and drain of mosfet connect directly to bananas. Put the load on the banana jacks and drive the BNC input with a function generator like one of the old protek ones with a DC control. Make your voltage divider so that the max voltage of the fn generator makes the mosfet 100% on. Now to test you just adjust the DC and if you want to see what the load regulation looks like play with the fn generator frequency / amplitude. No batteries. If the SMPS is isolated (if it's not I wouldn't use it) then you don't need isolation. But if you want further isolation you could add an opto isolator in front of the mosfet. Depending on how long and how much power your test is going to be for, use a die cast aluminum box and mount the mosfet through it to a heatsink. And maybe add a 10 ohm resistor to the source for safety / reduce temp effects / make DC adjust smoother.

Just thinkin' out loud of course.

Interesting. You don't have any feedback then to keep the current steady. The circuit you describe, if I read it correctly, is completely open loop?
Expat Audio Home: http://www.expataudio.com

squarewave

Re: Isolated Power Supplies
« Reply #3 on: June 27, 2019, 05:41:13 PM »
Interesting. You don't have any feedback then to keep the current steady. The circuit you describe, if I read it correctly, is completely open loop?
True. Linearizing would be better. But even small source resistor will linearize quite a bit. To make it more linear you would require a few more parts but for basic load testing I'm not sure if it's really necessary.


 

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