LM318 (LM317) get really hot... not as I expected - can someone help?

Thread Starter

rvflores

Joined Nov 20, 2015
7
I have small project where I used a power supply supplying 15Vdc wired to LM318T to output 5.1Vdc. I am trying to drive a load of 21 LEDs. Each LED is wired in parallel with 100 ohm resistor. I was trying to "turn on" the load with a transistor hook up with a photo-resistor so it turns the LEDs on at night. By my calculations: 3.2V with 100 ohm resistance = 19mA = ~20mA. Load total = 20mA x 21 = 420mA. P = 3.2V x 0.420A = ~1.3W. The transistor is 1300d. I hooked up 100 ohm resistor to same 5.1V supply to the base of the transistor. The load is between the 5.1V supply to the collector. Emitter to ground. The LM318T begins fine but then begins to heat up rapidly. I do have the LM318T mounted to a heat sink. Is it suppose to get really hot? Did I miss calculate?
 

wayneh

Joined Sep 9, 2010
16,114
So your op-amp is being asked to supply (5.1-0.7)V/100Ω=43mA into the base of the transistor. The data sheet says the absolute maximum output current for your amp is about 21mA, falling to 18mA at 70°C. You're surprised that it's getting hot?
 

Thread Starter

rvflores

Joined Nov 20, 2015
7
@Dodgydave - I used the wife's computer which came w/Win8. That was a bust. Upgraded to Win10. Hate it! I thought Win7 was finally a good, stable product then they put this garbage out.... but that's a different thread. ;-)

IMG_20151120_122627_800x511.jpg
 

Thread Starter

rvflores

Joined Nov 20, 2015
7
@wayneh Yeah, well, I guess I read the spec sheet wrong. Hate when that happens. I thought the transistor needed the 43mA Ib to turn fully on for 420mA load.
 

wayneh

Joined Sep 9, 2010
16,114
@wayneh Yeah, well, I guess I read the spec sheet wrong. Hate when that happens. I thought the transistor needed the 43mA Ib to turn fully on for 420mA load.
Well yes, it probably does. But that op-amp can't even supply the base current, let alone all the LED current as well. It's severely overloaded. As noted, if you want 5V, use a 7805. Or you could just use different resistors with your 15V supply voltage is, as long as it is somewhat regulated.​
 
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Thread Starter

rvflores

Joined Nov 20, 2015
7
I replaced the not-so-solar outdoor lamps with the LEDS so I don't want to re-solder new resistors! ;-)
Thanks again for the circuit analysis! Much appreciated!
 

#12

Joined Nov 30, 2010
18,159
The LM318 is an op amp.
The schematic in post #6 had me convinced it was a voltage regulator.
Sorry, I don't seem to be quite up to speed right now.:oops:
I've been crawling around a truck today, looking for a ground fault. Brain might be affected by overheating.:confused:
 

absf

Joined Dec 29, 2010
1,942
Refering to the schematic drawn by TS at post#6, I think he must have mistaken the LM318 (op-amp) for LM317 (voltage regulator). See simulation attached.:)

lm317T.PNG

There is no way an op-amp can fit in that circuit.:D


Allen
 
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ScottWang

Joined Aug 23, 2012
6,829
LM317 is a voltage regulator and LM318 is an op amp, the number as a series and relative, ha ha, these are the numbers easy to confuse.
 

Thread Starter

rvflores

Joined Nov 20, 2015
7
Yes, I missed typed - it is a lm317t. Which, after confirming the spec sheet says it can handle 1.5A loads.

So I am still puzzled?
 

Alec_t

Joined Sep 17, 2013
10,438
15V in, 5V out and a current of 420mA means that the LM317T is being asked to dissipate 4.2W. That will require quite a large heatsink if you want the IC to run cool. Sure it could handle 1.5A, but that would require an even larger heatsink.
 

spinnaker

Joined Oct 29, 2009
7,837
Yes, I missed typed - it is a lm317t. Which, after confirming the spec sheet says it can handle 1.5A loads.

So I am still puzzled?
18 posts later. :eek::eek::eek:
You weren't the only one that was puzzled.


If it is a lm317 and indeed wired as in your post then it is wired wrong.

In addition, you are dropping 10 VDC, It will get hot even if it is really wired correctly. Use a input voltage closer to you required output or use a switching power supply. They can be purchased on eBay.
 
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