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In need of some help!

A

Alistair

Jan 1, 1970
0
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
 
J

John Fields

Jan 1, 1970
0
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.

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.
 
T

the Wiz

Jan 1, 1970
0
Alistair said:
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

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.
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

More about me: http://www.jecarter.com/
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johnecarter [email protected] mindspring dot.dot com. Fix the obvious to reply by email.
 
C

CFoley1064

Jan 1, 1970
0
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

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
 
A

Alistair

Jan 1, 1970
0
Well thank you all for the overwhelming response to my tiresome problem!!!
:eek:) I will let you all know the outcome of the steps Ive taken.

Alistair
 
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