Voltage2Current conversion

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lanternfish

Senior Member
Hi all

Have searched high and low but can't seem to find a solution to this problem.

Input: 0V to 1V analog signal.

Output: 0 -20mA current (approx 5V). :Linearity not an issue at present.

I need a analog solution :mad:

Can't get my head around transconductance op-amps.:confused:

Any pointers to suitable info will be appreciated.
 
premelec

premelec

Senior Member
Can you put a sense resistor in series with your current? An op amp or differential amp can then sense the current and the current can be varied by unbalancing the op amp which drives the current producing series element - MOSFET easiest... this is easiest if you can have one end of the device to be driven to V+ the other to drain of MOSFET [or collector of bipolar].

Give more detail on your app...
 
L

lanternfish

Senior Member
Give more detail on your app...]
Click to expand...
Oops!. Forgot to mention this will be driving RGB LEDs for a VGA demonstrator. All sync pulses slowed down appropropriately but voltages to remain the same.
The input (control) voltage (0 - 1V) needs to dim the LED.
LED will be common anode and multiplexed
PWM is not an option so has to be analog.

Have been looking at datasheets for various ic's so there may be a ready made solution out there.

My op-amp knowledge is virtually nil (old and disused) thus the request.
 
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L

lanternfish

Senior Member
Thanks for that.

It is that same old thing - knowing the right 'question' to ask. And variable just didn't make into my thoughts. Duh!

I tried to get a free sample from Maxim for something else once but get told I am a reseller. Not sure what happened there.
 
premelec

premelec

Senior Member
>http://www.uchobby.com/index.php/200...-current-sink/

That's pretty much the circuit I was suggesting - however note that if you just want a constant current SINK you don't even need the op amp - just put a resistor in the emitter or source of the transistor and when the current through that resistor = about 4 volts the collector / drain current will be 4/R [Vgs or Vbe need to be overcome to start conduction of the transistor]. The current set voltage goes to the base / gate of the appropriate transistor.

There are complicated multi channel IC drivers such as the TI TLC5922 16 channel unit - however transferring the control data to them is a chore...
 
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lanternfish

Senior Member
The current sink needs to be variable. A 0 - 1V input providing 0 - 20mA (or 0 to 10mA) current sink. With the links you guys have provided I should be able to get simulating (qin ISIS) at least. Cheers
 
B

boriz

Senior Member
“Output: 0 -20mA current (approx 5V).”

I=V/R

With a constant V, you need to vary the series resistance to vary the current. So you need a Picaxe controlled variable resistance. Many ways of doing this. I’d Google digital potentiometer for a start.
 
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lanternfish

Senior Member
There are certainly any number of options to do it digitally. Have been asked to design an analog solution. I did explain that digital is easier to implement but the answer was "No!"

Despite the analog-ness, an 08M is used to provide sync pulses and 'dot clock'. I was told it was for a VGA simulator so that students could see how a TV/VGA display is generated.

And as usual, my mind is thinking of other projects based this.
 
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boriz

Senior Member
“so that students could see how a TV/VGA display is generated.”

Voltage-to-current is not hard, but you’re talking about very fast signals. Too fast for Picaxe I think.
 
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lanternfish

Senior Member
Refer my post #3. All the sync pulses, etc are slowed down.

And i have not yet received clarification as to why the RGB inputs have to be analog as per a normal VGA output. Time will tell!
 
D

Dippy

Moderator
I don't even understand #3 or most of the rest of it...
Maybe a really decent clarification will spur others into giving you a slick solution. Maybe even a diagram.
And if you haven't received a full clarification then the rest of us may be muggered.
 
L

lanternfish

Senior Member
@ Dippy - You are so right. All I really wanted was a pointer in the right direction for a voltage controlled current sink. Analog not digital.

Anyhow, a flurry of emails has resulted in the revaling of a ludicrous expectation.

What was wanted was a way to " ... slow down the VGA output of a laptop so that students could see the display being generated on a normal monitor ..."

What!!!!!!!!!!!!!!! Maybe some SciFi gadget could slow time sufficiently so that the observer could see it happening.

Project abandoned!
 
P

papaof2

Senior Member
You might consider building a device that puts out a few lines of pixels to draw characters or simple graphics on a graphic LCD. The concept can then be expanded with a "the computer's faster and does it in color" explanation.

John
 
L

lanternfish

Senior Member
Hi papaof2

apparently they thought there was some magical way of slowing down a VGA output.

All is not wasted though as I now have a better understanding of analog current sinks and a referesher on opamps.

And worse, ideas for more bizarre projects
 
D

Dippy

Moderator
If all they wanted to do was, in effect, show a slow motion of a video signal then you could write that in VB or whatever your code of choice is.

"And worse, ideas for more bizarre projects "
- uh oh. Danger Will Robinson.:eek:
It's not going involve massive cubes of LEDs to show how fast your washing machine is going round is it?


I've got a far more useful project for you.
And if you could flog this to the manufacturers you'd make a bomb - so I want 20%.
A humidistat in the Tumble Drier output pipe. When your pants are dry the thing switches off. It should be standard in EVERY tumble drier as it will save gigaWatts of power world-wide. Far more than 'smart' fridges will ever achieve.
Fluff will be a problem, but the Mk2 belly-button fluff filter may sort that.
 
M

MartinM57

Moderator
I think Bosch have already thought of that...and implemented it (our 10yo tumble drier does that - and you tell it how dry you want the stuff (using terms only the current Mrs MartinM57 understands)) ;)
 
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Dippy

Moderator
Darn, there goes another one :)


Ah well, make it a retrofit with nice bare wires. Maybe you can use arcing to determine humidity. Anyway, just a thought. I'm sure there are far more useful things to do.
 
L

lanternfish

Senior Member
Dippy said:
..."And worse, ideas for more bizarre projects "
- uh oh. Danger Will Robinson.:eek:
It's not going involve massive cubes of LEDs to show how fast your washing machine is going round is it?
Click to expand...
Actually, you are not too far off.

It would involve a truck load of RGB LED's, a trailer load of 74595's, a Fisher & Paykell Smartdrive washing machine motor rewired as a generator, a netbook, some interfacing electronics.

Oh! And another motor that can spin up to 9000 or preferably 18000 rpm and a picaxe to control rpm (accurately).

Want to know more?
 
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