Making a >12v power supply

I'm working on a project for which I need to calibrate a DC Buck convertor to provide a 12v output. The convertor can accept 17v max on its input.

Unfortunately I don't have a power supply which outputs more than 12v but less than 17v. I could of course buy a proper bench power supply, which would surely be an investment, but also total overkill just for the purposes of calibrating this convertor.

Any suggestions for cheaply and easily generating a >12v supply? Doesn't need to be too sophisticated or permanent.

I have considered:

  • Batteries in series (should work fine and may be the best approach),
  • Wall wart style SMPS in series (looks a bit fraught from reading on the internet, seems to depend on the PSU in question as to whether it is safe or not),
  • 24v SMPS (if I have one), into a voltage divider to drop it to a safe level for the buck converter

Any better solution I'm missing?

Thanks!

Why 17V?

What do you mean by calibrate for 12V? If it's a buck converter with a 12V output then it has a 12V output.

I feel that either you are not telling us something important or you are misunderstanding something important.

You can't use a voltage divider as you described.

Look at laptop power supplies. All the Dell ones I've ever had put out 19.5V, so obviously a voltage divider would get you in the range you need. Most folk seem to accumulate a collection of them.

A DC buck converter that outputs 12V and only accepts up to 17V as input ? That does not make sense. No manufacturer would make such a converter. Can you give a link to it ?
Is it perhaps a step-up-step-down converter ? Can be input be lower than 12V as well ?

If you have a laptop, then the adapter is a 19V power supply. Add four diodes to lower the voltage.

Switching power supplies in series :o I agree with what you already found. It might work or not, but it is not safe. Just for a test with a small current, then you can try it. If something starts smoking, then you know that something is wrong :o

A voltage divider can not supply a lot of current.

What else have you lying around ? Transformers ? Variac ?

if your only looking for voltage increase, and not concerned with current, then, yes a battery in series will increase your voltage, but will severely limit the current available.
Paul

Thanks all for your responses.

More on the convertor: its an eBay cheapie "Mini 360". There's a good write-up on it here:

I was posting based on memory earlier (mistake!): the module takes 4.75-23v in, and outputs 1-17v.

The module has a pot which controls the output voltage, that's what I mean by calibration. I need to feed it something > 12v in order to twiddle the pot to set the output to 12v (hope this makes sense).

By the way, I am assuming that the convertor regulates it's output, such that if the input varied between say 13v-15v, the output would still be a constant 12v. This may turn out to be an erroneous assumption.

In terms of calibration I don't need a lot of current.

The module has a pot which controls the output voltage, that's what I mean by calibration. I need to feed it something > 12v in order to twiddle the pot to set the output to 12v (hope this makes sense).

Ok, yes, that makes sense. Make sure there is some load on the output when you do it.

By the way, I am assuming that the converter regulates it's output, such that if the input varied between say 13v-15v, the output would still be a constant 12v.

That's kind of the point of a converter! Your assumption is correct, although it might not work with 13V in and 12V out, try it.

In terms of calibration I don't need a lot of current.

You need more than can reasonably be drawn from a voltage divider.

Buy a 12 volt solar cell. Use the solar cell as a 20 volt source. Later, get a charge controller and run the project off the solar cell.

It sounds like "find a laptop power supply (or similar) and drop voltage as required through N diodes" is the way to go then.

Thanks again everyone for your input

tomparkin:
Unfortunately I don't have a power supply which outputs more than 12v but less than 17v. I could of course buy a proper bench power supply, which would surely be an investment, but also total overkill just for the purposes of calibrating this convertor.

Any suggestions for cheaply and easily generating a >12v supply? Doesn't need to be too sophisticated or permanent.

The first statement is confusing. It could represent anything from a typing error to a fundamental misunderstanding. If you need to adjust ("calibrate") the output of a variable buck converter, you use a voltmeter.

Voltage dividers are fine (in some cases) for supplying reference voltages, but absolutely terrible for supplying power. For one thing, you are wasting power in the resistors. For another, they are not regulated. If either the supply or the load changes, so will the voltage.

It's already been mentioned that many laptop supplies typically output 15-20v at perhaps 2-4 amps. These are inexpensive (US$4-$8 on the surplus market) and readily available. I have several and use them extensively as input devices to feed regulators (which often are little more than a 78xx IC and a couple of caps).

S.

Oh! Forgot to add: the solar cell is an interesting suggestion I hadn't considered. This project is going to be hooked up to a decent power source though, so I'd just be buying a cell for the sake of it.

tomparkin:
It sounds like "find a laptop power supply (or similar) and drop voltage as required through N diodes" is the way to go then.

Thanks again everyone for your input

No need for any diodes as far as I can tell.

Tip for the future: collect power supplies from old electronic kit that's being thrown away, you will soon have a power supply for any project you can think of.

I actually have something in the region of 10,000 power supplies of various flavours, but they all seem to be 12v or less.

Maybe the simplest option is just "dig through the boxes until you find one of the right flavour"

Is that a typo? Ten thousand???

If you have lots of 12V power supplies why do you need another one?

Sorry, PerryBebbington, I was making a joke. Badly! Sarcasm is the lowest form of wit and all that.

So no, not really 10k :slight_smile:

I need > 12v because if I hook the convertor up to 12v I have to set the pot to full chat to get something in the region of 12v out. Then if the convertor sees more than 12v the convertor output rises accordingly.

Somewhere in those zillion power supplies there is probably a laptop adapter. When I look into my crystal ball, then I see one in the left bottom corner in a box that is sitting there since the Stone Age. It does not need to be a JigoWatt since those adapters can often supply more current for a short time. As I said before, you see that something is wrong when the magic smoke comes out.

Have you considered purchasing a Lab Power Supply? A new one with 0-30V and 0-5 or 10A can be gotten for less then $75.00 from your favorite china supplier. You can adjust the voltage and current limit from 0 to max rating. The analog ones will cost a bit more but they are the best when working with low level and analog signals, much less electrical noise. Used ones can be gotten for less.

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