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I have a load that has varying resistance and want to know the change of resistance by measuring the voltage with constant current. The figure below is known to provide constant current. But I have no idea what transistor, resistor, and zener diode I need to use. Assuming that 5V will be supplied, and the load will be in approximately range of 2.5 - 15 ohms, what transistor, resistor, and zener diode will I need to use? At least, would you please inform me the equations that i need to set up for the resistor, and etc.?

enter image description here

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  • \$\begingroup\$ Umm. If you want a constant-current source, what actual current do you want? And a bit of warning - while your circuit is a constant-current source, it's not a very good one. It's temperature sensitivity is high, and it's very difficult to predict exact current for a given set of components. Finally, running at 5 volts may be difficult or impossible, depending on the current you want. \$\endgroup\$ – WhatRoughBeast Feb 11 at 5:31
  • \$\begingroup\$ Is there any simple circuit you recommend other than this? \$\endgroup\$ – Min Sik Choi Feb 11 at 6:37
  • \$\begingroup\$ How about answering the question? What current do you want? Then think about how precise a current you need, and specify that precision. \$\endgroup\$ – WhatRoughBeast Feb 11 at 17:14
  • \$\begingroup\$ About 6mA will be good enough I think. \$\endgroup\$ – Min Sik Choi Feb 12 at 2:12
  • \$\begingroup\$ 1% precision will be good enough, too, if possible. And the response time has to be really fast. What I mean by this is, even though the resistance of the load changes abruptly, the current must catch up the change as fast as it can and stay constant. \$\endgroup\$ – Min Sik Choi Feb 12 at 3:03
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Okay look the circuit you have provided is the constant current circuit. Now to find the constant current you first need to find the voltage drop across Re (emitter resistor) and voltage across zener diode (suppose if the zener is of 2vV and supply i.e Vcc = 5 V then drop across zener will be Vz=2 V).

Now if Vz=2 V then Ve (i.e voltage drop across Re) is Ve= 2 V-0.7 V=1.3 V (considering Vbe=0.7 V).

Now that you have Re and Ve you can find Ie (emitter current) this is the constant current that will flow through the load.

So if the load is 2.5 ohm then load voltage VLoad=(2.5*Ie) and if the load is 15 ohm then the load voltage VLoad=(15*Ie).

Note: Here Re=R2

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  • \$\begingroup\$ Starting your answer with "okay look" is never a good sign. This answer needs some heavy formatting rework before it can be up to the standards of this site. \$\endgroup\$ – Hearth Feb 11 at 4:04
  • \$\begingroup\$ You should also state that Re = R2, to make the equations more clear. \$\endgroup\$ – Sparky256 Feb 11 at 4:16
  • \$\begingroup\$ Would you please go over my question again? The circuit I uploaded was wrong. \$\endgroup\$ – Min Sik Choi Feb 11 at 4:22
  • \$\begingroup\$ Yes please go through my answer i have updated it. \$\endgroup\$ – user212487 Feb 11 at 4:23
  • \$\begingroup\$ is there any requirement for diode and transistor in order to make this work? such as maximum current? \$\endgroup\$ – Min Sik Choi Feb 11 at 4:30

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