Trimmer on a power transistor base, then VCC to collector and 0v to emitter=
 !?! Put some DMMs in line to see the load and adjust the trimmer as needed=
 ?



Regards,

Steve Maroney
Business Computer Support, LLC
504.904.0266  Office
504.914.4704 Mobile
866.871.7797  Fax


-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of=
 Spehro Pefhany
Sent: Saturday, March 03, 2012 5:58 PM
To: Microcontroller discussion list - Public.
Subject: Re: [EE] Programmable R Load

At 06:07 PM 3/3/2012, you wrote:
>What I need to test & verify is the current sense on the power supply.
>Does it set a flag if the current  is outside of the limits?  If it is
>outside, what is the actual trip point?
>
>The problem is that I need to be able to vary the current through a load
>that is not referenced to ground.  It is reference to the +12v with the
>low side varying between 0 and 12v.

An active circuit that simulates a resistor will have a settling time
as well. It won't do you much good if the PWM rate is 1kHz and it
take 10msec to settle.

The idea is trivial (draw a current that has a fixed
proportionality to the voltage, where the proportionality is determined
from the reciprocal of the simulated load resistance). I'll let you
decide whether you could pull this off cheaper than buying an off the shelf
solution (hint- multiplying DAC). The control loop will probably have
to be analog. That it is not referenced to ground is only a minor
inconvenience.

>My specifications for accuracy are not the real problem.  I can measure
>the real current with enough precision but I need a way to vary the load
>repeatability.
>
>-Phil-

If discrete steps are acceptable, relay contacts switching resistors is a
very easy approach. 6 contacts & 6 resistors can give you 64 steps. It
should be very repeatable if done right.


>On 3/3/2012 1:50 PM, Spehro Pefhany wrote:
> > At 04:17 PM 3/3/2012, you wrote:
> >>     I am trying to build a controllable resistive load for a test syst=
em
> >> and I was hoping that someone has solved this problem before.
> >>
> >>     The over all design is meant to drive four parallel LEDs&  resisto=
r
> >> pairs and to detect if any of them are open or shorted. The driving
> >> circuit has a 12v supply with a current sense and the low side is an F=
ET
> >> that is PWM controlled to Gnd.
> >>
> >>     I need to design a variable load that can sink between 20mA and 30=
0mA
> >> and will emulate the LED/Res load.  This means that the resistance wou=
ld
> >> be 60 to 600 Ohms.
> >>
> >>     I have looked at the programmable resistance chips like the MCP401=
2
> >> or the AD5246 but the operating current range is too small.
> >>
> >>     I have also looked at commercially available ones but they are ver=
y
> >> expensive and meant for carrying 15Amps.
> >>
> >>     Cost is not a significant factor since I only need to build about
> >> three of these but I would hate to waste couple of thousand dollars wh=
en
> >> a couple of hundreds (or less) would do.
> >>
> >>     Any suggestions would be appreciated.
> >>
> >> -Phil-
> > What are your specifications for accuracy? The LED + resistor pairs
> > do not behave quite like a resistor. Are there multiple LEDs in series
> > in each pair (seems like you'd want to do  that with 12V available
> > and the max single junction voltage is only a few volts.
> >
> > Could you use signal relays (either electromechanical or MOSFET) and fi=
xed
> > LEDs and/or resistors to test at your limit values?
> >
> > --sp
> >
> >
> >
> >
> >
> >
> > -
> >> http://www.piclist.com PIC/SX FAQ&  list archive
> >> View/change your membership options at
> >> http://mailman.mit.edu/mailman/listinfo/piclist
>
>
>--
>--------------------------------
>Long ago when men cursed and beat the
>ground with sticks,  it was called witchcraft..
>
>Today, it's called golf.
>
>--
>http://www.piclist.com PIC/SX FAQ & list archive
>View/change your membership options at
>http://mailman.mit.edu/mailman/listinfo/piclist

Spehro Pefhany --"it's the network..."            "The Journey is the rewar=
d"
speff@interlog.com             Info for manufacturers: http://www.trexon.co=
m
Embedded software/hardware/analog  Info for designers:  http://www.speff.co=
m



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