Understanding Negative Temperature Coefficient (N.T.C)

This is a type of thermistor which has is resistance decrease when its temperature increases.

Because of this characteristics N.T.C are used in circuits which require low initial and then high current.

Example: When you first power on the TV (s.m.p.s) at first you notice the main power flickers, this is because in the television power supply there is usually a big capacitor (rated around 400Volts)

When the TV/monitor is off this capacitor usually discharges and as you know if capacitor has no charge on it, it has low resistance and therefore when you power ON the set, the power will see low resistance path and therefore there is inrush of current.

This can trigger the fuse to blow thanks to this component called N.T.C which is usually placed in series with the power supply positive rail.

When the set is switched ON this component is usually cold and hence high resistance and therefore this high resistance will hold (resist) this inrush of current briefly before the main capacitor charges and its resistance increases smoothly.

In some power supplies they don’t use this component (N.T.C) but instead they use low resistor (usually high wattage but low resistance) called surge limiter resistor and others call it dropper.

Testing N.T.C: Set you meter (digital or analogue) to low ohm range and if it is good usually will show very low resistance (remember this component is placed in series, component in series should never read open otherwise current will not flow)

If you don’t get any meter reading then it is open and considered bad (open).

If this component opens, this will cut the main voltage to the main capacitor because it is in series with the power supply.

You can also test it In-circuit by doing voltage testing. You should have same voltage on both pins. You can use the same method to test the surge limiter resistor, but in both cases be sure to use the main capacitor negative pin as your ground.

Note: When doing voltage testing and find no voltage at the main capacitor and the main fuse is not shorted (no short) then you can check components in-series with the supply which may be open using voltage testing.

Just get a schematic diagram of that equipment and look out for the components in series with the supply.

You should get voltage on both sides of the pins of all components in series otherwise the component should be considered open if you don’t get reading on both sides (pins)

You can confirm the component is actually open by using ohm meter.

When measuring resistance always, be sure the equipment is not powered and capacitors are discharged.

This is an extract from my basic electronics course and therefore if you want to go a notch higher you can click on book cover for more info.