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How to build the voltage controlled current source based on modified Howland current pump with FET?

DX400

New Member
Hi,

I have a problem with building of the voltage controlled current source with obligatory grounded load. The input control voltage is in the range from 0 to 5 V, the output voltage is up to 20 V and output current on the load up to 150 mA. The load is some semiconductor device( or material), the (light-emitting)diode for example. Other very important requirements are that the load must be grounded, the control voltage on input and the voltages on the output should be measured with respect to the one common point(ground).

I have already used the modified Howland current pump. The scheme is quite good for output current of 25 mA, but the suitable heat sink was required to protect the OpAmp. Also I have used the unipolar voltage supply of the OpAmp to obtain the 20 V of output. The another benefit of this scheme is that the output current does not depend on the load resistance.
118928


How to modify the Howland current pump with the grounded load for output current up to about 200 mA (150 required with about 10 -40 mA in reserve)?

I have found the example of such modification in the Ref. https://www.edn.com/design/analog/4412375/Choose-resistors-to-minimize-errors-in-grounded-load-current-source. The Howland current pump based, with n-channel FET switch.



I have tried this scheme with OpAmp LM324 and FET IRLB8743, using resistors R1=R2=R3=100 kOhm, R4=104 kOhm, and obtain the output current up to 200 mA by varying of R5 (pot of 5 kOhm) controlling resistor.

But as I found from the circuit calculations, presented in the Ref, the output current is dependent on the load resistance. Hence, this current source is quite bad.

How to improve the scheme to resolve this problem and to obtain voltage controlled current source with grounded load for output current up to about 150-200 mA?
 

rjenkinsgb

Well-Known Member
The second circuit is almost a differential amp, in an odd config.

I'd use a standard differential amp to sense voltage a across a suitably low value fixed resistor in series with the load (where you have R5) and convert that to a ground referenced voltage.

Then compare that to your control voltage with another opamp and use the output of that to control the regulator transistor.

The result should be independent of load resistance, subject to the commanded current being possible with the supply voltage and load resistance.

Conventional differential amp circuit: https://encrypted-tbn0.gstatic.com/images?q=tbn:ANd9GcSJfBUEgbVEraiQ-737rf9jMEY-FtJcFIspqWLDZqw-VrPprLkp

R1 & R3 are equal. R2 & R4 are equal. The ratio of R1 & R3 to R2 & R4 sets the circuit gain.
 

unclejed613

Well-Known Member
Most Helpful Member
a few years back, i designed a high power (up to +/-2A DC or 4A AC(peak to peak) Modified Howland current source.. the goal was to make an AC current source to directly measure speaker/crossover/amplifier output impedances. i posted the schematic and description here on ETO as a blog article, but i think it got lost when ETO migrated to new servers. basically it's a "current boosted" op amp with two output transistors and a bias stack, and connected up as a Modified Howland current source. i'll see if i can dig up the circuit.
 

DX400

New Member
a few years back, i designed a high power (up to +/-2A DC or 4A AC(peak to peak) Modified Howland current source.. the goal was to make an AC current source to directly measure speaker/crossover/amplifier output impedances. i posted the schematic and description here on ETO as a blog article, but i think it got lost when ETO migrated to new servers. basically it's a "current boosted" op amp with two output transistors and a bias stack, and connected up as a Modified Howland current source. i'll see if i can dig up the circuit.
I should be very obliged if you could find the schematics of the current source you had created. Thank you for the answers.
 

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