Silicon Diodes Across Panel Meters - Good Idea?

Discussion in '"Boat Anchor" & Classic Equipment' started by N2EY, Aug 1, 2017.

A popular mod to older gear with "analog" meters is to put diodes across the meter terminals. This idea is particularly popular with older amplifiers such as the Heathkit SB-200/1 and SB-220/1. Some folks use one diode, others two in a "back to back" configuration. Some use Zeners, others ordinary silicon rectifier diodes such as the 1N5408.

The usual stated purpose is that the diodes protect the meter in the event of a short that results in enormous plate or grid currents. The idea is that the diode(s) will conduct at a certain voltage (typically 0.7 volts or so for a silicon diode) and protect the meter from high currents - yet, in normal operation, will not conduct at all.

Is this a good idea? Yes and no.....

The key factor is what voltage appears across the meter in normal operation. This is easily computed by looking at the meter resistance and the full-scale current reading, and doing a little Ohm's Law.

Consider this meter:

http://www.ebay.com/itm/BRAND-NEW-5...-AMMETER-ANALOG-PANEL-METER-USA-/381934081775

The specification says it has a coil resistance of 1 ohm and full scale deflection of 50 mA. So, doing the math, E = IR gives 50 millivolts - 0.05 volts - across the meter terminals at 50 mA. To get 0.7 volts across the meter, we'd need 14 times that - 700 mA - through 1 ohm - before the protection diode would start to conduct. How long that 50 mA meter could tolerate 700 mA without damage is the question....

Now look at this similar meter, from the same supplier:

http://www.ebay.com/itm/BRAND-NEW-U...P-METER-USA-/380393962921?hash=item58913f3da9

The specs say the coil resistance is 1000 ohms. 200 microamps through 1000 ohms gives 0.2 volts. To get 0.7 volts, we'd need 700 microamps through the meter. That's 3.5 times the full-scale current, which is a lot less than the 14 times needed with the 50 mA meter.

In both cases, a diode across the meter terminals will be very far from conducting in normal operation, and will have no effect on the circuit IF the diode has a good "knee" characteristic (doesn't conduct below about 0.5 volt or so).

Looking at the meter used in the Heathkit amplifiers, its specifications say that it is a 200 uA movement with coil resistance of 1400 ohms. E equals I times R......0.28 volts gives full-scale, so a good silicon diode will have adequate "headroom".

Note that in amplifiers such as the SB-200, the metering circuits use both shunt and series multiplier resistors, both of which must be in tolerance to get an accurate reading. Note that the diode(s) go across the meter movement itself, right at the terminals.

Some folks say to use "back to back" diodes. This clearly means two diodes in parallel, with polarities opposite - with two diodes in series, with opposing polarities, neither would conduct until the voltage exceeded the PIV! Two diodes in parallel-reversed polarity are only needed if there's a chance of reverse current through the meter - it is left to the reader to determine if that is needed.

Which diode to use? Obviously, for a circuit where the currents are measured in milliamps or microamps, a 1 amp diode such as the 1N400x family is adequate. What's more important is how sharp the "knee" of the diode is, and the exact conduction voltage. Both are easily determined with a good multimeter, battery, and series potentiometer.

73 de Jim, N2EY

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2. K8ERVHam MemberQRZ Page

Here is a trick that may be helpful.

Arrange for the meter to read full scale, or as close as you can get in normal service. Now shunt it with your
protection diodes. If the meter dips just little bit you know the diodes are starting to conduct and should
provide good protection.

TOM K8ERV Montrose Colo

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Sure...but I suspect there will be very few cases where the diodes are that close to conduction.

Another method is to determine the meter resistance and then make up a simulator using a resistor of the same value and the diode(s), plus a voltage source and series resistor. Voltmeter across the diode, milliammeter in series with the whole shebang. Increase voltage until the voltage suddenly "knees off" at around 0.7 volt, and see what the series current is.

One more point:

The diodes should be right at the meter, and should be effectively bypassed for RF. Otherwise you may get all sorts of odd readings.

73 de Jim, N2EY

4. KA9JLMHam MemberQRZ Page

I think it is a good idea.

I am a diode and fuse nut too.

KD2ACO and N2EY like this.

It's a good idea IF one does the calculations and determines what the actual voltages across the meter are, in normal operation and under fault conditions.

For example, if a 2 or 3 times overload causes the diode to conduct, but it doesn't conduct under normal operation, it's great!

But if it takes 10, 20 or more times the normal full-scale current to make the diode conduct......there's no protection.

None of the Southgate Radio transmitters have their meters protected by diodes. They don't need it, because of the design.

73 de Jim, N2EY

6. KA9JLMHam MemberQRZ Page

If that is the case, The meter sends out smoke rings in hope for help.

Other parts do too.

AF6LJ likes this.
7. KL7AJHam MemberQRZ Page

It's best if you can use a diode with an abrupt conduction voltage, like a zener or avalanche diode. "Normal" diodes with a soft knee will conduct well below their "official" turn on voltage and will thus compress the voltage reading, affecting the accuracy (to varying extents). There are, of course, more elegant clamping circuits, but they get fairly complicated.

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8. WA7PRCHam MemberQRZ Page

The thing is, the meter shunt vs the current produces a voltage that may be above the point of diode conduction. More than one diode in SERIES may be required. 700mA through 1Ω current shunt produces 700mV. Even at lower IANODE current, a protection diode can start to conduct, resulting in a misleading meter indication.

Also, in the case of a typical ham grounded grid HF amplifier, the diode(s) should be placed between -B and chassis as close as practicable to the negative terminal of the bottom electrolytic capacitor. Tom W8JI discusses it here (link) and here (link). As well, to protect an expensive HV transformer, there should be a 'glitch' resistor. W8JI wrote here (link):

"Arc current is limited by the resistance and impedance of L2 [anode RFC], the fault protection resistor, the filter capacitor series resistance, and the impedance of the power supply negative-rail-to-chassis resistance.

Typical path resistance would be about 10-12 ohms without the fault resistor, and 20-22 ohms with it. There is also additional resistance inside the tube.

If the high voltage was 3000 volts, typical fault current would be under 150 amperes with a fault protection resistor included, and 100 amperes with the additional 10 ohms."

A HV fault can result in a yuuge discharge from the filter capacitor(s) in the hundreds of Amperes. A typical 1N4xxx 1A rectifier can handle a surge of about only 30A. A 1N54xx 3A rectifier can handle 200A. A 6Axx (6A) rectifier can handle 400A. The 6A10 can be found for pennies.

Sure. That's why you need to check how the circuit actually behives.

I agree 100% - BUT - what you're talking about is a slightly different thing from the thread title..

The practice in question is that of putting diodes directly across the meter terminals, not across a shunt resistor at the "bottom" of the capacitor stack.

Putting diode(s) between "B-" and chassis is a good idea anyway, Besides the protection of the meter, it also protects the meter shunt resistor. Also, if the meter shunt resistor should open, the diodes will (hopefully) limit the voltage rise of B-. Diode(s) across the meter won't.

W8JI's advice is, of course, excellent. The problem is getting people to read it, and, even more challenging, to understand it.

Which is why I mentioned setting up a "simulator" and checking the actual circuit behavior before finalizing anything. Such simulation is easy - just a couple of flashlight batteries, clip leads, a limiting resistor of appropriate size, and a known-accurate meter for comparison. Done correctly, you can verify the accuracy and protection of the entire metering circuit.

------

It's......almost amusing.....how the whole thing has evolved.

The Ancient Ones didn't have such fussy metering and protection circuits. They just put a plate current meter of appropriate range in the B+ lead and were done with it. B- was grounded solidly and that was that. If meter insulation was an issue, they submounted the meter behind a piece of glass or Lucite.

Glitch resistors and such were not a concern because they used choke-input power supplies, and the total filter capacitance was much lower. Such supplies had much lower fault currents. They also had much better power factors and crest factors, and didn't need fancy "soft start" protection. Of course they needed a time-delay circuit to prevent applying HV to the mercury-vapor rectifiers before they'd had their warmup time, but that was a simple thing to implement with a time-delay relay and a stick repeater relay.

Yes, they were big and heavy, but a good solid chassis and some casters would do the trick.

All this "miniaturization" and such....one sometimes wonders if it's worth it.

73 de Jim, N2EY

10. WA7PRCHam MemberQRZ Page

I did. I like behives... that's where we get hunny.
It's related -- it's worth protecting an unobtanium and/or VERY expensive transformer. Proper sizing of the protection diode(s) should be done.
Electrically, it's the same as putting the diode(s) across the individual meters except, it also protects the wiring. Placed at the meters, the wiring becomes a fuse. When the insulation burns up, it lets you know there's a problem.

^ ^ ^ A yuuge understatement ^ ^ ^