Short Circuit Protected Constant Current Supply

Why would anybody sane short-circuit protect a constant current source?

A short circuit isn't going to damage a properly designed constant current source. It's just going to deliver it's constant current into zero volts which it should be able to do indefinitely.

If you size C4 right, it can be the current limiter. Caps don't get hot!

You need the current to be adjustable. The 8N60 mosfets are insulated and mounted on a heatsink. For any load within the operating range, the current is adjusted for maximum voltage across the load. This minimizes the heat generated in the mosfets and minimizes the cooling needed.

The upper mosfet is only for brief shorts that may occur. It is not intended for long-term misconnections. Without it, of course, a short would destroy the voltge doubler diodes and source resistor.

I had to adjust Vto in the 8N60 model to 4V to reflect the datasheet value.

Thanks for taking a look at the circuit.

That's the one I meant. C4 could be picked to limit the current available from the voltage doubler.

Brilliant! Thanks

Well, it kind of works. Dropping C4 from 22uF to 1uF did indeed drop the short-circuit current to approximately 30 mA, the same as before. However, it greatly increases the supply impedance.

This causes a serious droop in the +HV supply. As the constant current demand increases, the +HV drops significantly. This makes it impossible to reach the desired 20 mA maximum with a large load impedance.

The other problem is the ripple on the +HV supply increases dramatically. The TL431 would probably clean it up, but it is a bit disappointing.

The original circuit using the Vbe of a 2N3904 is quite a bit more accurate and holds the +HV constant until the current limit is reached. Back to the original version.

Thanks for the help! A great idea and probably useful in some applications.

Oops. There used to be a TL431 in the current source but it was replaced with a LM358. I will have to correct the title.

Designing electronics is HARD:)

One common approach is to do resistive or capacitive current limiting inside a feedback loop. The two fight each other, and the point where the feedback loop loses is the current limit.

Cheers

Phil Hobbs

Sure. It depends on what the current limit is for. If it's to prevent explosions if the hv supply gets grounded, there's probably a value that works.

It would still increase the impedance of the +HV supply and degrade the voltage regulation under load. I need the current source to supply as high a voltage as is reasonable in order to drive current into a high resistance load at max current. As it is, the voltage drops from 330V to

This limits the maximum load resistance to 286/20e-3=14,300 ohms at max current. This is acceptable, since the area of the electrodes will be fairly large in order to achieve a low current density. The cell will be the largest I have ever built, so there is some uncertainty in the results.

During the electrolysis process, the conductance of the cell will increase by a factor of 10 or 20 as silver ions enter the solution. The current is limited to restrict the combination of silver and hydroxide ions in the Nernst Diffusion layer to produce silver hydroxide, AgOh. This is undesirable, since it limits the concentration of silver ions to low values, such as 4-5 ppm. It takes care to reach 18 ppm, which is about the best you can do at home.

The current supply will be connected to an external electrolysis cell, so the leads will be exposed. The current limit is to protect the supply in the event the positive lead accidentally strikes ground during the connection.

Normally the supply will be turned off and the capacitors discharged while it is being connected, but there could be some other accident that shorts the supply. For example, a DVM could be connected to monitor the voltage across the load to check for saturation. The leads could be connected together for an instant and short out the supply.

Also, the 1N4007 rectifier diodes have a peak surge current of 30A, but I'd rather not deal with that kind of current even for a half-cycle. The power supply switch is a miniature DPDT and I'm certain the contacts would not handle it. So the old uA723 style Vbe current limiter seems the best choice. It is only there for transients, so I don't need to go into voltage foldback and all its complications.

Thanks for your help.

Thanks, but sounds complicated. All I need is protection against accidental transient short circuits. The old ua723 Vbe limiter seems to be the optimum solution.

I don't know of any other current limiter that can handle 30mA at 330V, at a price I can afford:)

On a sunny day (Fri, 7 Jan 2022 21:05:50 -0000 (UTC)) it happened "Arnie Dwyer (ex Jan Frank)" snipped-for-privacy@not.com wrote in <XnsAE18A2F78FF27idtokenpost@144.76.35.252>:

? No idea about this project but I used BU??? HV 400V power MOSFETs in the eighties to do just that, on a big heatsink.