Today, we have a HM100 Digital Multimeter from our friends at farnell!
The multimeter gives a lot of features for not much money, costing only £31.23. Functions include:
- Autoranging (so you dont have to keep changing settings until you find if you should be in Ohms or MegaOhms)
- Backlit (for hacking in the dark)
- Auto Power off (who doesnt forget to turn their meter off)
- Diode Tester with Beep
- Measurements include
- AC & DC Voltage
- AC & DC Current
- hFe (Transistor Gain)
- Temperature (using supplied probe)
First impressions when opening it up are very good, the accessories are all well packed, i was surprised by the inclusion of remote thermocouple for temperature sensing, as that isnt a feature normally found on such cheap meters. The LCD screen is nice and BIG, so you can see the numbers even in low light, and the clip-out stand is a very clever addition.
Sensing voltage is fairly easy, switch the meter to AC or DC, and put the probes of the meter across the thing you want to measure the voltage across.
Current is also easy, interrupt the circuit, and put the meter in-line with the circuit you want to measure.
Resistance sensing is the same as in most multimeters, put the probes in parallel with the device you want to measure the resistance of. Bear in mind that if you’re measuring something thats already in a circuit, you’ll be measuring the resistance of the whole circuit between those two points, not just of the device you’re looking at!
To measure Capacitance, use the multi-function socket, and plug in your capacitor to the holes next to Cx. If your capacitor is polarised, make sure you plug it in the right way round! Measuring this capacitorshows it’s actual value is 50uF less than it is marked.
hFE is transistor gain, and is often misunderstood, so i’ll try and give a quick overview of it. Transistor Gain is the amount of amplification between the current flowing into the base pin (Base Current, Ib) and the current flowing into the collector pin (Collector Current, Ic). It can be calculated by dividing the Collector Current by the Base Current, and has no units. To get the total current rating of the transistor, we add together Ib and Ic, to get Ie, the Emitter Current.
Lets add some numbers! hFE = Ic / Ib, so if the current flowing into base is 100mA, and the current flowing into collector is 1000mA, the hFE will be 1000mA/100mA = 10.
This also works the other way round, so if we know the hFE transistor of our transistor is 10, and the Base Current is 100mA, we multiply them to get the Collector Current 10*100mA = 1000mA = 1A. If we need the total current rating we can now add together Ic and Ib, getting 1100mA (1.1A).
To test the gain of a transistor on the meter, first check the data sheet for the pinout of the transistor. Put the transistor into the multi-function socket. The meter will show the the gain of your transistor! Yay!
An unusual function on a low cost meter, the thermocouple allows you to measure temperatures from -20ºC to 1000ºC (!!!)
The Diode tester is used to measure the forward voltage drop of a diode. You can also use it to check the polarity of a diode by trying the diode in each way.
Sometimes, you’ve just gotta check that something is connected. The Continuity tester lets you ‘beep it out’, ie. it will emit an audible beep if the probes are connected. We often use this when making a PCB, as you can end up with tiny shorts or track breaks that can make really odd things happen!
General Usage & Verdict
First off, bad points! There are a couple of (very) minor oddities i’ve noticed while using the meter, the first being that you have to manually switch it to DC when measuring Voltage or Current. As a hobbyist, i’m usually using DC, so it’d be nice if this setting was remembered between function changes. Relatedly, when the auto power off is activated, and the meter turns off, turning it back on puts you back in AC mode. Its not a massive problem of course, but has led to a few head-scratching moments.
All in all, the HM100 is a fully featured multimeter, for a sensible price, with a few minor niggles. For 30 quid, it provides functionality only usually seen on a £150+ multimeter. If i was after a multimeter, i’d buy one!