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I am trying to convert 5V to a constant current source

circuit

for op amp I use AD822. for 250 ohm I use 180 ohm, so I should get a current of 28 mA. I verified when Rload = 0, 28 mA across negative terminal to ground.

But when I put Rload to circuit, 680 ohm, my current exceeds 28mA as Vcc increase.

Can anyone please point where might be the error?

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  • \$\begingroup\$ What does 'for 250 ohm I use 180 ohm' mean? \$\endgroup\$ – copper.hat Jun 9 '15 at 3:44
  • \$\begingroup\$ What is Vcc in your circuit?? \$\endgroup\$ – Kvegaoro Jun 9 '15 at 4:35
  • \$\begingroup\$ instead of 250 ohm I use 180 ohm. Vcc is at least +- 10V. \$\endgroup\$ – kevin0228ca Jun 9 '15 at 6:10
  • \$\begingroup\$ The maximum current from 0 to 10v is approx 15mA. This tells me the connections are not as shown. Assuming your supply is connected to the opamp as required and the ground is at mid point between the +/-10v rails. \$\endgroup\$ – Spoon Jun 9 '15 at 7:03
  • \$\begingroup\$ sorry I meant I was increasing Vcc to see how current go. 18 +- Vcc should give me 28 mA when Rload = 680? but at 18V I get around 20 mA, and continue increasing as Vcc increase, and went negative at Vcc = +- 25V. ground is created as mid between two power supply. V+ is from arduino 5V. \$\endgroup\$ – kevin0228ca Jun 9 '15 at 7:23
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The "operating output current" for an AD822 on a supply voltage of +/-15V is limited to 20mA (see page 9 of the data sheet).

Absolute maximum rating for the device is +/- 18V.

Either lower your expectations for this device or choose a more appropriate op-amp or an op-amp BJT combination.

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Have you considered a circuit like this ? Then you can keep the same opamp as the NPN (can be any general purpose type) can easily handle the current.

schematic

simulate this circuit – Schematic created using CircuitLab

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