Creating a voltage-controlled resistance

[Clint Sharp]

"David"


** Go get CANCER and die

Welcome to my killfile Phil. What an evil thing to say you worthless
piece of... Please feel free to flame the heck out of me, maybe while
you're battering against my kill file you'll leave everyone else alone.

 

[gearhead]

I'm testing rechargeable batteries and would like a voltage-controlled
resistance so I can easily adjust the load with a program.

I'd appreciate recommendations on how to design this.

Thanks,

David

view in fixed font:

batt +
|
|
|
s
,--------g n-channel
| d mosfet
| |
| /| |
| /-|----+
'--< | |
op amp\+|-, |
\| | Rsense
| |
Vref-----' |
gnd


This is a constant-current load,
controlled by Vref.
Current=Vref/Rsense
I know you asked for a "variable resistance,"
but you might want to reconsider.

 

[neon]

What you want is called dynamic load. any power transistor can be used for it just tie the collector to the output and vary the base input. there are limitations on big loads because beta decreases with hi current. you may actualy drive a sine ,trianglelar, square input if you wish. well that is the concept anyhow..

 

[Phil Allison]

"Clint Sharp"


** Go get CANCER and die


SHIT for BRAINS pommy PUKE !!





........ Phil

 

[Jamie]

I have a DAQ card to control the active resistance and sample the
current/voltage. Plan to use a shunt resistor to monitor the current. I
studied transistors in college years ago but am quite rusty. You're
suggesting using PWM to control the resistance? I'd like to drain the
batteries with a steady current, wouldn't that cause it to alternate?

Alternate?, Not really, the signal is just an ON/OFF, not a +/- signal.
for example.
setting a carry of 50khz for the PWM and adjusting the duty cycle and
passing it through a Resistor will be very smooth as long as the
batteries are connected.
You'll want to pick a resistor that will not allow the driving device
to exceed it's max I (current) if fully shorted.
this same resistor can also be used as the current monitor shunt
a nice like differential input would work nicely for this to give you
a linear current range.

P.S.
when deriving the signal around this resistor for the current sense,
you'll want some by pass caps at the op-amp input to smooth off the
PWM signal so that you can get a rather steady reading. WIth a carrier
of 50Khz, they should be rather small.

 

[David]

Alternate?, Not really, the signal is just an ON/OFF, not a +/- signal.
for example.
setting a carry of 50khz for the PWM and adjusting the duty cycle and
passing it through a Resistor will be very smooth as long as the batteries
are connected.
You'll want to pick a resistor that will not allow the driving device
to exceed it's max I (current) if fully shorted.
this same resistor can also be used as the current monitor shunt
a nice like differential input would work nicely for this to give you
a linear current range.

P.S.
when deriving the signal around this resistor for the current sense,
you'll want some by pass caps at the op-amp input to smooth off the
PWM signal so that you can get a rather steady reading. WIth a carrier of
50Khz, they should be rather small.

I should have said fluctuate. One of the factors I'm interested in studying
is current draw so using PWM, a constant current, and a constant resistance
will give me a few things to compare. I'm planning on using a few of the
circuits suggested so I can become more familiar with the components and the
differences in the designs.

Thanks for the suggestions all!

Dave

 

[Mike]

I should have said fluctuate. One of the factors I'm interested in studying
is current draw so using PWM, a constant current, and a constant resistance
will give me a few things to compare. I'm planning on using a few of the
circuits suggested so I can become more familiar with the components and the
differences in the designs.

Thanks for the suggestions all!

Dave

Maybe something like this for an adjustable constant resistance.


PNP Darlington
--------------- ------------
| | v / |
--- | --- |
Battery - | | |
| | | |
=== | .-. |
GND | | | |
| | | |
| '-' |
| | |
| | |
0.1 Ohm to 100 Ohm |--------/^\-------- |
Adjustable constant | /- +\ | |
Resistance | '-----' | |
| | | | |
| | | === |
| | | GND |
100k .-. | | |
| |<------- -------------|
Digital pot | | .-.
'-' | |
| | | 0.1 ohm
| '-'
.-. |
100 | | |
Ohm | | ===
'-' GND
|
|
===
GND

created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de


Mike


When truth is absent politics will fill the gap.