meter protection diodes - Antique Radio Forums

Post by Chris108 » Aug Thu 27, 9:53 am

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The thing you have to look at is the forward turn-on characteristics of the diodes. This information is not always given in spec sheets but it is easily measured on the bench. Many texts over-simplify things by considering a diode to be a switch that is open below 0.3 or 0.7 volts, and closed above those voltages. Reality is a bit different. Detector diodes like the 1N34A, 1N60, and the like are intended to begin conducting at low forward voltages to make them more sensitive as detectors. Below the "knee" in their VI curves they present considerable resistance which drops as the forward voltage increases; by 0.3 volts they are nearly full "on." There are certain "zero bias" Schottky detector diodes which begin conducting at -58 dBm, which is about 300 microvolts (RMS) of signal voltage. Such diodes are not good for meter protection as they may start conducting early and shunting the meter in normal operation, throwing its calibration off.

On the other hand, switching diodes like 1N914 and 1N have shorter "resistive" voltage ranges between when they first start to conduct and when they are fully on, and rectifier diodes are also designed to have short ranges of resistive behavior, since it contributes to heating in power situations. These are better for meter protection since they more or less stay out of the circuit until something bad happens. Problem is, most conventional switching and power rectifiers don't begin to conduct until about half a volt or so, which is too high for many sensitive meter movements. Schottky diodes begin conducting a bit lower, usually around 0.3 to 0.4 volts, and have very short resistive ranges, so they are ideal for protection of 100-mV and 200-mV meter movements. Some test equipment makers have made custom diodes with the right voltage characteristics for specific meters, but those are not universally applicable, nor do they tend to be readily available. So you have to do some circuit analysis on a case by case basis; putting standard diodes across meters may offer a worthwhile amount of protection in some instruments, in others it will be detrimental because it will impact calibration, and in yet others the meter will be damaged before the diodes conduct, which would be useless.

When protection diodes are used in engineered equipment, it is usually considered that most meter movements can safely withstand 100% overloads for brief periods of time, and some can tolerate quite a bit more than that. So one strategy is to use a diode that begins to conduct at 200% to 500% of full scale deflection voltage. This insures the diode will not interfere with calibration by conducting too soon. The meter pointer may slam if overloaded, but it will survive. In a design lab this would be tested by sacrificing a few meter movements, something not possible if you only have the one meter you are trying to protect.

One other thing to consider is, there are other ways of protecting meters which were sometimes used by designers. If the meter is driven by an electronic circuit, for example in a VTVM, chances are the amplifier was designed so it cannot deliver enough current to harm the meter regardless of what you do at the input of the instrument. That's why you almost never see a VTVM with a bad meter unless somebody knocked it off a workbench. Same is true of situations where there are heavy shunts across a meter, or lots of resistance in series with it. Protection diodes are superfluous if the electronic circuit driving the meter cannot deliver enough current to hurt it, or if other components in an instrument would be damaged before the meter would be.

"Hell, there are no rules here--we're trying to accomplish something!"

Thomas A. Edison

The two most important specs are forward current and reverse voltage. I usually buy a higher voltage version (1N) since they don't cost much more and I'm usually buying more than I need so I don't know what I'll be using them for.

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As a flyback (or flywheel) application the (peak) current through the diode will be the same as the current through the motor/coil, and the reverse voltage will be the applied voltage.* (So, in most low-voltage applications, voltage isn't an issue and you just have to check the current.)

I will be using it sometimes for rectification (max 24V)

When I'm building a power supply I usually use a [u]bridge rectifier[/u] which is 4 diodes in one package. I may have made a bridge from 4 1N's, but I don't remember. With a single diode, the power is "off" for half of the AC cycle which means you get more ripple (or you have to use a bigger capacitor) and you can only get half the current.

Or, if you have a center-tapped transformer you can get full-wave rectification with two diodes.

The forward voltage across a standard silicon diode is about 0.7V (depending on current) and in some applications where you want a smaller voltage drop, you can use a Schottky diode. Schottky diodes also switch faster than standard silicon diodes so they are sometimes used in high-frequency circuits.

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• When you disconnect an inductor/coil you get a high-voltage kick (back EMF). But, since the diode is "backwards", that back EMF becomes the forward voltage across the diode. That "kills" the high-voltage kickback, and your diode never sees high voltage.