Mickey said:
Does anyone have schematic for Output that is regulated with voltage (small
up to 2 V) and
otput is 0 - 2 Amps according to the input voltage?
Mickey
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
www.tech-chat.de)
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.