Current Source for Resistance Measurement

Here is a current source you can build for resistance measurement. When the current is held constant, you know as per Ohm’s Law the Voltage across Resistor is proportional to Resistance value.

Precision Current Source for Resistance Measurement

The supply is +12 and -12, The total voltage across R6 + R7 is 24V. Then 24V / 120K = 0.2mA. The voltage across R6 is (10K * 0.2mA) = 2V. The same is reflected across R5 in this feedback configuration. That means Q3 is a 2V / 1K = 2mA source. If my calculations are right.

There are sources of errors in this circuit. The temperature variation of all resistor values, which is 100ppm for general calculations in 1% MFR. Let us assume you use OP07 which is close to an ideal opamp, but for this application it is not needed. The second error is Ib, the base current of Q3 which may be 0.2mA / Hfe(200) = ~ 1 uA. Then the variation of Hfe, Vcc and Vdd w.r.t. Temperature, should not be overlooked. Use LM7812 and LM7912.

So you see, design knowing that all these components are not ideal. Leakage currents, Humidity, EMI, Stray Capacitance and Inductance and much more. It is just like, even when the motor is fixed firmly on the machine, some parts Vibrate and create a Noise due to Mechanical Resonance. So Build and evaluate your design in the real environment, to learn.

Discover how resistors are color coded – Interactive Java Resistors Tutorial.

  • Relay Driver – ElectromagneticRelay Driver – Electromagnetic
    This circuit is a standard way of driving Small Power Relays from Control Signals generated by Logic or Opamp Circuits. C2 and R5 form a [...]
  • Constant Current Source for LEDConstant Current Source for LED
    This article will explain the way a simple transistor based current source is designed, this will give an idea on how some components can be [...]
  • Differential to TTL convertor using LM339Differential to TTL convertor using LM339
    Some outputs are via Dual Differential Line Driver type DS8830. This device will interface with standard TTL systems. The differential outputs are balanced and are [...]
  • Constant Current Source LED DriveConstant Current Source LED Drive
    This is a Constant Current Source LED Driver, When the LED driver Upper-NPN is driven by a voltage thru 4.7K the LED lights up. Assume [...]
  • VCO with LM331VCO with LM331
    This Circuit is a simple Analog to Digital Interface with a capability of 10 to 12 bits resolution. 10 bits means 1024 counts or parts [...]
  • Connection Diagram of AC Control SSRConnection Diagram of AC Control SSR
    SSR or Solid State Relay or Electronic Relay Connection Method. This is a Thyristor Based Normally Open 230V Switch that can be turned on / [...]
  • Triac based Lamp Dimmer power controlTriac based Lamp Dimmer power control
    This is a phase angle control of a Triac which is a evolution from an SCR. The 50 Hz or 60 Hz Sine wave of [...]
  • Millivolt Meter using a LM3914 LED Dot DisplayMillivolt Meter using a LM3914 LED Dot Display
    Here is a easy to read ‘Analog’ Millivoltmeter. Just like the Moving Coil Voltmeter, but does not have that resolution. This gives a easy indication [...]
About delabs 152 Articles
Product Design - Industrial Automation and Instrumentation. -

Be the first to comment

Leave a Reply