Does a 1/4 watt resistor need any airflow if it is disapatin

S

SA Dev

Guest
Hi,

I've made a really small LED light that I'm planning to use and I was
thinking about shrink tube wrapping the resistor in with the leads on the
LED. The LED is rated for 30ma continuous, but I'm only putting around 12ma
through it. Would this be an issue for heat?

Thanks,

SA Dev
 
"SA Dev" <nospam38925@forme.com> wrote in message
news:pP2dnbMCtvjgvdnfRVn-sg@valortelecom.com...
Hi,

I've made a really small LED light that I'm planning to use and I was
thinking about shrink tube wrapping the resistor in with the leads on the
LED. The LED is rated for 30ma continuous, but I'm only putting around
12ma
through it. Would this be an issue for heat?

Thanks,

SA Dev
Click to expand...
If the voltage drop across the resistor multiplied by the current through it
is
close to the 250 mW resistor maximum, it is way too much and there is no
safety margin.
 
On Fri, 25 Mar 2005 08:30:21 -0600, "SA Dev" <nospam38925@forme.com>
wrote:

Hi,

I've made a really small LED light that I'm planning to use and I was
thinking about shrink tube wrapping the resistor in with the leads on the
LED. The LED is rated for 30ma continuous, but I'm only putting around 12ma
through it. Would this be an issue for heat?
Click to expand...
---
You don't say what your supply voltage is, or what ambient temperature
the device is supposed to work in, but if the resistor's only
dissipating 20mW I'd go out on a limb and say you'd be fine.

--
John Fields
 
Hi John,

You don't say what your supply voltage is, or what ambient temperature
the device is supposed to work in, but if the resistor's only
dissipating 20mW I'd go out on a limb and say you'd be fine.
Click to expand...
The drop across the resistor is 1.8v and the current is 11.3ma, if I
mutliply these I get 20.3mw, right? Ambient temp will be room temp ~75 deg
F.

Thanks,

SA Dev
 
On Fri, 25 Mar 2005 10:29:34 -0600, "SA Development"
<nospam38925@forme.com> wrote:

Hi John,

You don't say what your supply voltage is, or what ambient temperature
the device is supposed to work in, but if the resistor's only
dissipating 20mW I'd go out on a limb and say you'd be fine.

The drop across the resistor is 1.8v and the current is 11.3ma, if I
mutliply these I get 20.3mw, right?
Click to expand...
---
Right.
---

Ambient temp will be room temp ~75 deg F.
Click to expand...
---
Go for it!

--
John Fields
 
On Fri, 25 Mar 2005 10:29:34 -0600, "SA Development"
<nospam38925@forme.com> wrote:

Hi John,

You don't say what your supply voltage is, or what ambient temperature
the device is supposed to work in, but if the resistor's only
dissipating 20mW I'd go out on a limb and say you'd be fine.

The drop across the resistor is 1.8v and the current is 11.3ma, if I
mutliply these I get 20.3mw, right? Ambient temp will be room temp ~75 deg
F.
Click to expand...
Sounds about right and safe.

A question not for you, exactly, but to anyone else would be "What is
the C/W specification for a 1/4 watt resistor body in still,
relatively dry air?" It would be hard to imagine it being much worse
than the Rja=200 or so for a TO-92.

You'd usually just multiply that Rja spec by the watts the resistor is
dissipating to get the relative change in equilibrium temperature. If
the value were 200, then the change would be about 4 Celsius or less
than 8F. That would seem quite a safe change, over 75 F ambient.

I'd suppose that you *could* wrap the resistor in thermal insulation
and that if you really tried hard you might cause a problem. But the
Rja would have to be in the area of 2500 or so before a 50 Celsius
change would be seen. Hard to imagine that big a number for Rja
unless you really worked at causing a problem.

Jon
 
On Fri, 25 Mar 2005 08:30:21 -0600, SA Dev wrote:

Hi,

I've made a really small LED light that I'm planning to use and I was
thinking about shrink tube wrapping the resistor in with the leads on the
LED. The LED is rated for 30ma continuous, but I'm only putting around 12ma
through it. Would this be an issue for heat?
Click to expand...
If you're running it at ~20V or more, yes. Otherwise, no.

Cheers!
Rich
 
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