Thursday, June 13, 2013

DMM, VOM, DVOM, TVM & VTVM

Every radio engineer (and most any geek of the tech variety) needs two things: A soldering iron and a decent Multimeter. I've seen them called DVM for Digital Volt-Meter, or DMM for Digital Multi-Meter. Older engineers sometimes call them a VOM, an acronym for Volt Ohm Meter. Once I saw it in a catalog as a DVOM, Digital Volt-Ohm Meter.  Actually there are enough acronyms for these to just make a short list:
DVM - Digital Volt Meter
DMM - Digital Multi-Meter
VOM - Volt Ohm Meter
DVOM - Digital Volt-Ohm Meter
FET VOM -  Field Effect Transistor Volt Ohm Meter
FET VM - Field Effect Transistor Volt Meter
TVM - Transistorized Volt Meter
VTVM - Vacuum Tube Volt Meter 
DVTVM - Digital Vacuum Tube Volt Meter 
 This tool measures all the basic units of electrical power: AC voltage, DC Voltage, resistance, capacitance, ohms, and current. Nicer units will also measure frequency. Actually most geeks have a nice one and then a cheap crappy one for doing things that they know might immolate the device. Cheapo units are generally less accurate which can really matter when you're modifying a more complex circuit, like something you might connect to an Arduino. Even something as simple as a continuity test requires a multimeter. This can be used to test cables, fuses but also board soldered components. Does current pass between point A and point B?  You can't tell just by looking at it.

The VTVM is obviously an older device. The first volt meters used vacuum tubes, we didn't call them VTVMs because there were no transistorized devices. So that acronym was born later to distinguish those early analog models. Early VTVMs are about the size of a small guitar amp. There were numerous branded models. RCA had one absurdly named the "Voltohmyst." There were some early 50s units that actually were DVTVMS, employing both analog and transistor components. In the 1960s Heathkit made models you could assemble yourself. The use of that acronym dates back to at least 1949 even though the first truly digital voltmeter wasn't invented by Andrew Kay at Non-Linear Systems until 1954.  More here.

The first volt meter was a primitive galvanometer first invented by Hans Oersted in 1820. Oersted was just describing the effect of current on a compass needle. Later that year Johann Schweigger developed a moving coil galvanometer. Devices descended directly from this design were later used in the first telegraphs to detect the signals passing through the wire. It's been about 290 years since those first devices and now you can buy a cheap volt meter at a truck stop for under $10.
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