# Voltage to Current conversion? (Howland)

Discussion in 'The Projects Forum' started by eray, Oct 18, 2010.

1. ### eray Thread Starter New Member

Mar 31, 2010
7
0
hi,
i am working on a project and need to convert voltage input to current output with constant voltage. i am driving a power led which works at 4V and need to change its luminosity with 0-1 ampere.
0-1 voltage should be converted to 0-1 ampere linearly. my advisor told me to use howland current source for this application. what are your suggestions for this circuit? which op-amp should i use for current pump and for amplifiyng output current? thanks..

ps: graphically: http://img221.imageshack.us/img221/141/vtoc.jpg

2. ### wayneh Expert

Sep 9, 2010
12,394
3,246
Any general purpose op-amp will work fine. I'm using LM358 for something similar; find my last post regarding a "constant current" supply. Set your reference voltage from 0-1v with a resistor voltage divider, and monitor the voltage atop a 1 ohm shunt resistor between your LED and ground. The op-amp will try to keep those two voltages equal by driving the base of a transistor to control the current flow.

You'll have no problem dialing in the CURRENT and hence brightness you want. However, forget about the voltage, the LED will decide what voltage drop appears across it's poles in response to the current driven thru it.

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3. ### iONic AAC Fanatic!

Nov 16, 2007
1,422
68
So you want to use a 0 to 1V voltage adjustment to varry the current through a LED from 0 to 1A. The Howland current source is a novel way of doing just that. Wayne is right about the choice of opamp...a LM358 will be sufficient.

A linear scale for varying the brightness is not really practical since our eyes see light intensity logarithmically. I do believe they do make opamps that respond logarithmically.

iONic

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4. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Wayne's circuit will work, but it is not a Howland current pump. Here is a thread about a Howland current pump. The circuit in that thread is not suitable for what you want to do.

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5. ### gootee Senior Member

Apr 24, 2007
447
50
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