Tuesday, May 06, 2014

Of VTVMs and FET VMs

Anybody that has gotten into electronics in the past 10 to 15 years is familiar with the digital multimeter and the Volt Ohm Meter. The differences are obvious, the VOM can load down the circuit but allows you to see changes in the circuit on an analog scale. The DMM is rather sensitive and won't load down the circuit, but updates slowly and only provides a digital readout or at best a bar graph display. Can't we have the best of both worlds?

Prior to the digital multmeter and even transistorized electronics we had what was called a vacuum tube volt meter (VTVM). These high sensitivity instruments allowed us to take measurements on electronic circuits without loading them down like a VOM would and of course digital was a few decades away. The VTVM was the best voltage measuring device out there for electronics work. The big disadvantage of the VTVM was that even portable units required a power line connection and multiple vacuum tubes even though later hybrid units only required one (solid state rectifier and solid state diodes for the AC scale).  While I own a VTVM of the hybrid variety, I need to build a probe adapter to replace the missing switching probe that should have come with the unit. As technology progressed with the field effect transistor a VTVM no longer actually required tubes. VTVMs stuck around a while in the FET era until the DMM took over

This is where the FET VM or FET Multimeter comes in. The FET Multimeter is a solid state VTVM and can be made in a battery portable package such as the Sencore FE23 Little Henry Field Effect Multimeter. In fact I just acquired one of these at the local hamfest this past weekend. The FET multimeter gives you the portability of a VOM with the versatility of a VTVM. In addition this unit has a light bulb in it to illuminate the scale. These extremely versatile instruments were made in the 1960s and 1970s, but unfortunately like VTVMs fell victim to the new digital multimeter technology.

While these older instruments are no longer being manufactured they can be found on the used market and are a useful tool to have around the lab or shop.

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