Schematic Volt: Simple Neon Lamp Tester Circuit

Thursday, September 25, 2014

This tester can also be used to check IF transformers used in AC mains operated radio receiver.
Though this tester is not very accurate, it can give a reasonably good indication about the condition of the components being tested.
Neon lamps are ideal indicators for simple testers to check large resistances of leaky capacitors and mains lF transformers.
For accuracy, the component being tested should not have any shunting resistance. lf a shunting resistance is present, disconnect one end of it before testing.
Such a defective component cannot be easily tested with simple test equipment.
The tester circuit uses a relaxation oscillator, the frequency of which is determined by capacitor Cl and the external resistance (leakage resistance of the component under test) between the probes.
Two flashes per second on neon lamp indicates 200 megohm leakage resistance, four flashes l00 megohm, eight flashes 50 megohm, and so on.
Moreover, as it tests capacitors at a fairly high voltage—closer to that of the operating voltage-it offers an advantage over a VTVM which is operated at a low voltage and gives inconclusive results.
Being high-voltage, low-current devices, the neon lamps are very sensitive.
The grid of an IF amplifier usually has a high AC impedance and its primary is operated at a high voltage.
A small leak in the lFl` can cause considerable trouble.