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Science Forum Index » Electronics - Basics Forum » lm338 overheating
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| roadrunner_uk_98 |
 Posted: Mon Feb 12, 2007 10:26 pm |
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Hi all
i am using the lm 338 if i remember rightly the 5amp adjustable reg and have set it up to put out exactly 8.5v to runn my ps2 form the car, the problem is the ps2 only draws 2amps but the chip overheats in a matter of seconds, even when mounted on a mettal case, any ideas as to why this is happening. thanx all |
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| Phil Allison |
| Posted: Mon Feb 12, 2007 10:31 pm |
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"roadrunner_uk_98"
Quote: i am using the lm 338
** Err .... an LM338T or LM 338K ??
The K is in a TO3, all metal pack.
if i remember rightly the 5amp adjustable reg and have set it up to put out
exactly 8.5v to runn my ps2 form the car, the problem is the ps2 only draws
2amps but the chip overheats in a matter of seconds, even when mounted on a
mettal case, any ideas as to why this is happening. thanx all
** Err ... metal = what ?
An old tin can or a proper aluminium heatsink.
....... Phil |
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| roadrunner_uk_98 |
| Posted: Mon Feb 12, 2007 10:54 pm |
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sorry phil its the k package i actually mounted it onto an ali box so the whole box would be the heatsink but it just gets to hot to touch
regards scott |
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| Phil Allison |
| Posted: Mon Feb 12, 2007 10:59 pm |
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"roadrunner_uk_98"
Quote: i am using the lm 338
** Err .... an LM338T or LM 338K ??
The K is in a TO3, all metal pack.
if i remember rightly the 5amp adjustable reg and have set it up to put out
exactly 8.5v to runn my ps2 form the car, the problem is the ps2 only draws
2amps but the chip overheats in a matter of seconds, even when mounted on a
mettal case, any ideas as to why this is happening. thanx all
** Err ... metal = what ?
An old tin can or a proper aluminium heatsink.
Quote: sorry phil its the k package i actually mounted it onto an ali box so the
whole box would be the heatsink but it just gets to hot to touch
** What size box , what gauge metal ?
This is like pulling teeth ......
......... Phil |
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| roadrunner_uk_98 |
| Posted: Mon Feb 12, 2007 11:10 pm |
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| Its 50mm x 50mm x 50mm dont know the gauge but it look heavy duty, purchased from rapid electronics |
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| Eeyore |
| Posted: Mon Feb 12, 2007 11:37 pm |
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roadrunner_uk_98 wrote:
Quote: Its 50mm x 50mm x 50mm dont know the gauge but it look heavy duty, purchased from rapid electronics
Almost certainly not up to the task.
Your Output voltage is 8.5 V. The load current is 2 A. What's the input voltage ? Then you can
calculate the dissipation.
What are you using to mount the regulator with ? How does it transfer its dissipation to the metal case
? Does the case get hot as well as the regulator ?
Graham |
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| roadrunner_uk_98 |
| Posted: Tue Feb 13, 2007 12:19 am |
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the input voltage is between 11.5 and 13.8 volt, the reg has thermal paste between it and th ecase and the case is untouchable  |
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| John Popelish |
| Posted: Tue Feb 13, 2007 1:23 am |
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roadrunner_uk_98 wrote:
Quote: the input voltage is between 11.5 and 13.8 volt, the reg has thermal paste between it and th ecase and the case is untouchable
For the 13.8 volt case: The power from the regulator with a
2 amp load is (13.8-8.5)*2=10.6 watts. Enclose a 7 watt
Christmas tree bulb in the box and see how long you can hold
it. It is about a tenth as large as it has to be. |
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| Phil Allison |
| Posted: Tue Feb 13, 2007 3:02 am |
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"roadrunner_uk_98"
Quote: Its 50mm x 50mm x 50mm dont know the gauge but it look heavy duty,
purchased from rapid electronics.
** That will run very hot ( likely about 100 degrees C) with 10 watts
dissipation - especially if not in an open, free air environment.
The IC has internal thermal shutdown so will save itself.
Try a bigger box or add a small 12 volt a fan.
........ Phil |
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