In need of some help!

Hello again,

I still have a problem with this transistor setup. 4017 (5 Stage Decade
Counter) output to an NPN (BFX34) to drive 2 21 watt 12 volt lamps. I have
tried a suggestion of using a PNP FET (BUZ11A) whit a resisitor of 20 ohms
onto the base. The transistor gets hot and so does the resistor. I have also
tried a TIP42A, and found that with it mounted to the metal structure, it
shorts out to earth - which the lamps are connected to and eventually the
vehicle. So what I really need is a transistor that can be driven either
straight from the 4017 or via the NPN transistor from the 4017, capable of
sinking 5 amps without getting hot,so I dont have to mount it to the steel
frame.Please dont suggest using relays as they are totally out of the
question.

Many thanks

Alistair

 

A BUZ11 is an N channel MOSFET, (not a PNP FET???) and in order to use
it as a high side driver you need to connect the drain to +12V, the
source to your lamp load, and drive the gate to substantially higher
than 12V in order to get it to work without dissipating a lot of power.

If you use a MOSFET in a TO-220 package the tab will almost certainly be
connected to the drain, (check the data sheet) so if you're using a P
channel MOSFET with the tab connected to chassis ground and the source
connected to +12V what you'll be doing when you turn the FET on is
connecting +12V to ground, NOT a good thing. You might want to try
this:

+12 +12
| |
| |
[1K] |
| S
+--------+-----G IRF4905
| |K D
C [Z15V] |
4017OUT>--[10K]--B 2N4401 | [LAMPS]
E | |
| | |
GND>--------------+--------+------+

The 10k base resistor is assuming you're running the 4017 on 12V...

An IRF4905 has a channel resistance of 0.02 ohms, so fully enhanced the
MOSFET will dissipate about 1/2W with a 5A load, so you should need no
heat sink. The FET is rated for 260A max, (check the data sheet for
your 5A VS temp) so you should be OK even with a lamp inrush current of
50A.

 

That's why they sell insulated mounting kits with mica or teflon washers to
allow heat transfer to the metal and prevent an electrical connection.

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I have also

Hi, Alistair. You're almost there. You need a heat sink. Use the NPN
darlingtons you were directed to in another post, and purchase two RS Premium
Heat Sink $1.59 USD Catalog #: 276-1368. Since a darlington being used as a
switch has about 1V from collector to emitter, you will have to dissipate about
1V * 5A = 5 Watts of power. The heat sinks will do the job -- just make sure
they're not touching the chassis or anything that's grounded.

Good luck.
Chris

 

Well thank you all for the overwhelming response to my tiresome problem!!!
) I will let you all know the outcome of the steps Ive taken.

Alistair