CONTINUITY

       SECTION FIVE                             

                        CONTINUITY

Some multimeters have a "buzzer" that detects when the probes are touching each other or the resistance between the probes is very LOW. This is called a CONTINUITY TESTER. You can use the resistance scale "x1" or "x10" to detect low values of resistance. Set the pointer to "0" (right end of the scale) by touching the probes together and adjusting the "zero ohms" control.

When taking a reading, you will have to decide if a low value of resistance is a short- circuit or an "operating value." For instance, the cold resistance of a 12v car globe is very low (about 2 ohms) and it increases (about 6 times) to 12 ohms when hot. The "resistance of a circuit" may be very low as the electrolytics in the circuit are uncharged. This may not indicate a true "short-circuit."

The measurement across a diode is not a resistance-value but a "voltage-drop" and that is why the needle swings nearly full-scale. Leads and wires and cords have a small resistance and depending on the length of the lead, this small resistance may be affecting a circuit.

Remember this:

When a circuit takes 1 amp, and the resistance of the leads is 1 ohm, the voltage drop across the leads will be 1v. That's why a 12v battery supplying a circuit with these leads will have 11v at the circuit.

Note:

Turn off the equipment before making any continuity tests. The presence of even a small voltage (from an electrolytic) can give a false reading. You can determine the resistance of a lead very accurat ely by taking the example above and applying it to your circuit.

If the battery is 12.6v and the voltage across the circuit is 10v, when the current is 2.6 amps,  the resistance of the "leads" is 12.6 - 10 = 2.6   R=V/I  = 2.6/2.6 = 1ohm.  By making the lead shorter or using thicker wire, the resistance will be less and the voltage on the project will increase.

When taking readings in a circuit that has a number of diodes built-into IC's (Integrated Circuits) and transistors, some Continuity Testers will beep and give a false reading. The following circuit has the advantage of providing a beep when a short-circuit is detected but does not detect the small voltage drop across a diode. This is ideal when testing logic circuits as it is quick and you can listen for the beep while concentrating on the probe. Using a multimeter is much slower

 

CONTINUITY TESTER

You can build the circuit on Matrix Board and add it to your Test Equipment. You will need lots of "Test Equipment" and they can be built from circuits in this eBook.

TESTING FUSES, LEADS AND WIRES

All these components come under the heading TESTING for CONTINUITY. Turn off all power to the equipment before testing for shorts and continuity. Use the low resistance "Ohms Scale" or CONTINUITY range on your multimeter. All fuses, leads and wires should have a low, very low or zero resistance. This proves they are working.

A BLOWN FUSE

The appearance of a fuse after it ha s "blown" can tell you a lot about the fault in the circuit. If the inside of the glass tube (of the fuse) is totally blackened, the fuse has been damaged very quickly. This indicates a very high current has passed through the fuse.

Depending on the rating of the fuse, (current rating) you will be able to look for components that can pass a high current  when damaged - such as high power transistors, FETs, coils, electrolytics. Before re-connecting the supply, you should test the "SUPPLY RAILS" for resistance. This is done by measuring them on a low OHMs range in one direction then reverse the leads to see if the resistance is low in the other direction.

A reading can be very low at the start because electrolytics need time to charge-up and if the reading gradually increases, the power rail does not have a short. An overload can occur when the supply voltage rises to nearly full voltage, so you sometimes have to fit  a fuse and see how long it takes to "blow."

If the fuse is just slightly damaged, you will need to read the next part of this eBook, to see how and why this happens:

FAST AND SLOW BLOW FUSES

There are many different sizes, shapes and ratings of a fuse. They are all current ratings as a fuse does not have a voltage rating. Some fuses are designed for cars as they fit into the special fuse holders. A fuse can be designed for 50mA , 100mA, 250mA, 315mA, 500mA, 1Amp, 1.5amp, 2amp, 3amp, 3.15amp  5amp, 10amp, 15amp, 20amp, 25amp, 30amp, 35amp, 50amp and higher.

Some fuses are fast-blow and some are slow-blow. A "normal" fuse consists of a length of thin wire. Or it may be a loop of wire that is thin near the middle of the fuse. This is the section that will "burn-out."

A "normal" fuse is a fast-blow fuse. For instance, a 1amp fuse will remain intact when up to 1.25 amp flows. When a circuit is turned on, it may take 2-3 amps for a very short period of time and a normal 1 amp fuse will get very hot and the wire will stretch but not "burn-out." You can see the wire move when the supply turns on. If the current increases to 2amps, the fuse will still remain intact. It needs about 3 amp to heat up the wire to red-hot and burn out

If the current increases to 5 amp, the wire VOLATILIZES (burns-out) and deposits carbon-black on the inside of the glass tube. A slow-blow fuse uses a slightly thicker piece of wire and the fuse is made of two pieces of wire joined in the middle with a dob of low-temperature solder. Sometimes one of the pieces of wire is a spring and when the current rises to 2.5 amp, the heat generated in the wire melts the solder and the two pieces of wire "spring apart."

A slow-blow fuse will allow a higher current-surge to pass through the fuse and the wire will not heat up and sag. Thus the fuse is not gradually being damaged and it will remain in a perfect state for a long period of time.

A fuse does not protect electronic equipment from failing. It acts AFTER the equipment has failed. It will then protect a power supply from delivering a high current to a circuit that has failed. If a slow-blow fuse has melted the solder, it could be due to a slight overload, slight weakening of the fuse over a period of time or the current-rating may be too low. You can try another fuse to see what happens. You can replace a fast-acting fuse (normal fuse) with a slow blow if the fast-acting fuse has been replaced a few times due to deterioration when the equipment is turned on.

But you cannot replace a slow-blow fuse with a fast acting fuse as it will be damaged slightly each time the equipment is turned on and eventually fail.

TESTING COILS, INDUCTORS and YOKES

Coils inductors and yokes are just an extension of a length of wire. The wire may be wrapped around a core made of iron or ferrite. It is labeled "L" on a circuit board. You can test this component for continuity between the ends of the winding and also make sure there is no continuity between the winding and the core.

The winding can be less than one ohm, or greater than 100 ohms, however a coil of wire is also called an INDUCTOR and it might look like a very simple component, but it can operate in a very complex way. The way it works is a discussion for another eBook. It is important to understand the turns are insulated but a slight fracture in the insulation can cause two turns to touch each other and this is called a "SHORTED TURN" or you can say the inductor has "SHORTED TURNS."

When this happens, the inductor allows the circuit to draw MORE CURRENT. This causes the fuse to "blow." The quickest way to check an inductor is to replace it, but if you want to measure the inductance, you can use an INDUCTANCE METER. You can then compare the inductance with a known good component.

An inductor with a shorted turn will have a very low or zero inductance, however you may not be able to detect the fault when it is not working in a circuit as the fault may be created by a high voltage generated between two of the turns. Faulty yokes (both horizontal and vertical windings) can cause the picture to reduce in size and/or bend or produce a single horizontal line.

A TV or monitor screen is the best piece of Test Equipment as it has identified the fault. It is pointless trying to test the windings further as you will not be able to test them under full operating conditions.

MEASURING AND TESTING INDUCTORS

Inductors are measured with an INDUCTANCE METER but the value of some inductors is very small and some Inductance Meters do not give an accurate reading.

The solution is to measure a larger inductor and note the reading. Now put the two inductors in SERIES and the values ADD UP - just like resistors in SERIES. This way you can measure very small inductors.  VERY CLEVER!

TESTING SWITCHES and RELAYS

Switches and relays have contacts that open and close mechanically and you can test them for CONTINUITY. However these components can become intermittent due to dirt or pitting of the surface of the contacts due to arcing as the switch is opened.

It is best to test these items when the operating voltage and current is present a s they quite often fail due to the arcing. A switch can work 49 times then fail on each 50^th operation. The same with a relay. It can fail one time in 50 due to CONTACT WEAR.

If the contacts do not touch each other with a large amount of force and with a large amount of the metal touching, the current flowing through the contacts will create HEAT and this will damage the metal and sometimes reduce the pressure holding the contact together.

This causes more arcing and eventually the switch heats up and starts to burn. Switches are the biggest causes of fire in electrical equipment and households. A relay also has a set of contacts that can cause problems. There are many different types of relays and basically they can be put into two groups.

1. An electromagnetic relay is a swit ch operated by magnetic force. This force is generated by current through a coil. The relay opens and closes a set of contacts. The contacts allow a current to flow and this current can damage the contacts. Connect 5v or 12v to the coil (or 24v) and listen for the "click" of the points closing. Measure the resistance across the points to see if they are closing. You really need to put a load on the points to see if they are clean and can carry a current. The coil will work in either direction. If not, the relay is possibly a CMOS relay or Solid State relay.

2. An electronic relay (Solid State Relay) does not have a winding. It works on the principle of an opto-coupler and uses a LED and Light Activated SCR or Opto-TRIAC to produce a low resistance on the output. The two pins that energise the relay (the two input pins) must be connected to 5v (or 12v) around the correct way as the voltage is driving a LED (with series resistor). The LED illuminates and activates a light-sensitive device.