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oscarcot's Achievements


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  1. 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)
  2. Wow, audioguru. I really apreciate the patience you have to try to help me out. Thanks a lot. Ok, first of all, 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. 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). 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. 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. 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).
  3. 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
  4. Ok. Now I've got 2 replacements and a heatsink. If everything else was alright, I think it must work now already. I'll tell you later when I connect them.
  5. Agree. 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. 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... 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
  6. 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
  7. 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.
  8. 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
  9. 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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