Smokin' POTentiometer

"Paul E. Schoen" <pstech@smart.net> wrote in message
news:47f08019$0$17377$ecde5a14@news.coretel.net...
Given the symptom (the pot smoking when turned about 1/2 way)
and assuming things are wired correctly, it means the transistor
draws more base current than the pot can handle. A transistor
with higher gain, or a darlington transistor, or a circuit with
2 transistors would draw less current from the pot.
Click to expand...
"However, using a more powerful transistor to run a more powerful fan is not
normally a viable option. The gain will typically be much lower so the
needed base current very soon gets to a level which will damage the
potentiometer track."

The above three methods covered
http://www.cpemma.co.uk/ef.html

A swap to a MOSFET voltage follower isn't really a good option, the voltage
drop is several volts rather than 0.7 or so.

--
Yorkshire rules, OK
 
"Paul E. Schoen" <pstech@smart.net> wrote in message
news:47f08019$0$17377$ecde5a14@news.coretel.net...
Given the symptom (the pot smoking when turned about 1/2 way)
and assuming things are wired correctly, it means the transistor
draws more base current than the pot can handle. A transistor
with higher gain, or a darlington transistor, or a circuit with
2 transistors would draw less current from the pot.
Click to expand...
"However, using a more powerful transistor to run a more powerful fan is not
normally a viable option. The gain will typically be much lower so the
needed base current very soon gets to a level which will damage the
potentiometer track."

The above three methods covered
http://www.cpemma.co.uk/ef.html

A swap to a MOSFET voltage follower isn't really a good option, the voltage
drop is several volts rather than 0.7 or so.

--
Yorkshire rules, OK
 
K

kayvee

Guest
Repost, I believe the message got lost in the SPAM...

Hello everybody. About a month ago I made a post titled "How do I
hook up my potentiometer?"

[http://groups.google.com/group/sci.electronics.basics/browse_thread/
thread/c9b1f3cd98cb2e2b/7faa05598f647c54?
hl=en&lnk=gst&q=Potentiometer#7faa05598f647c54]

Now, I ended up hooking it up John Fields' way with the diode in
parallel with the motor [12V]:

..V+>---+---------------+
.. | |
.. [220] |
.. | |
.. P /c
.. O<------------| NPN Power xsistor
.. T \e
.. | |
.. +---[Motor]-----+
.. | |
..GND>--+---[Diode>]----+

Now, I have been using a 12V power supply and a NPN with a high power
rating and I got a hold of a 500ohm POT (unknown power rating). I
hooked it up and all and it runs fine when the POT resistance it
0ohms, but once I start to turn the POT around half way, thus slowing
down the motor, the POT starts to smoke, which in my experience is
bad. Do I just need a POT with a higher power rating, or is there
another way of doing this?

Thanks guys!
 
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Hash: SHA1

kayvee wrote:
Now, I have been using a 12V power supply and a NPN with a high power
rating and I got a hold of a 500ohm POT (unknown power rating). I
hooked it up and all and it runs fine when the POT resistance it
0ohms, but once I start to turn the POT around half way, thus slowing
down the motor, the POT starts to smoke, which in my experience is
bad. Do I just need a POT with a higher power rating, or is there
another way of doing this?
Click to expand...
I didn't see the original post so I may be way off here.

The first thing I would try is to use a pot with a higher resistance. If
that doesn't work, I suggest switching over to a MOSFET which will
require almost nothing to flow through the pot in use.

- --
Brendan Gillatt | GPG Key: 0xBF6A0D94
brendan {a} brendangillatt (dot) co (dot) uk
http://www.brendangillatt.co.uk
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kayvee wrote:
Repost, I believe the message got lost in the SPAM...

Hello everybody. About a month ago I made a post titled "How do I
hook up my potentiometer?"

[http://groups.google.com/group/sci.electronics.basics/browse_thread/
thread/c9b1f3cd98cb2e2b/7faa05598f647c54?
hl=en&lnk=gst&q=Potentiometer#7faa05598f647c54]

Now, I ended up hooking it up John Fields' way with the diode in
parallel with the motor [12V]:

.V+>---+---------------+
. | |
. [220] |
. | |
. P /c
. O<------------| NPN Power xsistor
. T \e
. | |
. +---[Motor]-----+
. | |
.GND>--+---[Diode>]----+

Now, I have been using a 12V power supply and a NPN with a high power
rating and I got a hold of a 500ohm POT (unknown power rating). I
hooked it up and all and it runs fine when the POT resistance it
0ohms, but once I start to turn the POT around half way, thus slowing
down the motor, the POT starts to smoke, which in my experience is
bad. Do I just need a POT with a higher power rating, or is there
another way of doing this?

Thanks guys!
Click to expand...
Several ways, but the original answer was general and described
a way to control "a lot more current" with a pot. It was not a
specific solution for your individual application.

Knowing the specs would be helpful: how much current does the
motor draw and what transistor are you using? With the specs in
hand, we can determine what should be used.

Given the symptom (the pot smoking when turned about 1/2 way)
and assuming things are wired correctly, it means the transistor
draws more base current than the pot can handle. A transistor
with higher gain, or a darlington transistor, or a circuit with
2 transistors would draw less current from the pot.

Ed
 
"ehsjr" <ehsjr@bellatlantic.net> wrote in message
news:vckHj.60$Re7.19@trnddc04...
kayvee wrote:
Repost, I believe the message got lost in the SPAM...

Hello everybody. About a month ago I made a post titled "How do I
hook up my potentiometer?"

[http://groups.google.com/group/sci.electronics.basics/browse_thread/
thread/c9b1f3cd98cb2e2b/7faa05598f647c54?
hl=en&lnk=gst&q=Potentiometer#7faa05598f647c54]

Now, I ended up hooking it up John Fields' way with the diode in
parallel with the motor [12V]:

.V+>---+---------------+
. | |
. [220] |
. | |
. P /c
. O<----[100]---| NPN Power xsistor
. T \e
. | |
. +---[Motor]-----+
. | |
.GND>--+---[Diode>]----+

Now, I have been using a 12V power supply and a NPN with a high power
rating and I got a hold of a 500ohm POT (unknown power rating). I
hooked it up and all and it runs fine when the POT resistance it
0ohms, but once I start to turn the POT around half way, thus slowing
down the motor, the POT starts to smoke, which in my experience is
bad. Do I just need a POT with a higher power rating, or is there
another way of doing this?

Thanks guys!

Several ways, but the original answer was general and described
a way to control "a lot more current" with a pot. It was not a
specific solution for your individual application.

Knowing the specs would be helpful: how much current does the
motor draw and what transistor are you using? With the specs in
hand, we can determine what should be used.

Given the symptom (the pot smoking when turned about 1/2 way)
and assuming things are wired correctly, it means the transistor
draws more base current than the pot can handle. A transistor
with higher gain, or a darlington transistor, or a circuit with
2 transistors would draw less current from the pot.

Ed
Click to expand...
Add a 100 ohm resistor to the pot as shown. Measure the voltage drop to see
how much current is being drawn. A 500 ohm 1 watt pot is rated at I =
sqrt(P/R) = 44 mA. If the motor draws 1 amp and the transistor has a beta
of 20 you will just start to overload the pot.

It would be much better to use a MOSFET and get rid of the 220 ohm resistor
so you can get closer to the supply voltage out to the motor.

Paul
 
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