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LM338/LM318 Not enough power. Vref drops!


oscarcot
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Hello.

I need a regulator to deliver 5 Amp, at 7.5V to a servo motor I'm trying to drive. I use nearly the same simple schematic as in LM138/338 datasheet (page 5) in Application Hints. I'm using an 11.1V Li-Poly Battery.

With no load, the regulator seems to work properly with 7.5 V at the output. But when I try to move the servo, output voltage drops down to 3 V. At that level, the current it delivers is just 1.33 Amp. (I'm measuring output voltage directly on the case -in the T-03 package- so cable gauge mistakes do not matter.)

The first thing I thought was "Well, maybe the battery is unable to deliver such a big current.", but then I checked the Vin battery level and it just drops down to 10.3V, so it doesn't seem to be suffering much. Even taking into account the regulator 3V dropout voltage with 5 Amp current , output voltage shouldn't drop much (10.3V- 3V= 7.3V).

The important thing I noticed was this: Vref (which is meant to be a 1.25V constant, regardless of the current), drops down to 0.5V when the load is present.

So well... this makes me think: The regulator is uncapable to supply the current I need. But well... the spec sheet says it can do so... so I don't know what to do.

For reference, I'm using R1=100 Ohm and R2= 500 Ohm.

I appreciate any help.
Thank you,
Oscar Cota

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Hi Oscar,
Welcome to our forum. ;D
I think either the LM338 is getting too hot and is shutting down or it is defective. They typically can supply 9A if they have only 5V across them and are cooled properly.

Maybe your servo is stalled and draws much more than 5A.


Hey. Thanks for your help.

--------------------
Well, actually I don't have a heat sink since I'm using a protoboard and there's not enough room for it. But well... in fact, it doesn't get that hot because it's just draining 1.33 Amp @ 3V (remember the huge drop in Vout).
---------------------

If you have experience using it, I would seriously take into account the possibility that it's defective because well...

1. Battery voltage only drops down to 10.5V (the battery's not having problems).
2. Nevertheless, Vref, which is supposed to be 1.25V constant, drops down to 0.5V.

----------------------
And my servo is not draining more than 1.33 Amp, haha (at 3V). I would be happy if it was stalled and draining more than 5A. At least I would know the circuitry can deliver enough power to it.  ;D
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By the way... I forgot something. The servo does move freely and happily, but when I try to stop it is when the voltage drop ocurrs.

And by the way!... output voltage has really big fluctuations when the servo is moving freely and happily. Capacitors (as in spec sheet) just smooth this fluctuations but do not eliminate them.

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No heatsink???

It has 10.5V-3.0V= 7.5V across it.
It has 1.33A through it.
Therefore it is dissipating 7.5V x 1.33A=10W.

Its max allowed internal temperature is 125 degrees C.
Its TO-3 package has an internal temperature rise of 35 degrees C/W without a heatsink.
Boy is it ever hot at about 375 degrees C! It is supposed to shut down at 150 degrees C or hotter.

Get a heatsink and replace the melted LM338.

How do you stop the servo? By shorting it and shorting the supply from the LM338?

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It has 10.5V-3.0V= 7.5V across it.
It has 1.33A through it.
Therefore it is dissipating 7.5V x 1.33A=10W.


Haha. Ok, sorry. Well, yeah... maybe it's dissipating a little too much power but I just leave it that way for about 3 or 4 seconds when I'm stopping the servo. I swear it has always been cool enough to put my fingers over it without getting them burnt.

But anyway, I'm buying a new LM338, because I do think it may be defective (although I have never let it melt down or be at 350
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We don't know how much time it takes for 375 degrees C on the chip inside the IC to heat up its case. Its case can be at room temperature and its insides could be too hot.
The datasheet for the LM338 shows it limiting its output current from 12.5A to 8A within 10ms when it has a good heatsink. That is 1/100th of a second.

If you reduce the output voltage of the regulator then it will get hotter with the same load unless you also reduce its input voltage.

When you stall the rotor of a DC motor then its current is much higher than when it is running becuse it is working much harder.
Measure its DC resistance and use Ohm's Law to calculate how many Amps it draws when it is stalled, or look on the motor's datasheet. A DC motor also draws a very heavy current when it starts running.

Of course the insides of your LM338 were at 375 degrees C. I showed you the calculation.

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If you reduce the output voltage of the regulator then it will get hotter with the same load unless you also reduce its input voltage.


Agree.


When you stall the rotor of a DC motor then its current is much higher than when it is running becuse it is working much harder.


Yeah, that's exactly what I want. I stall it on purpose to make it drain more current to see if the regulator works properly providing it 5 Amps. I'm measuring it with my ampmeter in series with the battery but current level only reaches 1.33 Amp when stalled.


Measure its DC resistance and use Ohm's Law to calculate how many Amps it draws when it is stalled, or look on the motor's datasheet. A DC motor also draws a very heavy current when it starts running.


Unfortunately, to do that with my servo, I would have to dissasemble it, so that I could access the motor's leads. But anyway, in the motor's datasheet, it says it should drain 5 A when stalled (hitec titanium gear robot servo http://www.hitecrcd.com/Servos/spec_sheets/HSR-5995TG.pdf ). It hasn't been able to draw that amount of current, because the supply voltage always drops.

And well, finally...


Of course the insides of your LM338 were at 375 degrees C. I showed you the calculation.


I know, man, but if it were that simple, what's the heatsink for? We could just argue that eventhough we put a heatsink for the IC, it's inside will be melting down at 375
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You had 10W of dissapation. The thermal resistance from the chip to the air is 35 degrees C/W so the chip was at 350 degrees C above the ambient temperature of 25 degrees C.
The thermal resistance from the chip to the case is 1 degree C/W so the case would reach 340 degrees C above the ambient temperature without a heatsink.

You said it wasn't on long enough to get hot so the IC must be faulty.

If you had a good heatsink then 10W of power would have the case at 50 degrees C and the chip at 60 degrees C. 

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Hi.

Well, well, well....

This is what happened now:

1.

I got 2 LM338 IC's with the T-03 encapsulate and a heatsink. I connected them to the circuit. (The Vout wasn't in direct contact to the heatsink, it had a plastic cover.) The servo moved alright (without stalling), drawing a current of 500mA-700mA. But when I stalled it, Vout dropped down to about 4V, drawing just 2.4Amps for a time of 2 or 3 seconds, and then the regulator shutted down forever, giving a Vout of 0V.

After the shut down, the IC and the heatsink didn't feel hot to touch. Their temperature shouldn't have been any hotter than 40

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I got 2 LM338 IC's with the T-03 encapsulate and a heatsink. I connected them to the circuit. (The Vout wasn't in direct contact to the heatsink, it had a plastic cover.)

The case must be in very good contact with the heaksink. The electrical connection can be made with a solder terminal bolted to the case.

The servo moved alright (without stalling), drawing a current of 500mA-700mA. But when I stalled it, Vout dropped down to about 4V, drawing just 2.4Amps for a time of 2 or 3 seconds, and then the regulator shutted down forever, giving a Vout of 0V.

After the voltage dropped, the amount of heat was (10.65V - 4V) x 2.4A= 16W. If the IC was fastened properly to a finned heatsink rated at 4 degrees temp rise per Watt then the chip temp would be 113 degrees C and the IC should have worked fine.

The resistance of the stalled servo is 4V/2.4A= 1.67 ohms. Before the voltage dropped then the current was probably 7.5V/1.67 ohms= 4.5A. The heat was (10.65V - 7.5V) x 4.5A= 30W and if the IC was properly fastened to a finned heatsink rated at 4 degrees C temp rise per Watt (not big enough) then the chip would be 190 degrees C which is much too hot. Its max operating temp is 125 degrees C and it shuts down at about 150 degrees C.

I tried it again stalling the servo, and IT DIDN'T SHUT DOWN! I could even leave it that way for about 10 or 20 seconds before I felt the heatsink was getting hot.

The heatsink is not supposed to get hot. It must be too small or not have enough fins if it gets hot. It is supposed to cool the IC. It is alright if it gets warm.

This experiment was done on the same protoboard, and it worked well for both of the IC's in the T-220 package, with and without the heatsink.

The IC needs a good heatsink and a TO-3 metal case.
If the TO-220 IC was properly fastened to a finned heatsink rated at 4 degrees C per Watt temp rise then the chip would be 280 degrees C which is way higher than its max allowed, so it should have shut down.

I got another pair of LM338 IC's. Now, instead of using a protoboard, I soldered them directly on a circuit board with the heatsink. The heatsink wasn't in direct contact with the case

It must be properly fasytened to a pretty big heatsink.

I got 2 new T-03 replacements. I thought maybe the heatsink and the plastic cover

What plastic cover? The metal case of the IC should have a thin film of thermal grease between it and the heatsink. Then they are bolted tightly together. Nothing is plastic.

DOES SOMEBODY HAVE THE FOGGIEST IDEA OF WHAT'S HAPPENING?? SOMEBODY COULD HELP ME??

Attach a schematic of the regulator circuit with parts values. Attach a photograph of the circuit, the heatsink and the plastic cover.
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Wow, audioguru. I really apreciate the patience you have to try to help me out. Thanks a lot.

Ok, first of all,


What plastic cover? The metal case of the IC should have a thin film of thermal grease between it and the heatsink. Then they are bolted tightly together. Nothing is plastic.


sorry for my lack of vocabulary (not a native english speaker). I think the thin film of thermal grease you say is the "plastic cover" I was talking about. I'll attach some pictures later. But yeah, it's transparent and thin. That must be what you were talking about.

What I meant is that the case was not in electrical contact with the heatsink. But it was firmly and tightly bolted to it, separated by the film.


The resistance of the stalled servo is 4V/2.4A= 1.67 ohms. Before the voltage dropped then the current was probably 7.5V/1.67 ohms= 4.5A. The heat was (10.65V - 7.5V) x 4.5A= 30W and if the IC was properly fastened to a finned heatsink rated at 4 degrees C temp rise per Watt (not big enough) then the chip would be 190 degrees C which is much too hot. Its max operating temp is 125 degrees C and it shuts down at about 150 degrees C.


Ok, your calculations must be right but they don't explain why the T-03 were permanently damaged (with Vout=0) though they didn't even feel warm to touch, while the T-220 ones still have Vout=7.5V and keep working although they have been working under a real temperature rise. And they also don't explain why the first LM338 I used (without a heatsink) still works (with the Vout drops I mentioned).


The heatsink is not supposed to get hot. It must be too small or not have enough fins if it gets hot. It is supposed to cool the IC. It is alright if it gets warm.


Well, I'm not sure about the difference between warm and hot. I just can say I don't burn my fingers when I touch it.


If the TO-220 IC was properly fastened to a finned heatsink rated at 4 degrees C per Watt temp rise then the chip would be 280 degrees C which is way higher than its max allowed, so it should have shut down.


Ok... probably. But it kept working at 4.8V with 2.5A for almost 20 seconds without changing its output!, while the ones in the T-03 package just got permanently damaged within 2 or 3 sec. and that is what doesn't make any sense to me.


Attach a schematic of the regulator circuit with parts values. Attach a photograph of the circuit, the heatsink and the plastic cover.


Ok, I'll do it as soon as possible. Meanwhile, I hope you take a look to my replies.

----------------------------------------------------------------------------------------------
But man... I wonder if sometime you could think on anything else besides me burning and melting my chips. The T-03 package was the one that got permanently damaged 6 times, and it didn't even feel warm. (And you know it's much harder to burn it than T-220).
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Hurray.

The problem of the 6 burnt LM338 is solved.

I bought one from a different store and soldered it into the very same circuit board where I had the others, and it wasn't damaged at all!

I conclude, all the LM338's from that store were defective.
-------------

I guess, now that we got over this, we can return to the issue of the "Not Enough Power".

Now that I've got this new IC, I get 3.0A at the output (a record max current for me), and the Vout now drops just down to 5.5 V.

(After some seconds of use -somewhat around 20- the heatsink felt warm to touch, and temperature stoped rising.)

The same comprobations were made
- Vin is constantly 10.5 and it doesn't drop
- There's no voltage drop from Vout to the motor connection
- Oscillations are much smaller (no more than 0.4V) because I'm using a 1000uF capacitor at the output (with protection diode).
- Vref drops from 1.25 to 0.9V.

-------------------------------------------------------------------------

Important note:

I measured short-circuit-current at the output and it was 2.8Amp (eventhough when did it with the motor it was 3.0A)

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Maybe you got counterfeight ICs.

Please post your circuit's schematic.
The LM338 is a voltage regulator with a rock-solid output voltage without oscillations. It doesn't need a huge output capacitor but needs to have a small one as shown on its datasheet.
The LM338 should pass about 9A before its output voltage drops.

Maybe you have its load current through a thin pcb track that also connects to one of its voltage-determining resistors. Then the voltage will drop when the load current rises.
Post your pcb layout for us to see.

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