Variable Wattage Box

_q12x_ · Sep 4, 2024

Hi everyone,
I want to make a variable Wattage box. Mainly for testing purposes.
I want some small scale, medium and if possible, some big scale variable wattage ranges.
Lets start with something easy, the small scale. From the back of my mind, I would use a small npn tr like BC548 or BD681 to variate between tens of miliwatt. (Without a Heatsink). For variation of hundreds of mW up to 1W, I remember from my tests, that my 2N3773 was the best choice in this range, again (Without a Heatsink).

So this is simple, right? The hard part is to switch between a couple of watts, not many, maybe 5W, maybe 10W, Im undecised.
One important design future is that I want a POT to scale the values, but also in // with it some sort of digital button inc/dec, and the simplest possible I can think of is with a counter IC like my favorite 4017. I was thinking originally on switches but a POT is more elegant.
I want from you a cct design, an idea, of how you will make it. I already give you half of the solution how I will make it. Im curious how you will do it. Its very important if you already made it in your past, and now its time to shine with its circuit here for me. I love practical solutions, not only the theoretical ones, although Im accommodating any weird ideas that I never tried. See if you can design something quick in your simulator or on paper, or search on the www. Or something that is already consecrated, and you know about it but I don't, will actually be the best, if possible.
Thank you !

James · Sep 4, 2024

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_q12x_ · Sep 4, 2024

No.
I want to make it myself with the components I have.
And to make it as customizable as I can.
So please share the circuit skematic next. Thanks.

mainline · Sep 4, 2024

Do you mean a variable current circuit ?

What sort of voltage ?

Ac or Dc ?

Has the voltage got to be regulated ?

_q12x_ · Sep 4, 2024

mainline said:
Do you mean a variable current circuit ?

What sort of voltage ?

Ac or Dc ?

Has the voltage got to be regulated ?

I mean variable Wattage, not current.
Yes, in a sense it will be current variation but the result should be wattage variation on the output wire.
No, its DC current, already regulated.

_q12x_ · Sep 4, 2024

This is how I think it should look like. Please correct any possible problems with it, because I "slap" this cct, from the back of my memory, without checking with anything. I didn't add the 1's Watts because that's where I need your expertise. I can solve it in 3 ways actually, but I'm not sure completely.

[ElectriciansForums.net] Variable Wattage Box

mainline · Sep 4, 2024

_q12x_ said:
I mean variable Wattage, not current.
Yes, in a sense it will be current variation but the result should be wattage variation on the output wire.
No, its DC current, already regulated.

Wattage = voltage x current.

What is the voltage max ? And is this dc voltage variable.

To make a variable constant current circuit is quite simple, using a regulator either wired for CV or CC.

You need to give some more info as in what do you want it to do ?

_q12x_ · Sep 4, 2024

A regulator is limited to 1.25V as it's very minimum V that can supply, and it will not go to the +VCC rail, but a bit lower with 0.5V or so, if I remember right. A regulator will also have to resist to the power surge through it, so this is a very limited option. Its better with tr of my choosing. But is an idea after all.
Please look on the img I provided and tell me if you think is good or bad.

mainline · Sep 4, 2024

All you need is a regulator ic in constant current mode.

The current will stay constant irrespective of the load.

You still haven't told me what you want to do and what sort of load you want to test.

_q12x_ · Sep 4, 2024

My UK friend, I feel you're avoiding my question...or maybe is my impression...

mainline said:
The current will stay constant irrespective of the load.

- Ah, you want to make a constant current regulator then. I see your point. Hmmm...This is not about the stability of the supply. It is about dealing with numerous/variable output values, switched or dialed, that can deliver x amount of power. They will be stable enough for my tests. Actually it will be a single output line.

mainline said:
You still haven't told me what you want to do and what sort of load you want to test.

- The load is fixed, not variable - theoretically. Initially I was thinking to insert many switches for each 10mW then 100mW then 1W ranges. So in total should be 30 switches. But if Im using a POT for each category, I will escape with only 3 POTs and also finer resolution, even 0.5 steps, in between the planned values.
- Ideally this circuit should power a load of maximum 10W. Usually will be smaller power elements connected for test.
Lets say for now that the max is 1W for simplicity. Check my circuit I posted and tell me if it will be good enough for a max 1W, and what I should add more to it?
thank you

mainline · Sep 5, 2024

_q12x_ said:
My UK friend, I feel you're avoiding my question...or maybe is my impression...

The circuit you are proposing is basically a variable resistor amplified by the common emitter transistors.

_q12x_ said:
- Ah, you want to make a constant current regulator then. I see your point. Hmmm...This is not about the stability of the supply. It is about dealing with numerous/variable output values, switched or dialed, that can deliver x amount of power. They will be stable enough for my tests. Actually it will be a single output line.

- The load is fixed, not variable - theoretically. Initially I was thinking to insert many switches for each 10mW then 100mW then 1W ranges. So in total should be 30 switches. But if Im using a POT for each category, I will escape with only 3 POTs and also finer resolution, even 0.5 steps, in between the planned values.
- Ideally this circuit should power a load of maximum 10W. Usually will be smaller power elements connected for test.
Lets say for now that the max is 1W for simplicity. Check my circuit I posted and tell me if it will be good enough for a max 1W, and what I should add more to it?
thank you

Are you wanting an electronic load rather than current limiting so for instance can put a power supply under a variable load ?

_q12x_ · Sep 14, 2024

An opamp problem:
- At +input I have a 424uV!!! signal. I diminuate it as much as I could to be clear that it is not the interference problem.
You can see the value at the (left) blue probe.

At -input I have a constant 2.3mV. Also see the coresponding (right) blue probe. And here is the problem. I can not lower it down more than this value. This is the minimum possible, for this cct and this kind of setup. My problem is that I want to make it read 1mV instead of 2.3mV. That 2.30mA is the same as the voltage because of that 1R resistor. They are interchangeable V and A at this point.
I managed to change the -input to 1mV actually, by changing the 1RLoad into a 2.22RLoad. See my notes in the cct.

But I dont want to change this critical value that I depend of getting my accurate readings over it. Basically transforming V into A.
- My suggestion is to insert something on the -input line, from P1 to the actual -input. But what?
Or some other circuit that can deal with this limitation? I dont think my solution is the only solution. If it is, be clear to me, and tell it to my face that it is. Im not sure it is. I trust in your experience more than mine, especially with opamps, although I start to get more better used to them.