# How do I make a current source using transistor and zener diode?

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#### skymin0202

Joined Feb 11, 2019
15
I have a load that has varying resistance with external pressure. In order to measure the change, I am trying to measure the voltage on the load, when constant current is applied. And the circuit in the attachment is known to supply constant current using a transistor and a zener diode. But I have no idea what resistor/ transistor/ diode I need. I do not know if the circuit will burn them out.
The current on the load has to be approximately 6 mA, I think. And the supply voltage will be 5V. If needed, the transistor and zener diode I purchased are KTC3198, and 1N5231B-5.1V, respectively. The Vz (breakdown voltage of zener diode) will be 3.3V. Please comment, if any further information needed

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#### skymin0202

Joined Feb 11, 2019
15
and the DC CURRENT GAIN of the transistor will be 70

#### dl324

Joined Mar 30, 2015
8,891
Why did you start a new thread?

I'd operate the zener at Izt, that's the current where Vz is valid. Data from OnSemi datasheet:

For the zener you're now going to use, that would be 20mA.

$\small R = \frac{V}{I} = \frac{10V-3.3V}{20mA}=335\Omega$

The closest 5% value is 330 ohms.

If your supply voltage is stable, you could use a lower current.

#### skymin0202

Joined Feb 11, 2019
15
The zener diode has been changed. 1N5226B would be the one I will use. And the voltage across the diode will be 3.3V.

#### ScottWang

Joined Aug 23, 2012
6,822
Please don't do double post, this thread will be locked, the original posted is here.

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