Bipolar Microammeter Design

N

Nick Naylor

Guest
Hello Everyone,

I have a device that can go from -25Vdc to +25Vdc. I want to attach a
device to the output of this that can give me a current measurement.
Basically, I want to have an analog signal that goes from 0Vdc to
5Vdc. When the signal is at 5Vdc, I have full power (1mA). This
analog signal should be linear according to the output current (0Vdc =
0mA, 1Vdc = 0.2mA, etc, etc). My concern is the changing polarity on
the output of my device that the meter is trying to read. Any
thoughts on this?


Thanks,
Nick
 
On 28 Apr 2004 13:43:45 -0700, the renowned naylor_nick@yahoo.com
(Nick Naylor) wrote:

Hello Everyone,

I have a device that can go from -25Vdc to +25Vdc. I want to attach a
device to the output of this that can give me a current measurement.
Basically, I want to have an analog signal that goes from 0Vdc to
5Vdc. When the signal is at 5Vdc, I have full power (1mA). This
analog signal should be linear according to the output current (0Vdc =
0mA, 1Vdc = 0.2mA, etc, etc). My concern is the changing polarity on
the output of my device that the meter is trying to read. Any
thoughts on this?
Click to expand...
I think that you need to provide more information or you'll waste a
lot of time with useless guesses as to what you need. Preferably just
tell us exactly what you are trying to do and be done with it.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
Spehro Pefhany wrote:
On 28 Apr 2004 13:43:45 -0700, the renowned naylor_nick@yahoo.com
(Nick Naylor) wrote:


Hello Everyone,

I have a device that can go from -25Vdc to +25Vdc. I want to attach a
device to the output of this that can give me a current measurement.
Basically, I want to have an analog signal that goes from 0Vdc to
5Vdc. When the signal is at 5Vdc, I have full power (1mA). This
analog signal should be linear according to the output current (0Vdc =
0mA, 1Vdc = 0.2mA, etc, etc). My concern is the changing polarity on
the output of my device that the meter is trying to read. Any
thoughts on this?


I think that you need to provide more information or you'll waste a
lot of time with useless guesses as to what you need. Preferably just
tell us exactly what you are trying to do and be done with it.

Best regards,
Spehro Pefhany
Click to expand...
If it's just the polarity that worries you make a current-sensing
circuit and follow it with a precision rectifier. If there's more you
don't know, tell us.

How are you going to sense the current?

How are you going to convert the sensed current to a voltage?

Etc.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
 
Tim Wescott <tim@wescottnospamdesign.com> wrote in message news:<1090a8f658kujda@corp.supernews.com>...
Spehro Pefhany wrote:
On 28 Apr 2004 13:43:45 -0700, the renowned naylor_nick@yahoo.com
(Nick Naylor) wrote:


Hello Everyone,

I have a device that can go from -25Vdc to +25Vdc. I want to attach a
device to the output of this that can give me a current measurement.
Basically, I want to have an analog signal that goes from 0Vdc to
5Vdc. When the signal is at 5Vdc, I have full power (1mA). This
analog signal should be linear according to the output current (0Vdc =
0mA, 1Vdc = 0.2mA, etc, etc). My concern is the changing polarity on
the output of my device that the meter is trying to read. Any
thoughts on this?


I think that you need to provide more information or you'll waste a
lot of time with useless guesses as to what you need. Preferably just
tell us exactly what you are trying to do and be done with it.

Best regards,
Spehro Pefhany

If it's just the polarity that worries you make a current-sensing
circuit and follow it with a precision rectifier. If there's more you
don't know, tell us.

How are you going to sense the current?

How are you going to convert the sensed current to a voltage?

Etc.
Click to expand...


I have an output that goes from -25Vdc to +25Vdc. That's all I have
at the moment. Everything about the current sensing circuit isn't
even started yet. I'd like to get ideas about making the sense
circuit before I start it.
You know what I have right now, and what I want is a 5Vdc analog
signal that I can attach to a multimeter. Ideally, the signal should
be 1Vdc/0.2mA. That's exactly what I am trying to do. I have no idea
how I want to do it yet, but any ideas would help get the ball
rolling.
 
Subject: Bipolar Microammeter Design
From: naylor_nick@yahoo.com (Nick Naylor)
Date: 4/28/2004 3:43 PM Central Standard Time
Message-id: <f0a720cc.0404281243.22a7e873@posting.google.com

Hello Everyone,

I have a device that can go from -25Vdc to +25Vdc. I want to attach a
device to the output of this that can give me a current measurement.
Basically, I want to have an analog signal that goes from 0Vdc to
5Vdc. When the signal is at 5Vdc, I have full power (1mA). This
analog signal should be linear according to the output current (0Vdc =
0mA, 1Vdc = 0.2mA, etc, etc). My concern is the changing polarity on
the output of my device that the meter is trying to read. Any
thoughts on this?


Thanks,
Nick
Click to expand...
It's possible your description of your problem could be a little better. I
hear you saying that you've got an input signal that goes from +25V to -25V.
You want to build a device that will read that signal, and will output a
current proportional to the input signal, ie 0.2 mA per volt. The catch is
that you want the output signal bounded at +/- 1 mA, so that any signal above
+5V will read as 1 mA, and any signal below -5V will read as -1 mA.

If you've got a floating milliammeter, this will do the trick (view in fixed
font or M$ Notepad):

Clipped Voltage to Current Converter
.-----.
| mA |
|meter|
.--o+ -o--.
| | | |
| '-----' |
1K 1 Watt 4K | |
___ ___ | |\| |
o--|___|--o------|___|---o----|-\ |
| | >---'
/-/ Vz = 3.3V .--|+/
^ | |/|
|2 X 1N4728A |
| ===
V Vz = 3.3V GND
/-/
|
===
GND

This circuit assumes you've got a low impedance input signal. If your signal
has impedance, subtract it from the 1K resistor. Just about any op amp will do
the job here -- provide a bipolar power supply to the op amp.

If this isn't good enough, provide a better description of what you need.

Good luck
Chris
 
Nick Naylor wrote:
Tim Wescott <tim@wescottnospamdesign.com> wrote in message news:<1090a8f658kujda@corp.supernews.com>...

Spehro Pefhany wrote:

On 28 Apr 2004 13:43:45 -0700, the renowned naylor_nick@yahoo.com
(Nick Naylor) wrote:



Hello Everyone,

I have a device that can go from -25Vdc to +25Vdc. I want to attach a
device to the output of this that can give me a current measurement.
Basically, I want to have an analog signal that goes from 0Vdc to
5Vdc. When the signal is at 5Vdc, I have full power (1mA). This
analog signal should be linear according to the output current (0Vdc =
0mA, 1Vdc = 0.2mA, etc, etc). My concern is the changing polarity on
the output of my device that the meter is trying to read. Any
thoughts on this?


I think that you need to provide more information or you'll waste a
lot of time with useless guesses as to what you need. Preferably just
tell us exactly what you are trying to do and be done with it.

Best regards,
Spehro Pefhany

If it's just the polarity that worries you make a current-sensing
circuit and follow it with a precision rectifier. If there's more you
don't know, tell us.

How are you going to sense the current?

How are you going to convert the sensed current to a voltage?

Etc.




I have an output that goes from -25Vdc to +25Vdc. That's all I have
at the moment. Everything about the current sensing circuit isn't
even started yet. I'd like to get ideas about making the sense
circuit before I start it.
You know what I have right now, and what I want is a 5Vdc analog
signal that I can attach to a multimeter. Ideally, the signal should
be 1Vdc/0.2mA. That's exactly what I am trying to do. I have no idea
how I want to do it yet, but any ideas would help get the ball
rolling.
Click to expand...
Why not go +/-5V then, and you can see the direction as well as the
magnitude of the current?

The easy way to sense the current is with a small shunt resistance;
sense the voltage and amplify it way up (pay attention to the offset
voltage of your op-amp). There are various chopper-stabilized op-amps
out there if you want something with a really low offset.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
 
On 28 Apr 2004 18:59:46 -0700, naylor_nick@yahoo.com (Nick Naylor)
wrote:


I have a device that can go from -25Vdc to +25Vdc. I want to attach a
device to the output of this that can give me a current measurement.
Basically, I want to have an analog signal that goes from 0Vdc to
5Vdc. When the signal is at 5Vdc, I have full power (1mA). This
analog signal should be linear according to the output current (0Vdc =
0mA, 1Vdc = 0.2mA, etc, etc). My concern is the changing polarity on
the output of my device that the meter is trying to read. Any
thoughts on this?
Click to expand...
....

I have an output that goes from -25Vdc to +25Vdc. That's all I have
at the moment. Everything about the current sensing circuit isn't
even started yet. I'd like to get ideas about making the sense
circuit before I start it.
You know what I have right now, and what I want is a 5Vdc analog
signal that I can attach to a multimeter. Ideally, the signal should
be 1Vdc/0.2mA. That's exactly what I am trying to do. I have no idea
how I want to do it yet, but any ideas would help get the ball
rolling.
Click to expand...
---
The sensing circuit isn't a problem, but you're not being at all clear
about what you want the output of the circuit to look like.

There are a few ways you can do what you want, one being to generate a
bipolar signal which you can feed to your multimeter, and another to
generate a scaled unipolar signal which you could feed to your
multimeter. In the first instance, 5V out would correspont to a 1mA
load and -5V out would correspond to -1mA. In the second instance, 0V
would correspond to -1mA, 2.5V would correspond to 0mA and 5V would
correspond to 1mA.

Which of these would you like to do, or do you have something else in
mind?

--
John Fields
 
Buy a zero-center meter. It will read either negative or positive dc
currents. I happen to have a few that I'll be happy to part with for
close to the cost of shipping.

Norm Strong


"Nick Naylor" <naylor_nick@yahoo.com> wrote in message
news:f0a720cc.0404281243.22a7e873@posting.google.com...
Hello Everyone,

I have a device that can go from -25Vdc to +25Vdc. I want to attach
a
device to the output of this that can give me a current measurement.
Basically, I want to have an analog signal that goes from 0Vdc to
5Vdc. When the signal is at 5Vdc, I have full power (1mA). This
analog signal should be linear according to the output current (0Vdc
=
0mA, 1Vdc = 0.2mA, etc, etc). My concern is the changing polarity
on
the output of my device that the meter is trying to read. Any
thoughts on this?


Thanks,
Nick
Click to expand...
 
John Fields <jfields@austininstruments.com> wrote in message news:<nqp190dbmhu3cb2sjm7orutqf7hlqhviob@4ax.com>...
On 28 Apr 2004 18:59:46 -0700, naylor_nick@yahoo.com (Nick Naylor)
wrote:


I have a device that can go from -25Vdc to +25Vdc. I want to attach a
device to the output of this that can give me a current measurement.
Basically, I want to have an analog signal that goes from 0Vdc to
5Vdc. When the signal is at 5Vdc, I have full power (1mA). This
analog signal should be linear according to the output current (0Vdc =
0mA, 1Vdc = 0.2mA, etc, etc). My concern is the changing polarity on
the output of my device that the meter is trying to read. Any
thoughts on this?

...

I have an output that goes from -25Vdc to +25Vdc. That's all I have
at the moment. Everything about the current sensing circuit isn't
even started yet. I'd like to get ideas about making the sense
circuit before I start it.
You know what I have right now, and what I want is a 5Vdc analog
signal that I can attach to a multimeter. Ideally, the signal should
be 1Vdc/0.2mA. That's exactly what I am trying to do. I have no idea
how I want to do it yet, but any ideas would help get the ball
rolling.

---
The sensing circuit isn't a problem, but you're not being at all clear
about what you want the output of the circuit to look like.

There are a few ways you can do what you want, one being to generate a
bipolar signal which you can feed to your multimeter, and another to
generate a scaled unipolar signal which you could feed to your
multimeter. In the first instance, 5V out would correspont to a 1mA
load and -5V out would correspond to -1mA. In the second instance, 0V
would correspond to -1mA, 2.5V would correspond to 0mA and 5V would
correspond to 1mA.

Which of these would you like to do, or do you have something else in
mind?
Click to expand...
I think the unipolar design would be better. 0V on my signal would
indicate 0mA, and 5V on my signal would indicate 1mA. I am a little
concerned about trying to measure a current so small, but I think I
can get something working.
 
You must log in or register to reply here.

Welcome to EDABoard.com

Sponsor

Back
Top