Which Multimeter augments the capabilities of Analog Discovery the best?
From the simple connection of a few resistors to a large complicated circuit, every design needs to be tested for its electrical properties. The two channel scope and function generator software instruments in the Analog Discovery are great for making time domain and frequency domain measurements. The Waveforms software even includes a simple two channel voltmeter instrument. However, it is difficult to measure current and resistance with Analog Discovery. A good DC multimeter can be an important debugging tool in an electronics designer's arsenal. Yet making sure you have the right meter at the right price might take some research.
Multimeters are electronic devices that measure voltage, current, resistance and sometimes even parameters like capacitance, inductance and transistor gain. Although there are many different types of multimeters with different functions and benefits, your first consideration should be analog vs. digital.
Analog Versus Digital
Analog multimeters, as you might guess, use analog meter movements for displaying voltage, current and resistance. Digital multimeters by contract have LCD numeric displays. Figure 1 shows what generic analog and digital meters might look like. Multimeters with analog displays are not as commonly used and are becoming harder and harder to find. Simple analog meters passively measure voltage and current by extracting a small amount of energy from the circuit being probed to deflect the meter needle. Often this is only a minor effect but can adversely affect sensitive nodes in a circuit. This can lead to a lower input resistance than meters with active amplifiers as in most digital meters and some high end analog meters ( some of you may remember the old VTVM, vacuum tube voltmeters ).
Figure 1 Analog vs. Digital multimeters
Both kinds of meters generally have a switch to select the various functions and ranges, internal circuitry for signal conditioning and in the digital meter an analog-to-digital converter and display driver.
Generally speaking, DMMs are recommend, but there are still hundreds of models to choose from, so you need to narrow down the options and consider various factors when selecting the correct DMM for your needs. Here are some things to keep in mind:
Price: DMMs are available in a wide range of prices staring from less than $10 and going up to $1500 or more. This depends on the manufacturer and the included features. Higher priced meters tend to have more resolution and be more accurate than the lower cost models. Bench top multimeters tend to be more expensive when compared to a portable or handheld DMMs.
In general, the best recommendation is to look at where you will use the meter and the features that are needed. If it is going to be used along with the portable Analog Discovery module in an “anywhere, anytime” scenario, a lower priced battery operated hand held meter is probably a good option over a bench top model.
Display Count: The resolution of a meter, or display digits. It refers to how wide a range of reading the meter can display or the total number of digits that are displayed. The higher the display count, the better. Accuracy is generally specified as a percentage of the range +/- some number of counts. The Scope inputs on the Analog Discovery have a DC accuracy of a few mV so a DMM with similar accuracy is probably about right.
Versatility and Functionality: Multimeters can have standard features such as measuring AC and DC voltages, current, resistance and even more advanced capabilities to measure frequency, capacitance and inductance. Many meters offer various other tests such as the forward voltage of diodes, battery test, continuity, transistor gain. Special functions like auto range, analog bar graph, USB interface to a computer, true RMS, can make them more useful than a standard multimeter. Check out the meter's user manual to review the functions offered and select your meter based on the kinds of measurements you will be making.
Typical specifications for 3½ digit hand held DMMs in the $10 price category:
- 1999 count meter digital display
- DC Voltage: 200mV, 2000mV, 20V, 200V, 1000V
- AC Voltage: 200V, 750V
- Measures DC current: 200uA, 2000µA, 20mA, 200mA, 10A
- Measures resistance: 200Ω, 2000Ω, 20KΩ, 200KΩ, 2000KΩ
- Diode forward voltage
- NPN and PNP hfe ( gain)
- Test 1.5V and 9V batteries under load: 9V @ 5mA, 1.5V @ 50mA
- Provide 2.7V p-p reference square wave output
- Input Impedance: 1MΩ
- DC Voltage: ±(1.5% + 2)
- AC Voltage: ±(2.5% + 15)
- DC Current: ±(2.5% + 10)
- Resistance : ±(2.5% + 5)
We don’t necessarily endorse any of the following suppliers or products and can’t guarantee their price or availability but here are a few possibilities:
The Harbor Freight tool company offers a couple of low cost 3½ digit hand held DMMs such as this:
It is often on sale for $5.99.
Big box stores like Sears and Walmart etc., have a number of multimeters on-line and in their stores. Home improvement stores like Lowe’s and Home Depot often have a number of low cost meters such as this analog display model for $4.99:
MCM Electronics ( a Premier Farnell company ) offers a wide range of digital multimeters ranging from 1,999 count 3½ digit hand held meters like this one priced at $9.99:
To this 40,000 count 4½ digit, true RMS, 10Mohm input impedance, bench top multimeter for $233.59:
I have owned and used a wide range of DMMs including the $5.99 one from Harbor Freight and the $233.59 one from MCM. The portable hand held models are very useful and handy to have around for general measurements when doing experiments with the Analog Discovery module but when you need more accuracy ( and more features ) the bench model is indispensable.
As always I welcome comments and suggestions from the user community out there.