Lab supply on a shoestring

[rikkitikkitavi]

I see no switch/relay or sensing circuit in the posted schematic, so I assume this is done on AC side of the rectifier, i e a transformer with two windings. That would of course prevent any voltage dip, given that the rectifier caps are large enough.

[Elvee]

Originally Posted by powerflux

i'm (when i feel like i've earned a break) working on a PCB.
Since i'm doing this for the first time following a different strategy than i'm used to (in/out on same side, path folded in U shape and defined ground return paths instead of super-short paths and a ground plane) it will take a while. also maybe a little because of exams@school.

where do you measure I? R6?

Yes.
If you make a PCB, it is probably a good idea to include all the options, to maximize its flexibility and fully benefit of its versatility.
I will redraw a suitable schematic.

Originally Posted by powerbob

This is a very nice project, but I am having trouble following some of this. Are these mods you made to your original schematic? If so can you modify the original schematic and repost it.

The topic has in fact two parts: the first deals with the presentation of a simple supply concept, illustrated with an example having numerical values, and the second shows a practical application, the replacement of the control board in an existing supply.
I understand that this can lead to some confusion, especially since the "general" circuit is so much like the application example.
That's no coincidence of course: when I designed the circuit, I had in mind that particular application.
I first had to build my prototype as a stand alone supply, to make sure it was proper and healthy before I could blend it with the existing circuits, otherwise, in case of problems I wouldn't have any reference point and would have had diificulty tracing the origin of problems.
As I said above, I will post a more general schematic, including all the options and mods.
At present, all the correctives except for Q6 apply to the particular issues arising from grafting the circuit onto an existing supply.

I am also wondering how that “automatic 15/30V transformer switching board" works. As you increase the voltage from 0 to 36vdc, it must sense the output voltage and then at the right time switch from 15v to 30v. But as it switches do the relays momentarily give zero voltage, as in 15v zero for a fraction of a second and then 30v.

The original supply was equipped with a very common accessory, a voltage-sensing relay to switch the transformer's winding when the output voltage is below 15V, to reduce the dissipation in the ballast. I kept this part almost unchanged.
The switchover time is short enough to allow the filter cap to ride through, as rikkitikkitavi rightly remarks:

Originally Posted by rikkitikkitavi

I see no switch/relay or sensing circuit in the posted schematic, so I assume this is done on AC side of the rectifier, i e a transformer with two windings. That would of course prevent any voltage dip, given that the rectifier caps are large enough.

[Elvee]

Here is the schematic, updated with all the options I could think of.

You can pick and choose what you like best.

[fra]

Hey
Can this schematic work for higher voltage?

[Elvee]

Originally Posted by fra

Hey
Can this schematic work for higher voltage?

I am pretty sure it can: I have successfully simulated some examples, see above, BUT simulation is no substitute for reality, especially in the high voltage field.

I am 99.9% confident I could have a high voltage version working, but realistically the working sim schematic would probably need some tweaking and adaptations to be really safe and effective in reality.

I will probably test a HV version in the coming days or weeks (or months).

In the mean time, if you have sufficient experience with HV, you can try it (carefully) for yourself.

[Elvee]

I made a quick and dirty breadboard test of this HV version:



It worked immediately (except I had connected the 330µF/400V filter cap in reverse and had a narrow escape).

There are some differences with the schematic: transistor types were different, with a 2SC2922 as a ballast for example, but that is secondary.

In order to reduce the quiescent dissipation, I also doubled the values of all DC-affected resistors: for example, R9 & 26 became 6.8K, the 680 ohm became 1.2K, the voltage and current pots became 2.2K and 1K, etc.

That is acceptable since the output current is half of the low-voltage version.

In order to keep some advance warning with the current limit, I changed D4 for a 5V6, and the optimum output resistance compensation R19 was 22K, but this did not affect the functionnality of the supply, and at all times it was stable both in current and voltage modes, despite the breadboarding.
Thus, it looks pretty safe.

For this kind of voltage, it essential to keep in mind the SOA limits of the pass element(s).
Neglecting that will certaily result in molten silicon.

I also dropped in a 2SK794 NMOS as a replacement for Q1, and it worked transparently, with just R1 raised to 680ohm. All the parameters remained unchanged, including the compensation resistor.

I even played the sorcerer's apprentice by removing completely the compensations C3 and C5, and it remained stable, both in voltage and current, but I don't recommend it, as they look necessary in the sim.

[powerbob]

ELvee, I looked over your post 13 were you posted your latest schematic. I noticed that you left several values off of the schematic. Was this an over sight or are we supposed to calculate these for our specific use?

Components with no values.
R32, R28, R26, R29, C8, R17, R20, D1, C7, R21, R30, R31, C10.
Should Zero adjust R and I Adj, be a resistor or a POT?

Thanks

[Elvee]

Originally Posted by powerbob

ELvee, I looked over your post 13 were you posted your latest schematic. I noticed that you left several values off of the schematic. Was this an over sight or are we supposed to calculate these for our specific use?

Components with no values.
R32, R28, R26, R29, C8, R17, R20, D1, C7, R21, R30, R31, C10.
Should Zero adjust R and I Adj, be a resistor or a POT?

Thanks

These are optional components you have to use (and dimension) in specific cases.
For example, with a higher voltage version, it is convenient to split the dropping resistors in two or three to spread dissipation, and it is possible to add supplementary filtering in the middle, etc.

You pick and choose want you want, and calculate it accordingly.
The latest 150V 1A example uses some of these options.

Should Zero adjust R and I Adj, be a resistor or a POT?

You won't find a pot for the Iadj resistor, it will be a few ohm, and anyway, additional resistors are much more stable than pots. That's slightly more inconvenient, but once it's done, you gain peace of mind, it doesn't vary anymore.

[fra]

hey.
If anyone need shemaich and pcb in eagle.

[Elvee]

Originally Posted by fra

hey.
If anyone need shemaich and pcb in eagle.

Very nice, thanks, I am sure it will help the community.