Is there anything wrong with putting a pot on base of transistor

Thread Starter

Mrdouble

Joined Aug 13, 2012
107
image.jpg I'm trying to build a ALL TRANSISTOR (BJT) small motor stall detector circuit. I have the cutoff portion working great but I'm having a heck of a time making current threshold variable. It's working pretty good finally but I was concerned about stability of transistor. Since its operating as a switch I'm not even sure stability needs to be considered. The circuit is below. Any improvement or suggestions are welcome. Thank you in advance.
 

MikeML

Joined Oct 2, 2009
5,444
Thermal stability might be a problem if the circuit is subject to large temperature swings.
What is the value of R1? What is the motor stall current?
 

Thread Starter

Mrdouble

Joined Aug 13, 2012
107
Thermal stability might be a problem if the circuit is subject to large temperature swings.
What is the value of R1? What is the motor stall current?
I have several unmarked motors. Some standard toy motors, some relatively high current and even a very tiny motor. Resistor is 1 ohm
 

crutschow

Joined Mar 14, 2008
27,018
Below is your circuit modified to allow better adjustment of the trip point.
The circuit triggers when the pot output voltage reaches about 0.65V.
(Note that the temperature change of the base-emitter voltage will reduce this current trip point about 0.3% for each degree C increase in temperature)
For the 1 ohm resistor in series with the motor (R2), the minimum trip point then would be about 0.65A.
Change the value of R2 to get the trip point range you want.
The simulation shows the trip point for wiper settings of 0% (green, wiper on top), 50%(blue) and 75% (red).

Motor Monitor.gif
 

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MikeML

Joined Oct 2, 2009
5,444
I was working on this too, but I had an interruption, so Cruts got his done sooner.

Mine is a bit different. I increased the gain by making the load resistor higher. I also used the pot as a rehostat (two terminals connected).

Note that the shunt resistor R1 should be tailored to the motor current per the formula R1=1/(stall current). I show V(out) as a function of R2, reading from left to right, R2 going from 1K to 10K in steps of 1K...

92.jpg
 

AnalogKid

Joined Aug 1, 2013
9,189
LT Spice. Linear Technology has free on their website an ***excellent*** SPICE circuit analysis program. Many people around here use it for general purpose schematic drawing as well as for its analysis tools. It is not a high performance schematic capture program that generates netlists for pc board layout, it is strictly for circuit analysis. I putter around in it; Wally and Mike are wizards.

ak
 
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