# 1.2 - 32 volt power supply

#### walcen

Joined Aug 6, 2009
4

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Why do you feel that you need such a large voltage range?

If you're going to be using the supply mostly in the 25v to 30v range, then that's OK.

However, if you are also planning on using it for lower voltage projects (like 3v-10v) then you will have a power dissipation problem.

Let's say just for example that you have a reasonably stable 35v on your filter capacitor after rectification, and it'll stay within a few volts of that whether your regulator is putting out 10mA or 5A (it would have to be powered by a transformer rated for >10A to do that...)

Let's also say that you are powering a project that requires 5A @ 30V.
So, you have 35v in, 30v out, 5A current.

The load will be dissipating 30v*5A=150 Watts of power, and the regulator will be dissipating (35v-30v)*5A = 5v*5A = 25 Watts of power.

Let's also say that you want to run the supply in a room that is nominally 25°C/77°F.
The LM338T (TO-220 package) has a thermal resistance to a heat sink of 4°C/Watt.
The LM338T (TO-220 package) has a thermal resistance to ambient (no heat sink) of 50°C/Watt.
The LM338K (TO-3 package) has a thermal resistance to a heat sink of 1°C/Watt.
The LM338K (TO-3 package) has a thermal resistance to ambient (no heat sink) of 35°C/Watt.

Both will shut down at 125°C.
So, with no heat sink and operating at ambient 25°C, the LM338T will shut off when it is dissipating 2 Watts of power (50°C*2w+25°C ambient=125°C), and the LM338K will shut off when it is dissipating under 3 Watts of power (35°C*2.86w+25°C ambient=125°C).

You will need a heat sink big enough to make up for the difference between the 25 Watts and the 2 or 3 Watts that they can dissipate themselves.

Now let's try powering a 3V load that needs 5A current. We still have 35V on the filter capacitor.

The load will dissipate 3v * 5A = 15 Watts of power, and the LM338 will dissipate (35v-3v)*5A = 32v * 5A = 160 Watts of power. Now you have a really, really big problem of heat dissipation.

Let's just look at the LM338K-package (TO-3 case). As a review, the LM338K (TO-3 package) has a thermal resistance to a heat sink of 1°C/Watt. Remember, the regulator will shut down when it's internal temp reaches 125°C.

With the TO3 package, dissipating 160 Watts means that you will need a PERFECT heat sink that can keep the outside of the package at 125°C-160°C, or -35°C just to keep it from going into thermal shutdown. Basically, you'd need to run the thing at the North Pole - with a huge heat sink. Even freezing it into a block of ice would not be good enough.

You might use water cooling. Water boils at 100°C at sea level. However, that only gives you the basic 25 Watts of dissipation from the steel TO-3 case.

So, what is your plan to get rid of all of the heat you're going to subject your regulator to?

I too am preparing to build a bench power supply such as mentioned above using a rewound MOT to cater for heavy load, and I, as a layman!!! also was under the assumption that the lm338 could simply adjust voltage from 1.2 to 32 volts with a maximum supply voltage, Doh.

I must farther educate myself.

Using multiple regulators would increase the load capacity but if the load requirements are say 5 amp and one used three or more regulators, would they share the heat dissipation?

And if one were to wind multi taps on a transformer to apply varying input voltages to the rectifier via a rotary switch, and then used a pot to vary the voltage on the regulator, would this be an acceptable method?

And what would be the maximum allowable voltage between taps for the regulator to operate efficiently between 1.2 to 32 volts.

So many questions, so little time .

Kindest regards to all

Walt

#### k7elp60

Joined Nov 4, 2008
562
I understand what you say and the problems you face with such a powersupply. I wanted to build one similar to your idea. I used a LM338K on a large heatsink and I discovered that at times it would shut down when I thought it sould not. I finally realized that even thou I thought everything was okey, the internal circuitry of the LM338K sensed a overload condition. I went back to a LM723 and a darlington series pass transistor 2N6284 with external current limit resistors. I decided I didnot need to go below 6V so I have 2 ranges. 6-12V@ 8A and 12-28V@ 4A. I used the transformer in FW center tapped and FW bridge for the two ranges.

#### DickCappels

Joined Aug 21, 2008
9,853
I have used such a bench power supply (LM317 + LM337) to obtain the range of ±1.25 to approx 25 VDC since those ICs became available. They work fine as long as I don't draw too much current. In the rare case of overloading, the thermal protection and sometimes the current limit on the chips took over. Most of my circuits don't take much power.