Re: Power transistor overheating

Sjouke Burry wrote: 
ehsjr wrote:

talisman wrote:

I am building an electric fence using a 12 volt battery to energise a
NE555 timer to drive a TIP41C transistor into a car ignition coil . Coil resistance is 1.7 ohms as measured.
Drive pulses to TIP41 are about 30ms duration at one pulse per
second.
Problem is that TIP41 gets hot enough to destroy itself after about 30
pulses .
Difficult to measure current as spark upsets multimeter.
Using a lamp as load I measure 1.6 amps and TIP41 still gets hot
enough to burn.
What am I doing wrong !!!
Should anyone volunteer to help me I can email the circuit which I got
from the net but with no contact address.


Use a BU941ZT on a nice heatsink.  It's a darlington made for driving
ignition coils.  The data*** is here:
http://eu.st.com/stonline/products/literature/ds/5288.pdf

Ed 

If you have the on/off the wrong way around,it gets very hot,
else if the transistor is not saturated(enough base current),
it dissipates more ,and last ,30 msec might be a bit long,and
if so, the high voltage might become dangerous.(And drain your
battery faster then you would like).So make the pulse time as
short as possible,for the output voltage to shock in stead of
kill.It also saves the battery.


All excellent points.  With the transistor I mentioned, he'll
have 1.8 V C-E (from the data***) at 1.6 amps (from his
measurement), for 2.88 watts.  He needs ~100 mA base drive
(from the data***), and the 555 can source or sink 200 mA
(from the data***), so the BU941ZT will saturate easily.
It  can handle 150 watts (from the data***), so with a nice
heatsink 2.88 watts won't be a problem.  So it should be able
to handle the op's requirements, easily.  I wouldn't want
to run the base drive the wrong way round either! However,
more to the point, the 30 mS figure is not necessarily the best
choice. If it drives the ignition coil into saturation
or if the voltage produces is too high that needs to be
reduced.  I think 30 mS is too long - I'd think 1 mS would be
a good target figure.

Ed
.

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