There are three components routinely used in the power input section of many linear and switching power supply that have confused me over the years and have often been difficult to identify. In this blog I would like to call your attention to them, show how they are used in the circuit, and give some tips on how to identify each of them. Since I am not an expert on any of these devices, comments, clarifications and corrections to this blog by my friends here on the forum who are the experts will be appreciated. The triplets of my analogy all look very similar but of course they are very different and serve quite different purposes. Here is how they look:
Across the Line Capacitor
The "across the line capacitor", as its name implies, sits between the line and neutral power as it enters the power supply. The value of this capacitor is typically 50 nF to 100nF with a voltage rating in excess of 275 volts AC. If your line supply is 220 volts it is likely that the voltage rating on the capacitor would be closer to 400 VAC. The purpose of this capacitor is to decouple short duration spikes and higher frequency signals that may be on the incoming power line. The capacitor is most effective against short duration spikes and high frequency interference but not much else. Long duration transients or low frequency noise just blow right past it and into the circuit. The "across the line capacitor" is very durable and will not fail unless it suffers a direct, high voltage, pulse in excess of its rating, like that provided by a lightening strike. The capacitor is the most easily identified of the triplets as it is usually clearly marked as a capacitor with a value in micro farads and also a breakdown voltage. When out of the circuit it will read as an open circuit on an ohm meter and can be measured with a capacitor meter. Here is a Farnell Data Sheet on one type of across the line capacitor:
The Metal Oxide Varistor
The Metal Oxide Varistor also designated as a MOV also sits across the input voltage just like the "across the line capacitor". It is however not a capacitor but rather a device whose resistance is designed to vary with the level of the voltage that is applied to it. The MOV is composed of a matrix of materials that act like a large number of diode junctions in parallel and series. It is also bidirectional so that current can flow in either direction. While it may not always be clearly marked the MOV is typically rated with a voltage level at which the device will conduct called the "Clamping Voltage" and also a power rating in Joule/seconds that it can absorb. The MOV will have a very high resistance up to the point of its clamping voltage. When the clamping voltage is exceeded the resistance will very rapidly (40 to 60 nano seconds) drop to a low level and effectively short the excess voltage to the neutral side of the line. Unlike the capacitor that can only deal with very short transient spikes the MOV will continue to clamp the excess voltage until it has absorbed too much energy and fails. The MOV has no effect on spikes or signals that do not exceed its specified voltage. Since many of the transients have a very short time duration the MOV can absorb very high amounts of current and energy. Since the MOV and Capacitor are complementary it is quite common to find both of them across the input power line of power supplies. The clamping of spikes and over voltage transients can have an accumulative damaging effect on the MOV and the failures can be catastrophic with lots of smoke and fire. Of the triplets the MOV is the most likely to fail in the circuit. MOVs are commonly used as surge suppressors in protected outlet strips and many electronic appliances. The MOV can be identified by its position across the line and its near open circuit resistance when checked with an ohm meter. It will usually not be marked with directly readable values. If the MOV isn't marked the only way to determine the voltage is to hook it up to a high voltage variable source in series with a 10 K or higher as appropriate power resistor and read the voltage across the MOV while slowly raising the voltage. The voltage will rise until the Clamping voltage is reached at which point it will not rise any higher. Be aware that at this point any voltage in excess of the clamping voltage will be across the series resistor which has to be able to handle the power. Also be aware that most of these clamping voltages are well into the dangerous voltage region and extreme care needs to be taken. Here is a data sheet from Farnell on a line of MOVs:
The In Rush Current Limiting Thermistor
Unlike the Capacitor or the MOV the "In Rush Current Limiting Thermistor" is wired in series with input power. We have come to think of thermistors in terms of their properties that make them usable as temperature sensors and regulators. This however is a special type of thermistor that has a negative temperature coefficient and has a fairly low resistance at room temperature. This means that as it gets hotter its resistance to current flow goes down. Some power supply circuits with their high capacitive secondary demand a very high initial input current. The Thermistor allows the input circuit to start off with a series resistance that ameliorates this high current demand and then the resistance drops out as the IR drop across the Thermistor heats it and causes the resistance to drop to a stable low level. By the time the Thermistor drops to the lower resistance the initial current demand to power the capacitors has passed and the power supply operates within it specifications. The Thermistor can be identified by its series position in the circuit. If we test the Thermistor with an ohm meter at room temperature we will usually get a reading in the lower single or 10s of ohms. If we heat the thermistor the resistance will drop. Here is a Farnell Data Sheet on one type of Current Limiting Thermistor:
This is a power supply from a DVR which has an example of each of the triplets in its circuitry. The Yellow Disk is an MOV that is across the power line. In this case the Capacitor doesn't look like one of the triplets and is rectangular and yellow. The Green disk is an in rush current limiting thermistor and is in series with the power line.
This interesting little circuit board has the power line come into the board on the orange terminal strip. The black cylinder is a fuse and the blue disk is an MOV. The switch is DPST and switches both Line and Neutral. The switch is followed by a common mode inductor and finally the two white rectangular capacitors. The conditioned power exits at the gray terminal strip.
Here is one final example with the MOV and a Capacitor. This is the more common arrangement with the use of an In Rush Current Limiting Thermistor limited to situations where there is a high current demand on the input supply power when the board is first turned on.
Watch for these components and how they are used when you have the opportunity to inspect a power supply circuit board. Remember the Capacitor has infinite DC resistance and sits parallel to the input power supply. It will commonly have markings associated with a capacitor to help identify it. The MOV will also likely have no measurable resistance. I tried to measure the resistance of the red MOV in the picture at the top of this Blog and found it to be beyond the range of my Fluke which means it is over 50 meg. The MOV will also sit parallel to the incoming power. If your power supply has a thermistor it will be in series with the input power and will show a resistance at room temperature probably less than 25 ohms.
Once again, I remind everyone that any additions or corrections will be appreciated. This is a learning process, most of what I post comes from experience and observation. If I got it all right this would be the first time.