Bench power supply problem

[chrish]

Thanks, that does help. Will try out on breadboard this evening.

Cheers,

Chris

[FenderBender11]

Originally Posted by AndrewT

What about replacing Vr1 & Vr1 with switched resistors for a range of fixed current limits.
A 2pole 6 position rotary switch would parallel a fixed value of say, 100r. With the switches open circuit, position 0, the current limit would be ~12mA,
then add progressively lower resistances in parallel to that 100r to reach your maximum limit of ~1A.

This allows the Vr4a and Vr4b to be omitted.

Now go back to post1, top figure and use that pair of voltage regulators.

Take care a range of 1v2 to 25V will require a 25Vac transformer and that could give 38Vdc as the input to the current regulator. The Vdrop of the voltage regulator could approach 35V and this combined with a 1A output would result in each voltage regulator dissipating 35W. That is very hot !!!!

Exactly how I'm going about building mine. Maybe if I was smart enough to use op-amps I would, but because it's dual tracking (split +/-), I've gotten thrown off a bit. So I decided to take the lazy way out and use a 6 pos 2 pole rotary and then use an LM317 and an LM337 each in current limit mode. I chose 1A, 500mA, 250mA, 100mA, 50mA, 25mA. Should be easy to dial in. Mine will only go to around 15V however= less heat.

This will go before my LT1033/LT1085 voltage regulator stage, which are pretty much pin-pin compatible with LM317s/LM337s but have better ripple rejection, transient response, and stability. They are quite expensive though. Like $8 between the two of them.

[chrish]

Thanks for the input guys,

I breadboarded the schematic with the LM317 and LEDs to set a bias as i don't have any LM385 at hand. It initially did not work, but I discovered a wiring error with the schematic. When I fixed it - all fine.

Here is the final working schematic:



Will integrate with the voltage reg to make sure all OK then wire it up in the enclosure that is ready to go.

Cheers,

Chris

[FenderBender11]

Originally Posted by chrish

Thanks for the input guys,

I breadboarded the schematic with the LM317 and LEDs to set a bias as i don't have any LM385 at hand. It initially did not work, but I discovered a wiring error with the schematic. When I fixed it - all fine.

Here is the final working schematic:



Will integrate with the voltage reg to make sure all OK then wire it up in the enclosure that is ready to go.

Cheers,

Chris

Hey that looks like a good idea! I wonder if there is a way to use a single pot to achieve that...But otherwise, do like!

[magnoman]

Chris

So if you set the current limit to something well below the 1.2V /1.2 Ohms max and short the 2 outputs together does the limiting current stay at this reduced level?

Of course you dont need to run these test at full input voltage if you have the capability to run off either another power supply or variac.

Hope this helps
-Antonio

[FenderBender11]

I'd also like to know. Also, do the LEDS turn on when it starts current limiting?

[magnoman]

Chris

One more thought.
With bipolar supplies (and especially with independent current limiters) you need to protect against one polarity supply pulling down the other below ground. This can happen when either is in current limit even on turn.
This can cause problems to the load as well as regulators and any polarized capacitors.
Simple schottky diodes across the outputs to ground will limit the reverse voltage. This applies not only to the output regulators but also the current limiting regulators since with the large input supplies you can easily exceed their rating.

p.s. Most of my suggestions are just painful memories of my many many mistakes.

Hope this helps
-Antonio

[chrish]

Hmmm, you would have to ask a difficult question Antonio I hooked up the meter I'm Amps mode between positive and negative, and it did not like it, a few mA of current and the LEDs did not light up at all. Interestingly, my rails measure +/- 39 volts prior to the regs with a floating ground. measuring after the regs, I get +39 volts on the positive rail to ground, but only -19 volts on the negative rail to ground (both measured with the current set pot at full, with reducing current set on the pot, voltages reduce correspondingly). Sound like what you describes re pulling voltages might be happening?

Don't have any schottky diodes in the box, would regular 1N4004 diodes work with their higher voltage drop?

Do I have to tie the common rail to ground? I was hoping to have a floating supply.

[magnoman]

Chris

Seems like the negative limiter was in current limit, possibly turned the pot the wrong way or a wiring error somewhere.
I still doubt the circuit will work satisfactorily under shorted conditions as there is just not enough voltage left to properly maintain the bias across the pots.
The simple solution is to simple tie the LED's from output back to the input of the opposite regulator (then the current will half in going from on to a full short). Not very elegant and I'm sure there are better ways to do it.
Or if you use 2 current sources to ground (since these are only rated to 40V) one for each polarity, and maybe add a power diode in series to the next regulator then the minimum voltage at which the output regulator will no longer conductor current maybe high enough (~3V) to maintain sufficient bias across the limiting pots.

Hope this helps
-Antonio

[chrish]

Thanks Antonio, I am away overseas with work at the moment, but will breadboard when I get home. Your input has been very helpful!

Cheers,

Chris