Searching \ for 'Constant Current source' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: www.piclist.com/techref/power.htm?key=current
Search entire site for: 'Constant Current source'.

Truncated match.
PICList Thread
'Constant Current source'
1997\07\02@122617 by vanes

flavicon
face
Hi!

Anyone know how to build a constant current source using Op-amps. I know
how to do it with transistors, but I want to try it with op-amps.

Thanks.
--
eric van es
spam_OUTvanesTakeThisOuTspamilink.nis.za
cape town, south-africa
http://www.nis.za/~vanes/

1997\07\02@143412 by Bob Fehrenbach

picon face
EJA van Es <.....vanesKILLspamspam@spam@ilink.nis.za> wrote:
>Anyone know how to build a constant current source using Op-amps. I know
>how to do it with transistors, but I want to try it with op-amps.

Hi Eric,

Try this, from a National Semiconductor Ap Note:


                         R3
                   -----/\/\/\----
                 |                |
                 |                |
          R1     |    | \         |
 E_in ---/\/\/\-------| - \       |      I = E_in * R3
                      |    \------          -----------
                  ----|+   /      |           R1 * R5
                 |    | /         |
                 |                |          R3 = R4 + R5
                 |                \          R1 = R2
                 |                /   R5
                 |                \
                 |                /
                 |       R4       |
                 |-----/\/\/\-----|
                 |                |  <---Constant current = I
                 |                |
                 \              -----
                 /             |     |
            R4   \             |     |  Load
                 /             |     |
                 |              -----
                 |                |
                GND              GND


--
Bob Fehrenbach    Wauwatosa, WI     bfehrenbspamKILLspamexecpc.com

1997\07\04@004405 by Harold M Hallikainen

picon face
On Wed, 2 Jul 1997 18:24:28 +0200 EJA van Es <.....vanesKILLspamspam.....ilink.nis.za> writes:
>Hi!
>
>Anyone know how to build a constant current source using Op-amps. I
>know
>how to do it with transistors, but I want to try it with op-amps.
>


       If you can put up with a floating load, just put the load between
the output of an op-amp and the inverting input, then put a resistor from
the inverting input to ground.  Apply a voltage to the non-inverting
input.  Since the inputs will "follow each other", the inverting input
will be at about the same voltage, causing I=V/R through the resistor to
ground.  Since an ideal op amp has no input current, this current also
goes through the load.
       You can also ground the noninverting input and apply a voltage to
the end of the resistor that was grounded.  The "happy op amp" will keep
the two inputs together, keeping the inverting input at ground.  The
current through the resistor and the load will be I=V/R, since the
voltage at one end of the resistor is zero.  This is the basis of an
inverting integrator.  The controlled current flows through the capacitor
(the load).  Since the inverting input is at ground, the op amp output
voltage is the same as the capacitor voltage.
       Another circuit with a grounded load is described below (my ascii
art isn't very good).  Imagine two equal value resistors in series.
Connect the load from the junction of the two resistors to ground.
Connect the reference voltage to one end of the series resistors.
Connect the junction of the resistors (and the load) to the input of a
noninverting amp with a gain of 2.  Connect the output of the amp to the
other end of the two resistor network.  Clever circuit!  Try a few ohm's
law analysis runs on it.  Start with a load of zero ohms to see that the
current is Vin/R.  Then assume the circuit works and use ohm's law to
find the voltage at the top of the load (assuming a resistor), then
determine the voltage at the output of the amp (twice the load voltage).
Apply Kirchoff's current law.  It works!

Harold

More... (looser matching)
- Last day of these posts
- In 1997 , 1998 only
- Today
- New search...