Power transistor overheating

Discussion in 'Electronic Basics' started by talisman, Dec 22, 2005.

1. talismanGuest

I am building an electric fence using a 12 volt battery to energise
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 pe
second
Problem is that TIP41 gets hot enough to destroy itself after about 3
pulses
Difficult to measure current as spark upsets multimeter
Using a lamp as load I measure 1.6 amps and TIP41 still gets ho
enough to burn
What am I doing wrong !!
Should anyone volunteer to help me I can email the circuit which I go
from the net but with no contact address

2. Guest

recheck your wiring....and heatsink the transistor....

3. John PopelishGuest

A 30 ms pulse is way too long. The coil has reached a steady state
maximum current, long before that. Think of a coil driving the sparks
for an 8 cylinder engine turning 3000 RPM. That is 3000/2 * 8 sparks
per minute or 200 per second. Even if the charge time was the whole
time between sparks (and it is actually more like half that), that
would mean that the coil had 12 volts applied about 1/200 s = 5 ms.
And it still makes sparks just fine.

The second thing to be sure of is that the transistor has enough base
drive current that it is well saturated at 12 volts / 1.7 ohms = 7.1
collector amperes. That may be hard to accomplish with a TIP41.

According to its data sheet:
http://www.onsemi.com/pub/Collateral/TIP41A-D.PDF
the gain falls off dramatically for collector current above 3 or 4
amperes, especially if you expect it to stay in saturation.

Another potential (pardon the pun) problem, especially if you are
using a TIP41 (not suffix B or C) is that it may stand only a little
more 40 volts before breaking down right after turn off, when the coil
puts a large voltage stress on it. This puts a lot of power into the die.

You may email me the schematic, if you wish, and I will critique it
and make suggestions for better performance.

4. Pooh BearGuest

The TIP41 is a power transistor. It can handle the volts and current but
it needs the help of a *heatsink* to get the resulting heat away.

Used in free air with no heatsink it'll overheat if much over 3 watts is
dissipated in it typically.

If you don't understand what I'm talking about you need to go learn some.

Graham

5. Rich GriseGuest

I'm with John Popelish on this one. The 555 isn't providing enough
base drive. I'd try to find a darlington that's rated for a much higher
voltage, and put a transzorb that's rated at the transistor's BVCEO across
it, to snub the spike at whatever the breakdown voltage of the transistor
is.

Or, you could do it completely different - lose the transistor, and
buy or build a little DC-DC converter to give you a few hundred
volts, charge a cap with it, and periodically fire an SCR to discharge
the cap through the coil.

Good Luck!
Rich

6. ehsjrGuest

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

Ed

7. Sjouke BurryGuest

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.

8. ehsjrGuest

All excellent points. With the transistor I mentioned, he'll
have 1.8 V C-E (from the datasheet) at 1.6 amps (from his
measurement), for 2.88 watts. He needs ~100 mA base drive
(from the datasheet), and the 555 can source or sink 200 mA
(from the datasheet), so the BU941ZT will saturate easily.
It can handle 150 watts (from the datasheet), 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

9. ehsjrGuest

Grumble. Left off a sentence:
He can work up from there, increasing the pulse length until
he gets the desired voltage, without overheating or saturating
the coil.

Ed

10. talismanGuest

some with modest success
However I have triumphed. My circuit is now fully functional an
scaring the hell out of the possums which attack my vegetabl
garden
The problem was that the TIP41 was conducting for the 970 ms and of
for 30ms. I did not know how to find this out. The LED flashed, tha
is all
I invested in a PC sound card oscilloscope and while difficult t
interpret because of the AC coupling, it enabled me to see the pulse
and set the pulse length.
It now conducts for 2ms and rests for 998. It does not overheat, i
fact it does not even heat at all!
2ms is the optimum, shorter weakens the spark, longer does not improv
it
Now to more exciting projects for which I will no doubt again solici