# Cheap Variable Power Supply

#### T.Jackson

Joined Nov 22, 2011
328
I made this thing a while back.

Now I require something a bit better. Something with:

• 0 - 30VDC
• Current limiting
• Dual rail tracking

I realize that I cannot use an LM3XX IC to produce 0 volts and provide features such as variable current limiting, so I am leaning towards a design that uses OP-AMPs with series pass transistors.

Are there any other, perhaps more modern approaches?

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#### MMcLaren

Joined Feb 14, 2010
851
I passed along a link to EEVBlog #221 in another one of your threads where Dave is designing a lab' grade variable voltage current limited power supply. Did you take a look?

#### T.Jackson

Joined Nov 22, 2011
328
Sounds good. No I haven't seen it.

#### MMcLaren

Joined Feb 14, 2010
851

#### T.Jackson

Joined Nov 22, 2011
328
I cannot see anything on that page besides comments.

UTube video presentation is it?

#### Wendy

Joined Mar 24, 2008
22,206
I have something very similar. After using it for many decades I've decided to upgrade it. One way is to get an external power supply or wall wart. There are many out there to choose from. Just look around. I have found a switching wall wart that is 24 V. I'll use a jack to allow it to use the internal transformer or the external power supply.

One way to make it tracking is to create a pseudo ground. Something like this...

Creating a Virtual Power Supply Ground

I also have another article that may help (or not)...

Basic Bench Top Power Supplies

#### T.Jackson

Joined Nov 22, 2011
328
You won't host mains powered projects correct? Must use a plug pack right?

#### Wendy

Joined Mar 24, 2008
22,206
Yep, no mains projects unless it has a transformer. The transformer does have to have a mains plug, but provides the safe isolation we require.

I have picked back up on the article for the AAC book. Lots of writing left to do. Some of what is left will apply for your ideas.

Submission: Power Supply Circuits

BTW, you do good work hardware wise. Not the same as I would do it, but it is attractive and well done.

You scavenging the old hardware, or starting from scratch?

#### tom66

Joined May 9, 2009
2,595
I've been playing with a design in LTSpice for a while, but it's not dual rail.

It implements CC/CV limiting. It keeps a constant output voltage set by one input until the current exceeds the adjustable limit; it then decreases the voltage until the current reaches the desired level.

It's very stable, and has low noise (<2mVp-p), even at high loads. It handles load transients well, a load step from 0A to 3A only causes a brief drop by 8mV and it returns to the original output in <20ms.

The LEDs indicate the mode it is in; CV or CC.

More than two transistors may be needed as they dissipate ~25W each under full load.

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#### T.Jackson

Joined Nov 22, 2011
328
BTW, you do good work hardware wise. Not the same as I would do it, but it is attractive and well done.
LOL 'good but not the same as I would have done it' You just reminded me of my grandfather for a second there. That was one of the last things that he said to me when were half way through building a work bench.

You scavenging the old hardware, or starting from scratch?
I usually prefer to go with all new parts when I do things. Cars, wood work, electronics, I tend to always buy new parts that are specific for the task.

#### T.Jackson

Joined Nov 22, 2011
328
I've been playing with a design in LTSpice for a while, but it's not dual rail.
Looks good, but I am hoping for something a bit more straightforward than that. Something for under $50 using all new parts, requiring only a few hours to build. You can buy one brand new in a profesional looking case for not much more, so in my mind it makes no sense to budget a few hundred on a DIY one that is likely going to be on sub par performance with one already made. You know what I mean? I will post a schematic soon of something that cuts straight to the chase. Unfortunately, I don't use simulators, so you'll have to excuse any illustrative errors, as I seem to be doing plenty of lately. #### bountyhunter Joined Sep 7, 2009 2,512 I made this thing a while back. Now I require something a bit better. Something with: • 0 - 30VDC • Current limiting • Dual rail tracking I realize that I cannot use an LM3XX IC to produce 0 volts and provide features such as variable current limiting, so I am leaning towards a design that uses OP-AMPs with series pass transistors. Are there any other, perhaps more modern approaches? Depends on how much current you need. If I wanted a 0-30V supply at decent current (like 2A or more) I would build a basic unregulated DC supply (FWB) to generate about 35VDC unregulated, then build a linear/switcher design such that the switcher output tracks the linear output and always stays 2V above the output of the linear which minimzes power dissipation. If you want dual output, get a power Xformer with dual secondaries and create two unregulated supplies and build two of these. You can build control circuitry for dual track or parallel (auto) track. Thread Starter #### T.Jackson Joined Nov 22, 2011 328 Do we agree on this much? #### Attachments • 45.3 KB Views: 152 Last edited: Thread Starter #### T.Jackson Joined Nov 22, 2011 328 Depends on how much current you need. If I wanted a 0-30V supply at decent current (like 2A or more) I would build a basic unregulated DC supply (FWB) to generate about 35VDC unregulated, then build a linear/switcher design such that the switcher output tracks the linear output and always stays 2V above the output of the linear which minimzes power dissipation. Looking for very clean safe power suitable for a wide range of general circuits, would be linear for this. #### bountyhunter Joined Sep 7, 2009 2,512 Looks good, but I am hoping for something a bit more straightforward than that. Something for under$50 using all new parts, requiring only a few hours to build. .
I should have read that first and saved time.

It's impossible to meet those specs.

#### T.Jackson

Joined Nov 22, 2011
328
I should have read that first and saved time.

It's impossible to meet those specs.
Maybe only a few amps, but it will be done.

Enjoy the ride.

#### bountyhunter

Joined Sep 7, 2009
2,512
Looking for very clean safe power suitable for a wide range of general circuits, would be linear for this.
No offense, but I just had a deja vu moment from when I designed power supplies.

Looking for very clean safe power suitable for a wide range of general circuits
That actaully says absolutely nothing to help you define the power supply or narrow in on a topology. You have to figure out exactly what you need (V/A), what features it must have (parallel/stack, etc).

#### T.Jackson

Joined Nov 22, 2011
328
That actaully says absolutely nothing to help you define the power supply or narrow in on a topology. You have to figure out exactly what you need (V/A), what features it must have (parallel/stack, etc).
The transformer and the degree of heat sinking with be the imposing limitations. 0-30VDC @3 Amps on the low end would be nice.

We need to get the basic abstract done first.

#### bountyhunter

Joined Sep 7, 2009
2,512
Maybe only a few amps, but it will be done.

Enjoy the ride.
No offense, but you said you wanted:

0 - 30VDC
Current limiting
Dual rail tracking
Add to that you now say it has to output "a few Amps", then you have a dual output 0-30V/3A (90W) dual linear power supply (180W total) which you think you will build in a few hours for a cost of under $50. I can tell you that's impossible. You might be able to buy the transformer and filter cap for$50, but I doubt it. You will also need a massive heatsink/fan that can dissipate 200W.

Good luck.