Schematic for 0-2 Amps voltage regulated output

So you need a current source that goes from 0 to 2 amps, and is controlled by an input voltage of 0 to 2 volts, right?

The typical way to do this is with an opamp. The current through a transistor is measured using a resistor, and that measurement is fed back to control the output.

If you measure the output using a 1 ohm resistor, then the current through it has a 1 to 1 relation with the voltage across it. So, 0A to

2A will create a voltage of 0V to 2V across the resistor.

If you get an opamp, you can use it to control the output given the input.

Now, it sounds like you want to have a ground referenced current source, meaning that your load will be between the source and ground. To do that, you need some way to measure the current coming out of the transistor (which is at some higher voltage) and compare that to your 0 to 2 volt control voltage (which is referenced to ground). Thus, you need a way to 'level shift' the control voltage appropriately. Thankfully, an opamp can do this for you.

Here is a schematic: (please view with a fixed-space font. Othewise, the lines don't line up)

Vin=10V o---o------------------o----------o------. | | | | | | | | .-. | | .-. | | 100k | | | | 1 ohm | | | | | | 0.1% '-' | | '-' | 100k | | | | ___ | .----)------o | .-|___|--)--. | | | | | | | | | | | 100k | | | | |\| | CTRL | ___ | |\| | '--|-\ |< ---)--|___|--o------|-\ | | >---| PNP 0-2V | | >-o-----|+/ |\ up to 2W o----------------|+/ |/| | | |/| | | | | | | .-. | | '----------o Out 0-2A | | 100k | | | | | | '-' | | | | | | | | o----o------------------o----------o-----------------o Out GND GND (created by AACircuit v1.28.5 beta 02/06/05

For this to work, the opamps will need to be 'rail-to-rail' input and output opamps. You can get these in ICs that contain two (dual packages).

The first opamp takes your 0-2V input, and maps it to 20-18V. The second opamp takes that, and tries to make the voltage across the resistor equal to it by controlling the voltage on the PNP transistor's base.

The voltage 'compliance' is defined as the voltage range the thing can produce. This will produce a voltage output between 0 and perhaps 7.8V. If you need more voltage, you can increase the Vin to something larger. It'll always have a voltage drop of about 2.2V at full current.

Also, there is no limit on the current at 2A. Thus, your voltage source should limit the current it'll supply to 2A.

--
Regards,
   Robert Monsen

"Your Highness, I have no need of this hypothesis."
     - Pierre Laplace (1749-1827), to Napoleon,
        on why his works on celestial mechanics make no mention of God.

Correction, from 10 to 8 V. Also, make sure the 1 ohm sense resistor is rated for at least 2.5W.

--
Regards,
   Robert Monsen

"Your Highness, I have no need of this hypothesis."
     - Pierre Laplace (1749-1827), to Napoleon,
        on why his works on celestial mechanics make no mention of God.

(small

Hi, Mickey. The simplest way to have a voltage-controlled current source requires that your load can be floating. Given that, you can do the whole thing, including perfboard, with Radio Shack parts like this (view in fixed font or M$ Notepad):

' VCC ' + ' |Iout = 0-2A ' |---> + ' '------o ' VCC ' + .------o 'Vin | | - '0-2V ___ |\| | ' o-|___|-o----o-------|+\ ___ |/ ' 750 | | | >---o-|___|--| TIP120 ' .-. |.1uF.--|-/ | 220 |>

' 220| | --- | |/| --- | ' | | --- | | --- | ' '-' | | | |.01uF | ' | | | === === | ' === === | GND GND | ' GND GND '--------------------o ' | ' .-. ' 0.22 ohm | | ' 5 Watt | | ' '-' ' | ' === ' GND ' (created by AACircuit v1.28.5 beta 02/06/05

Here's the drill. The 750 and 220 ohm resistors divide down your

0-2VDC signal to about 0-0.44V (plus or minus -- use 1% resistors for better precision). The op amp (use 1/4 of an LM324 if you're doing the Radio Shack thing) is set up as a voltage follower with the darlington transistor in the feedback loop. The .01uF cap is in the feedback loop to help prevent oscillations. Your 0-0.44V signal is impressed across the 0.22 ohm power resistor, which translates to 0-2A through the resistor.

This should work well with assembly on a perfboard. The power dissipation across the transistor is dependent on the power supply you choose. The above will work with a commonly available 5VDC regulated power supply. With a 5V supply, your transistor could dissipate as much as 9.5 watts for a short circuit load at 2 amps, so be sure to get the big TO-220 heat sink.

Have fun, and good luck Chris

Fun with oscillation Mickey may well have with the 10nF capacitive load on the opamp, and good luck he'll need fixing it! But luck is here - I've fixed the cap connection in the drawing below. :>)

' + VCC ' | Iout = 0-2A ' | ---> + ' '------o ' VCC ' + .------o ' Vin | | - ' 0-2V ___ |\| | ' o-|___|-o----o-------|+\ ___ |/ ' 750 | | | >--o--|___|--| TIP120 ' .-. |.1uF.--|-/ | 220 |>

' 220| | --- | |/| --- | ' | | --- | | --- | ' '-' | | === | .01uF | ' | === | GND | ___ | ' | GND '--------o--|___|----o ' | 4.7k | ' | .-. ' | 0.22 ohm | | ' | 5 Watt | | ' | '-' ' |______________________________| ' | ' === ' GND

I've also shown the mandatory 4-terminal Kelvin connection that should be used with low-value resistors at high currents.

--
 Thanks,
    - Win

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Thanks, Mr. Hill. I saved your earlier ASCII art, but couldn't find it. While batting it out from memory, I munged it (ouch! -- not paying enough attention, I guess).

Here's from Mr. Hill's earlier post on s.e.d.:

.. Rload=8ohm Lload .. 12V |---------/\/\/\------88888-------. .. | .. 5V | .. --- .1uF | .. |---| |---. | .. | | +------. .. | /// | | .. 10k |\| | | ..---/\/\/\--+------|+\ 220 C | .. | | >--+--/\/\/\--- B _|_/ .. | .--|-/ | E /_\ .. --- | |/| --- | | .. .1uF --- | | --- 0.01uF +------' .. | | /// | | .. | | | 10k | .. | '--------+---/\/\/\-----+ .. | | .. | \ .. | / 0.2 ohm .. | \ 5W .. | / .. | | .. '---------------------------+ .. | .. ///

(Zener added for inductive load)

Thanks again. Chris

Thanks all.

Best regards, Mickey