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# Circuit Simulator - Howland Current Source

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Circuit Description:

This circuit shows a current source. The current through the load is the same regardless of the position of the switch.

The op-amp attempts to keep the voltages at both terminals the same, so V = V+. Call V1 the voltage across R1, and I1 the current across R1. Then V2 = V4. Since I1 = I2 and R1 = R2, V1 = V2 = V4.

V = V+ = V1 - 5V

I3 = V+/R3 = (V1 - 5V)/R3.

I4 = V4/R4 = V1/R3.

V1/R3 = (V1 - 5V)/R3 + Iload

Iload = 5V / R3 = 1.67 mA

 Nick said: (Fri, Jan 27, 2012 12:25:22 AM) That's Widlar's current source. Still nicely done.

 Tom said: (Fri, Jan 4, 2013 04:52:58 PM) Why does the current stop in resistor R3 when the switch is closed? Is it just so small that it does not show? There should be some current unless these are theoretical Zero resistance switch and conductors ("wires").

 Someone said: (Wed, Apr 17, 2013 02:46:55 PM) It stops because when you flip the switch, you created a path where resistance equals 0.