Using a multimeter

The example meter that we are demonstrating is typical of many economical digital multimeters that you can purchase, hopefully these instructions will be of some use to you no matter what brand or type of meter you use.

The Basics

• Powered by 9V battery
• Black probe in COM, Red in VOmA (COM and 10ADC only used for high current)
• Dial measurement required, connect probes to circuit, read value
• DC voltage: 0.2V - 600V in x10 steps
• AC voltage: 200V, 600V ranges only
• DC current: 0.0002A - 0.2A in x10
• DC current (high range): 10A
• Resistance: 200 - 2,000,000ohms in x10 steps
• hfe test (NPN/PNP transistor)
• Continuity buzzer
• Price: $26.95 + $5.50 post (Aust)

Warnings

Respect the circuitry you are working on - If you touch exposed metal, or cause a short circuit, or connect to high voltage, you may expose yourself to hazard - or damage the circuit or your meter.

Measuring the short circuit current of a battery or power supply is usually not safe - it is likely to damage the meter or battery and possibly cause the battery to overheat so much that it may be dangerous.

You can read all the other important warnings in the instructions that come with your meter, including perhaps: "Do not run the equipment under water or in water shower for fun or any other reason".

Quick facts for the experts

• Voltage Measurement: Input impedance 1M
• Current Measurement: Voltage drop 200mV max
• 10A Current Measurement: Voltage drop 100mV
• 20k,200k,2M resistance measurement: 0.35V 22uA (note this means that 20k and above ranges will not falsely read diode drops as resistance)
• 200 ohm, 2k resistance measurement: 2.7V 0.6mA (note this is sufficient to dimly light a good quality Red/Yellow/Green/Orange LED on test)
• AC Voltage Measurement: Averaging only, not RMS (however responds somewhat to change from square to sine, but small 2.0VAC reads as 1.7V)
• Diode test (2k range): Not good, value shown is 2x actual fwd voltage drop
• Hold switch: Holds the last measured value, as you would expect
• Backlight: Illuminates the display for 5 seconds (But is anyone mad enough to try and use the meter on a live circuit in the dark?!)
• Voltage/Current reference: Internal to main AME7106Y IC
• Voltage/Current calibration: Internal 200 ohm cermet preset (VR1)
• Resistance: Resistance measurements are ratiometric
• Battery consumption: 1.57mA typ (But 4.4mA continuity, 40mA backlight)
• Battery life at 1.57mA: Standard 100-150hr typ, Alkaline 200-300hr typ
• Low battery indication: 7V typ, with voltage/current accuracy lost at 6.3V
• Manufacturer: Mastech MAS830L, Full meter schematic
• Transistor test: 9V Vce, 220k base resistor (approx 40ua IBE), 10 Ohm load
• Note 10 Amp current range is Unfused (as in most economical DVMs)
• Current range protection Fuse: 200mA (20mm) no spare provided :-(
• Voltage range protection: Inherent high impedance
• Resistance/continuity range protection: PTC resistor
• Continuity: Standard - buzzer sounds for less than 1500ohm or diode :-(
• Continuity if R29=22k - sounds for less than 100 ohms, but not diodes :-)
• Construction: 'Transistor radio style', Gold flashed PCB switch contacts (Note: Open switch construction - this means keep away from moisture!)
• Active circuitry: LM358, AME7106Y chip-on-board, 1N4007, 4 transistors
• Probes: 18AWG 70cm long, right angle banana, one red, one black

Measurements for Managers

As we state above - Have the Black probe in COM, Red probe in VOmA, and dial the measurement you want before you probe the circuit.

Voltage Measurements - DC - definitely the easiest of the lot, Black probe on Ground, Red on the voltage to measure - done. If the value reads negative, the voltage is negative with respect to ground. If the value reads '1.' the voltage is higher than the range setting that you have selected.

Continuity Measurement - definitely the most used setting. Dial the position, and put the probes across the two points to be tested, with the circuit unpowered. The value displayed is the resistance in ohms. If no connection is present, the value reads '1.' More usefully, the buzzer sounds on low resistance, so that you can quickly probe for continuity without looking over at the meter each time.

Resistance Measurement - as for continuity, again, '1.' means no connection or a resistance larger than the range setting that you have selected.

Voltage Measurement - AC - as for DC voltage, however AC voltage does not have a distinct polarity. Be very careful when measuring high voltages. Just because you are holding a meter does not mean you are now immune to electric shock. Exercise caution.

Current Measurement - DC - to measure the current flowing through a circuit you must insert the meter in series with the circuit. This means you must disconnect a wire, and insert the meter in its place. Normally you would connect the Red VOmA probe in the positive direction, and the Black COM probe in the negative direction. Remove the power from the circuit before doing this. When you have the wires secure and are sure they will not drop off and short circuit, reattach the power. If the value reads negative, the current flow is negative (that is, the VOmA probe has been connected in the more negative direction). If the value reads '1.' the voltage is higher than the range setting that you have selected. For currents higher than 0.2A use the 10A range.

Current Measurement - 10A DC - as for Current Measurement DC, but using the COM and 10ADC terminals. Exercise caution - the 10A DC range is unfused, if the current exceeds 10A damage to the meter will result. Make the measurement quickly - the connecting leads will get warm at 10A current!

Real world Measurements

Diode Measurement - Select the 2k (Or, the position). Connect the Red VOmA lead to the anode (unmarked) end of the diode, and the Black COM lead to the cathode (Bar or stripe marked) end of the diode. If the diode is good, a value representing the forward voltage drop in millivolts will be displayed. It happens that our example meter is not very good at this, as R12 PTC is not well selected for the design. The value displayed is approx 2x the actual voltage drop of the diode. In any case, diodes rarely fail by exhibiting a higher forward voltage than they should, they most usually fail by going close to short circuit. The example meter is useful for such a Go / No-Go diode test.

LEDs - Red VOmA probe to anode or longer lead, Black COM probe to cathode or flat-marked side of package. On 2k, 200ohm and continuity settings 0.7ma flows through a Red/Green/Yellow/Orange LED - just enough to dimly light a good quality LED. White and blue LEDs generally will not light - they require more than the 2.7V or so supplied by the meter.

1.5V Batteries - A Fresh battery reads 1.56V, A battery is perhaps half used at 1.35V, and is pretty dead by the time it reaches 1.1V. (Matching figures for 9V: Full = 9.36V, Half = 8.1V, Dead = 6.6V). However, not every item is the same, and some equipment needs a battery with low internal impedance. The amount of current a battery can deliver is not reliably reflected by its open circuit terminal voltage. The best way to check, if in doubt, is to measure battery voltage while the equipment is running.

Lead acid batteries - A 12V lead acid battery generally measures 13.4 to 14V while on charge. When off charge, a full battery will be up at about 12.8V, a discharged battery at about 11V. If a lead acid battery is in the range of 6-8V it is probably in pretty bad shape, and over discharged.

Elements/Heaters - The resistance of a heater can be calculated by first finding its rated current, and then finding its resistance using ohms law.
The heaters current is:
Power / Voltage = Current e.g. A 1200W 240V heater uses 5 Amps.
Then the heaters resistance is:
Voltage / Current = Resistance e.g. A 240V 5A heater is 48 ohms.
Knowing an expected resistance, if the heater was measured and found to be wildly different from these values, you would guess that it was faulty. However, the ohms law calculations are not useful for lights or transformers. The 'cold' resistance of a light bulb is frequently as little as a tenth of its running resistance - so a 120W 240V light bulb, which you would calculate to have a resistance of about 480 ohms, actually has a 'cold' resistance that measures closer to 48 ohms.

Power supplies - please note our warning at the start of this document - it is usually quite dangerous to measure the short circuit current of a battery or power supply - don't try it! If a power supply is unregulated, frequently the output voltage will be up to a third higher than its rated voltage when there is no load connected. '12V' regulated supplies for equipment that would normally be battery operated are usually 13.4 - 13.8V, not really 12V at all! Power supplies for digital logic are normally regulated to +/-5% or better, so you would expect a 5V power supply to be 4.75 to 5.25V without exception.

Components in circuit - Normally you have to remove a component from a circuit in order to reliably measure its value. For instance, if a 400 ohm relay coil had a 1N4002 diode across it, it would be quite difficult to reliably measure either the resistance of the relay coil, or the foward voltage of the diode - any measurement of one is likely to have the other components interfere with the reading. Almost all circuits are like this to some degree. Most components cannot be measured in circuit. The only thing that can be reliably be said is that you will not measure a resistance greater than a resistor value - the circuit can only lower the resistance reading. Contacts, because they generally read as a dead short: '0.0' are not much affected by being in circuit. To some extent it is safe to test contacts in circuit. Ofcourse there is an exception: If there are two or more contacts in parallel, and they read as continuous, you know that atleast one is closed but you cannot be sure which (if any) of them is open.

Fast changes - If a measurement is not stable for a second or so, it is difficult to get a reliable reading with a meter. An oscilloscope is needed for this.

Cables - Cables are easily tested with a meter on continuity setting. Usually cable faults will either be an open circuit or short circuit. It is a rare though possible fault to have a conductor show a significant resistance. Most often, the continuity test on a meter is used to check if the cable pins are going to the expect place on a connector, by buzzing each circuit in turn.

Voltage drop - A meter is the ideal tool for diagnosing voltage drop. Generally, cabling should not drop a significant portion of supply voltage. If a nominal 12V power supply is feeding a device through a long cable, with say 13.6V at the power supply, you would expect to have 90% of the power (or 12.2V) available at the powered device.

Low voltage - Teamed up with Voltage drop is the droop in supply voltage that can occur when a power supply is overloaded. Again, if a power supply is significantly lower than the expected value (90% is a good starting point) then it is likely that the load is drawing too much current. The only way to find this out is to use a meter.

Monitored circuits - Most alarm systems use monitored circuits - basically contacts in series and parallel with resistors. While two wires may come back to the alarm panel, instead of being just a closed circuit or open circuit to indicate the condition, the wires will have two values of resistance for the two states. During installation, this is easily checked with a meter on resistance setting, but it relies on disconnecting the wires from the panel first so that the panel circuitry does not interfere with the measured value.

Transistor test - The hfe test on a multimeter is rarely used. However, each bipolar transistor has the circuit equivalent of two diodes within it, and testing these is often done. The common terminal of the two diodes is the base of the transistor - for an NPN transistor, the two anodes are at the base. For a PNP transistor the two cathodes are at the base. Many transistor failures will be accomanied by one or both of the diode equivalents within the transistor failing.

Jumper leads / Crocodile clips / Test leads - Called different names in different places, basically a 30cm length of insulated wire with a clip on each end. Two (or more) of these make connecting a meter into a circuit a lot easier.

Low battery voltage in a multimeter - The low battery indication in the example multimeter comes on when the battery voltage falls below 7V. It pays to replace the battery quickly. If the voltage falls below (about) 6.3V, then voltage and current measurements are no longer accurate. Typically, the meter will indicate that the voltage or current is higher than it is in reality. Interestingly, resistance measurements stay accurate to far lower supply voltages, as the resistance measurements are made ratiometrically and so compensate for the lowered battery voltage in the meter.

Autoranging meters - Many multimeters are available with autoranging functions. Instead of selecting the required voltage range, just DC voltage is selected, and the meter selects the best range itself. Also meters are available that will connect to a PC to download readings, or with functions such as capacitance, inductance and frequency measurement, all of which are quite useful. There is also temperature, light, sound level and humidity measurement - really the sky is the limit if you want to get a meter with extra features!

A second meter - Perhaps the most useful hint of the lot is to have a second meter. I personally swear by it - I am constantly measuring voltage and current at the same time, or using one meter for buzzing out a circuit board when the power is off, while the other is set up to monitor voltages in the same board when the power gets turned on.

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AirBorn Electronics: www.airborn.com.au
Phone: (61)(2)9925 0325
Mail: P O Box 1491, North Sydney, NSW 2059, Australia
Contact: Steven Murray, Manager