# Making a power supply from bits...

#### Dodgy Geezer

Joined Nov 30, 2009
177
I have often thought that it would be nice to have a small bench power supply - but I don't like spending money...
I do, however, have a SMPS of 36V 5A, and an LM338 in my bits box. What would be the issues to consider if I wanted to connect them together to provide a variable voltage supply (with the appropriate circuitry, of course)? And could I add a current control to the same LM338, or would I need another one?

#### MrChips

Joined Oct 2, 2009
29,853
Let's be certain that it is understood what you mean by current control.

You cannot control voltage and current at the same time. The voltage and current adjustments on a bench power supply are limits. You set a maximum limit on voltage and current. One of the two limits will be reached, either voltage or current limit and not both at the same time.

#### Dodgy Geezer

Joined Nov 30, 2009
177
My (admittedly poor) understanding of a bench power supply is that it will have an output voltage that you can set, and a maximum current drain that it will allow.

And there appear to be many published simple circuits for using an LM338 as a variable voltage regulator - typically using a couple of caps, a couple of diodes, a couple of resistors and a pot. You seem to be able to use an LM338 to provide current limiting as well, but, I suspect, not at the same time as a variable voltage...

#### MrChips

Joined Oct 2, 2009
29,853
A 36V 5A PSU is a decent 180W PSU already.
Yes, you can use LM338 to provide voltage control with up to 5A output.
Bear in mind that any linear voltage regulation will have to dissipate a max of 180W and will get very hot. You will need a very large heat sink and perhaps even a cooling fan.

#### Dodgy Geezer

Joined Nov 30, 2009
177
Ah. I wonder if there are any other cheap variable regulators which would have less of a thermal problem? Or perhaps I would be better off splitting the output between two of them? That is certainly a downside to my idea....

I presume that proper bench supplies do not have this problem because they use different (and more complex) circuitry.

#### MrChips

Joined Oct 2, 2009
29,853
I presume that proper bench supplies do not have this problem because they use different (and more complex) circuitry.
A bench power supply (or any power supply) has this problem. It is either SMPS or a linear regulator with a big heat sink and a fan.

If you are drawing 5A from a linear regulator at any voltage output, 180W will be dissipated with 36V source, either in the load or in the PSU or shared between the load and the regulator.

The other solution is to go to a switched moded power supply (SMPS) (which is what you already have). Find a way to control the output voltage of the SMPS.

#### Dodgy Geezer

Joined Nov 30, 2009
177
Alas, a fixed-voltage SMPS will have a complex set of matched components - I would probably need to redesign the circuit. Which has 2 problems:

1 - I wanted a cheap and simple answer...
2 - I couldn't do it anyway...

#### Dodgy Geezer

Joined Nov 30, 2009
177
Would I be correct in thinking that:

a) If I want to have a variable voltage and limited current from an SMPS using LM338s, I would need two LM338 stages in series, one configured for voltage control and one for current?

b) And if I were to do this, the heat dissipated would be generated in each LM338 according to the work each one is doing, dependent on the settings chosen....

#### dl324

Joined Mar 30, 2015
16,162
If I want to have a variable voltage and limited current from an SMPS using LM338s, I would need two LM338 stages in series, one configured for voltage control and one for current?
Do you want current limiting? Or constant current?

EDIT: I see that you want current limiting.

If you intend to draw multiple amps at a low voltage, you should use a switching preregulator. For current limiting, you can use a simple hiccup mode limiter.

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#### Dodgydave

Joined Jun 22, 2012
11,152
Alas, a fixed-voltage SMPS will have a complex set of matched components - I would probably need to redesign the circuit. Which has 2 problems:

1 - I wanted a cheap and simple answer...
2 - I couldn't do it anyway...
Another option is to modify the smps to variable output, depending on the chip design, But this is Not for amateurs.

#### Dodgy Geezer

Joined Nov 30, 2009
177
Another option is to modify the smps to variable output, depending on the chip design, But this is Not for amateurs.
I would need to go on a lot of training courses before I became an amateur!

So I expect that the best answer is an SMPS feeding two lm338s in series - one set up for voltage selection while the other is for current limitation...

#### dl324

Joined Mar 30, 2015
16,162
So I expect that the best answer is an SMPS feeding two lm338s in series - one set up for voltage selection while the other is for current limitation...
It's a poor solution if you plan to draw high amperage at low voltage. Power dissipation in linear regulators will be high.

Using a switching regulator to lower the voltage would be a better idea.

Hiccup current limiting is easy to implement. Constant current can be done with LM338, but power dissipation could be a limiter.