Variable DC power supply?

[Andrew TinsleyParticipant @andrewtinsley63637]

I have a regulated DC power supply unit which gives 13.8 volts DC, at up to 20 amps. I require to use this to give me a variable power supply. Ideally 0 to 12 V, however I will only use 10 and 12 volts. in practice and max current will be 6.5 amps

The last time I was regularly designing electronics was around 45 years ago and despite having some very fancy electrical and electronic qualifications, no doubt that knowledge is now totally redundant!! I probably have the components to do this job using 45 year old design techniques. However it is about time I was dragged into the 21st century! So how would one go about this, using modern up to date components and methods?

Andrew.

[Peter Cook 6Participant @petercook6]

Probably the simplest is to buy one of these 400W 15A DC-DC Step Up Down Boost Buck Voltage Power Converter. or one of many similar at <£8 off a well known auction site.

I doubt you could source the components for that price.

 

[Les Jones 1Participant @lesjones1]

Looking at the specifiction of this power supply at the end of the advert it is only a boost converter.

Les.

[Andrew TinsleyParticipant @andrewtinsley63637]

I am at a loss to see how Peter’s suggestion could possibly work, but happy to be shown to be wrong. Using my outdated knowledge I could design a much simpler system using some old component stocks that I still have.

Andrew.

[Peter Cook 6Participant @petercook6]

Feed your 13.8v DC in, adjust the output pot to give 10 or 12v at the output – result. Yes the device is complex, but at £7 delivered is it really worth your time and effort to  design something simpler, dig out the components rom stock, wire them on a  board with screw connectors etc. etc. etc.!!

I have a smaller one I use as a battery replacement when working on clocks. 5v in from a USB power supply or 6v battery, tweak the output to give 1.5 or 3v depending on need. I think I paid £3 delivered for mine.

An alternative, better cased might be this one  which is step down only. You get over current protection, reverse polarity protection etc. Design & build of gadgets like these is simply no longer worth it!

[Robert Atkinson 2Participant @robertatkinson2]

Be aware that many of the cheap switch mode DC regulators coming out of the far east use counterfit IC’s. There have been reports on amateur radio forums of them failing and causing damage to expensive equipment. Even if the chips (often National Semiconductor (Now Texas Instruments) “SimpleSwitchers” they cannot produce the output voltage and current range claimed with a single value of inductor.

I’d suggest that the simplest and most effective solution is to modify the feedback resistor(s) in the exsitind circuit to give the desired range. Possibly with switched Variable/10V/12V.
The unit will almost certainly use a LM723 voltage regulator IC so fairly easy to work out the circuit.
There will be more power dissipated at the lower output voltage but this is more than compensated by the lower required current. Nothing wrong with a 45 year old 723 regulator. It’s still made and out performs many newer devices. It particular it has considerably lower noise on the output than any switcher.
Do you know the make and or model number of the powersupply?

Robert.

[Andrew TinsleyParticipant @andrewtinsley63637]

Peter, I do not want to buy a cheap off the shelf unit. I wanted to see what advances have been made in PSU design and if necessary build a modern circuit to do the job. I don’t care what it costs, it is in gaining some modern experience, that I am looking for.

Robert, I am disappointed that the LM723 is still king! I expected some modern whizz kid of a regulator to be the in thing. Having said that I took a look at the interior of the 13.8 V regulated PSU and lo and behold, there is what appears to be an LM723!

So after all, my 45 year old electronics is more than sufficient to modify the existing PSU. I agree with you about some of the current switch mode PSUs coming out of China. I innocently purchased one from Ebay, It didn’t last long, but fortunately no resulting damage to the circuit it powered.

Andrew.

[Robert Atkinson 2Participant @robertatkinson2]

Hi Andrew,

Other than ever increasing frequencies and corresponding reduction in size of switchmode supplies these has been little change in powersupplies and DC-DC converters. There was / is a bit of a drive to modularity in higher end supplies used for industry and aerospace. Vicor do “bricks” here you have an input module with about 350V DC out with power factor correction if required and then 350V DC to 24, 12, 5 or whatever output(s) you need that you pick & mix. Another aproach is having planar “windings” on a flexible PCB on each output module that aer clamped into a ferrite core stack with a single primary. These are a bit advanced for what you want. You could replace the 723 and 2N3055s with a switching regulator But most of the newest devices are in very small surface mount packages.
One thing Ive done with this type of supply when the output devices have failed is to replace them with MOSFETs You do need enough gate voltage. This can mean logic level devices. I’ve also designed from scratch with lower AC voltage mains transformer (14 or 15V rather than 18) with a schotky diode bridge and standard MOSFETs but using a simple diode capacitor voltage doubler to supply the 723 and thus gate voltage. The current is minimal unlike a the base drive for a bipolar. Add a resistor / zenner regulator between the doubler and regulator and it also isolates the regulator from load variation.
This results in a very low noise supply and laower power dissipation because of the lower transformer voltage.

Robert.

[Peter Cook 6Participant @petercook6]

Sorry Andrew my mistake. I read the original posts query “…I require to use this to give me a variable power supply…. So how would one go about this, using modern up to date components and methods?” as a request for the best way to achieve the goal of providing a variable power supply.

My interpretation of “best” was clearly at fault. I read it to mean “the quickest and cheapest” way of achieving the desired variable voltage supply, and that was the basis of my answer – Buy a pre built unit fairly cheaply.

While you were actually asking for updated technical design information. Sorry again for the confusion.

[Andrew TinsleyParticipant @andrewtinsley63637]

No problem Peter and thanks for your input. More often than not, it is I who gets the wrong end of the stick, probably due to aging in my case.

Andrew.

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