HI, I built a standard power supply from the datasheet for a LM338 regulator. The 24vdc output is fine with no load. Add a 350mA load and the power supply voltage slowly drops down and then out. I noticed the 338 was very hot. The power supply has a 120 to 28 vac transformer feeding a fullwave bridge rectifier, then a 1000uf filter cap before going to the 338 regulator. When I checked the secondary voltage of the transformer I found 30vac. The output of the bridge rectifier was at 40vdc. I believe this is two high and heating up the 338 causing it to drop out under load. Is the output of the rectifier so high because the filter cap is smoothing the rectified peak to peak voltage? What can I do to correct this?

 

Are you using a heat sink on your LM338?
If not, you will want to.
Also, you can use a diodes after the rectification to drop the voltage prior to regulation, or you can use some power resistors to help with the voltage drop to get rid of some of the wattage so the 338 doesn't have to.

Also, have you scoped the 338 to see if oscillations have occured?

 

Your LM338 is going into thermal shutdown because it is getting too hot.

With a 350mA load, 40VDC in, 24VDC out, your 350mA load will be dissipating 24v x .35A = 8.4 Watts, and your LM338 will be dissipating (40v-24v) x .35A = 5.6 Watts. You would need a big heat sink to dissipate that power; and as the output voltage decreases, your heat dissipation in the regulator will increase significantly.

Use a transformer with a lower secondary voltage.

Use a larger filter capacitor. 1000uF is not enough.

[eta]
For 60Hz using a full-wave rectifier, the rippled DC output will have a frequency of 120 Hz.
With a 1000uF capacitor and a load of 1.2 Amperes, you will have 10v ripple on the filter cap.
Doubling the capacitance cuts the ripple in half.
10,000uF with a 1.2A load would have 1v ripple.

At 4.8A load current, you would need 20,000uF to get the ripple down to 2v.

 

Or do that.

 

I am not using a heatsink on the proto board, but will when I finalize the circuit board. Do I calculate the transformer to have slightly over 24vac peak to peak on the secondary, like 26vac? Also what size filter cap would be enough- 4700uf? Have not scoped it yet. I kinda figured the 338 was disappating the the difference of 40v -24v as heat.

 

You were right. With no heat sink, thats disastrous.

 

Would a transformer with a 18vac secondary or 24vac secondary be the right choice? My transformer is supposed to have a 28vac secondary. I assume that is not the peak to peak rating. If that is the case a 18vac secondary would give me about 25.42 peak. Also when installing the heat sink I have the mica insulators and heat sink compound, do I put the heatsink compound between the 338 and mica insulator and then between the mica insulator and the heatsink?

 

do I put the heatsink compound between the 338 and mica insulator and then between the mica insulator and the heatsink?

You can use it between both, and you should. The paste helps heat transfer from one material to another, or one surface to another.

 

hi all
i have built a power supply using lm338T with NO HEAT SINK ...but then i tried it after i have finished building it, and it works perfectly with a motor 24vdc and 2.5-3 A.

Unfortunately the day after that i tried it again but it constantly delivering 42v out, after checking each component i found that the lm338T Vout and Vin pins are shorted .

i changed the lm338T and the trans 220 to (16-0-16) 3A with trans 220 to (12-0-12) 3A, But still the same..

This is circuit diagram
http://powersupply88.com/simple-vaiable-power-supply-1-5-30v-5a.html/15-to-30v-5a-power-supply
But without using the lm317 section (Current limiter)

Then i found that i was wrong in using LM33T without using heat sink (50C/W),
and found also that i must use LM338K with a suitable heat sink (1C/W)
Then i have decided to rebuild the following circuit:
http://www.eleccircuit.com/adjustable-power-supply-1-2-30v-5a-using-lm338/

Is there any advices before I start??

Im using a Trans 220 to (12-0-12) 3A
LM338K with heat sink
I have made calculations according to SgtWookie post in another thread and found that outputting 15v to 30v with 3A will be OK (15v through connecting a fixed Resistor series with the variable resistor).

Am I right??

 

I am not using a heatsink on the proto board, but will when I finalize the circuit board.

you need a heatsink