Please help me!

A

Alistair

Guest
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


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There are many things to consider..
1. 4017's can sink about 10mA with a 10v supply but have very little source
current.
2. Power transistors have very little DC gain so will need one or more
stages to get enough current gain.
3. If the FET or BiPolar output stage is not saturated the device will get
hotter than necessary
4. The output device and the driver stages will get hot as the device is not
perfect and will have an "ON" resistance...
by calculating the thermal resistance of the device, heatsink compound etc
the size of the required heatsink can be calculated to maintain the junction
temperature typically less than 125 degrees C. It is possible that the
heatsink may have to force cooled with a fan. Driver stages will probably
require heatsinking as well.
5. All biasing must be such that all devices are fully saturated, and in the
case of MOSFETS a voltage higher than the supply voltage may be needed to
bias it.
6. When driving incandescent lamps a limiting resistor is a good idea as the
inrush current can be up to 10x the running current, as a cold filament has
a much lower resistance than when hot.

It may be easier to purchase a readily available solid state relay which are
usually opto isolated, can probably be driven directly by the 4017 and have
an isolated heatsink.........

Hope this is of some help, Dick

"Alistair" <alistair.echlin@btinternet.com> wrote in message
news:bmp6vo$qgu$1@titan.btinternet.com...
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


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Outgoing mail is certified Virus Free.
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Click to expand...

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I would use a darlington transistor.I think the TIP42A is one. cant see how
you are going to get around not using a heat sink. Calculate the base
resistor to give you about 1ma of base current.


"Alistair" <alistair.echlin@btinternet.com> wrote in message
news:bmp6vo$qgu$1@titan.btinternet.com...
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


---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.512 / Virus Database: 306 - Release Date: 12/08/2003
Click to expand...
 
Alistair wrote:

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


---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.512 / Virus Database: 306 - Release Date: 12/08/2003
Click to expand...
Hi,

You should use something like a TIP112 (NPN darlington transisitor).
Use a 1k resistor to drive the base, ground the emitter, and connect the
collector to one side of the lights. The other side of the lights goes
to 12 volts.

You must either insulate the transistor from its heatsink, or 'float'
the whole heatsing on insulators to prevent the collector (case) from
shorting to ground.

Good luck,
Luhan
 
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