# calculating heatsink size

Discussion in 'General Electronics Chat' started by Fenris, Nov 9, 2007.

1. ### Fenris Thread Starter Active Member

Oct 21, 2007
288
2
hi all

having succesfully got my sound to light circuit
working, thanks to the forum, i know have to address
heatsinking. im hoping i have understood the maths of
heatsink calculation so here goes.
circuit supply 12VDC

A TIP41 with a max of 6A continuous, Switchs 2*12V/20W
halogen G4 bulbs.

the bulbs draw 3.33A leaving 2.67A across the TIP41

2.67A*12V=32.04W

i need to keep the temp of the TIP below 100oC inc
ambient tempreture. so......................

100oC/32.04W=3.12oC/W
meaning the spec of my heatsink must be =<3.12oC/W

this seems ok i dare say i have it wrong and explianing it to me wont help
my english is better than my maths, my concern is i cant find a heatsink
of the value or less as calculated.

Regards

Fenris

2. ### nomurphy AAC Fanatic!

Aug 8, 2005
567
12
Are you using the TIP41 as a switch, or as some kind of regulator. I don't understand the 6A - 3.33A = 2.67A ??? Also, why would the power be 12V * 2.67A ?? Isn't the 12V across the bulbs, not the xstr. Are you using 24V and dropping half across the xstr?

If you are using the TIP41 as a switch, and the total load is 3.33A, then the current through the TIP41 is 3.33A. Power dissipated in the xstr would be the CE "on" voltage of the transistor times the current through it (provided it's in saturation, and is not being used as a linear regulator). However, it would be easier to look at the TIP41's dissipation curves for 7A or so (this is about twice the current and provides headroom).

3. ### Fenris Thread Starter Active Member

Oct 21, 2007
288
2
hi heres a daigram of the circuit
for the TIP41C, from your
comment its obvious i have
failed to understand the
maths and relevant data
i need

regards

Fenris

File size:
22.8 KB
Views:
73
4. ### nomurphy AAC Fanatic!

Aug 8, 2005
567
12
So, it appears you are varying the bulb brightness based upon the input?

If so, then worst-case would be at 6V across the xstr or:

3.3A = 2 * 20W / 12

3.64 ohms = 12V / 3.3A

1.65A = 6V / 3.64 ohms

10W = 1.65A * 6V

The TIP41 will dissipate 10W if it is constantly half-on, the 13mA of LED current is not significant.

5. ### Fenris Thread Starter Active Member

Oct 21, 2007
288
2
hi

thanks for that so if i have followed the calculation side
correctly im going to need a heatsink =<10oC/W?????
lots to choose from in that case, cheers

regards

Fenris

6. ### nanovate Distinguished Member

May 7, 2007
665
1
What is the max ambient temp?

7. ### Fenris Thread Starter Active Member

Oct 21, 2007
288
2
hi nano

ambiant temp for the uk is less than 25oC most of the time
the unit will be in a variaty of enviroments as in indoors, out-
doors. something to take into acount also is that the unit will
probably only operated in short bursts, so i think its way over
spec'd for the heatsink but well covered.

Regards

Fenris

8. ### cumesoftware Senior Member

Apr 27, 2007
1,330
11
You should consider an ambient temperature "inside the case" of 40ºC. It is a good safety factor.

9. ### Fenris Thread Starter Active Member

Oct 21, 2007
288
2
hi there

the case will be well vented with slots top and sides near the heat sink
to allow good airflow. that said i should probably go for a heat sink
rated @ =<5-6oC/W? im figuring off the top of my head that that will
do the trick (cant do the maths im inumerate with anything over and above
normal maths)

regards

Fenris