Circuit : Andy
Collinson
Description:
A low resistance ( 0.25 - 4 ohm) continuity tester for
checking soldered joints and connections.
Notes:
This simple circuit uses a 741 op-amp in differential mode
as a continuity tester. The voltage difference between the
non-inverting and inverting inputs is amplified by the full
open loop gain of the op-amp. Ignore the 470k and the 10k
control for the moment, and look at the input of the op-amp.
If the resistors were perfectly matched, then the voltage
difference would be zero and output zero. However the use of
the 470k and 10k control allows a small potential difference
to be applied across the op-amp inputs and upset the balance
of the circuit. This is amplified causing the op-amp output
to swing to full supply voltage and light the LED's.
Setting Up and Testing:
The probes should first be connected to a resistor of value
between 0.22 ohm and 4ohm. The control is adjusted until the
LED's just light with the resistance across the probes. The
resistor should then be removed and probes short circuited,
the LED's should go out. As the low resistance value is
extremely low, it is important that the probes, (whether
crocodile clips or needles etc) be kept clean, otherwise
dirt can increase contact resistance and cause the circuit
to mis-operate. The circuit should also work with a MOSFET
type op-amp such as CA3130, CA3140, and JFET types, e.g.
LF351. If the lED's will not extinguish then a 10k preset
should be wired across the offset null terminals, pins 1 and
5, the wiper of the control being connected to the negative
battery terminal. A pin out for the 741 can also be found
here |
