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 Post subject: METER MOVEMENT PROTECTION DIODES
PostPosted: Feb Mon 18, 2013 5:04 pm 
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Location: Georgia
Rebuilding a Hickok TV3/BU and before buttoning it up thought about paralelling the meter movement with a couple of paralell opposing diodes to "clamp" down the voltage across the meter in case anyone "pushed the wrong button". The meter movement is typical in that it is
1000 ohms and 50 microamps which translates to a full scale voltage of 50 millivolts (.05v).
If I used silicon diodes they do not conduct until about .6 - .7 volts which represents about a 14X overload across the meter. I think using germanium diodes in this application is a better choice since they conduct at about .3 volts which represents only about a 6X meter overload.
I did an empirical application where I deflected the meter about 3/4 of full scale and placed the germaniums across the meter and the meter did not change whatsoever so I do not believe this would introduce any error. Anybody out there tried this??


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 Post subject: Re: METER MOVEMENT PROTECTION DIODES
PostPosted: Feb Mon 18, 2013 5:41 pm 
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The idea is great. I use two silicon diodes in parallel, back to back, in this type of circuit. Even clamping at .7 volts is enough to save a meter.

Although germanium diodes clamp around .2 volts they have leakage than can have an effect on a meter reading. Since it didn't change your meter reading use germanium diodes.

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 Post subject: Re: METER MOVEMENT PROTECTION DIODES
PostPosted: Feb Mon 18, 2013 6:06 pm 
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Wow that is a great idea!

I just had to replace a bad meter in a B&K 650 - I'm going to see if I can't implement something like that to make sure the same thing doesn't happen again. :D


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 Post subject: Re: METER MOVEMENT PROTECTION DIODES
PostPosted: Feb Mon 18, 2013 11:02 pm 
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I have a few AVO testers with 30uA meter movements which are sensitive to overload damage. There is a very good posting on protecting the meter and the use of both 600 mV Silicon clamping diodes and 200 mV Schottky 1N5711 clamping diodes. The type of Schottky diode will depend on the uA (mV) of the movement, so do a full scale check with/without the diode protection. The reason for using both, is the schottky has a faster response, and should it overload, the silicon diode would block catastrophic damage. I also use a MLCC non-polar 100uF 25V capacitor (AVX ST203C107MAJ05) to dampen the meter movement, which has benefits over conventional electrolytics.

http://www.vintage-radio.net/forum/show ... ter&page=3
A pair of 1N4001 standard Silicon rectifier diodes has been connected in parallel to the 1N5711 Schottky pair. A combination of Schottky and standard Silicon diodes is superior to Silicon diodes alone:
• The maximum current through the two Schottky diodes is greatly reduced.
• The Schottky diodes have a much higher differential resistance than the Silicon diodes but they begin conducting at a lower voltage. This additionally reduces meter overshoot during normal operation compared to Silicon diodes alone.
• Due to their small differential resistance the Silicon diodes safely carry even high currents and thus protect the entire meter circuit also in cases of valve inter electrode shorts or erroneous electrode selector switch settings.


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 Post subject: Re: METER MOVEMENT PROTECTION DIODES
PostPosted: Feb Mon 18, 2013 11:14 pm 
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Is there a white paper on this anywhere?

My B&K has 100 uA movement, my Triplett VTVM 200uA. It would be nice to have a quick reliable method of calculating the correct protection.


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 Post subject: Re: METER MOVEMENT PROTECTION DIODES
PostPosted: Feb Tue 19, 2013 12:20 am 
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Quote:
I also use a MLCC non-polar 100uF 25V capacitor (AVX ST203C107MAJ05) to dampen the meter movement, which has benefits over conventional electrolytics.


A good choice. This is not an electrolytic, it's a ceramic capacitor designed for switch-mode power supplies. I think most any electrolytic would have enough leakage that it would affect the meter reading accuracy.

The diode choice for meter protection also depends on how much fault current is available. When I built an 813 power amplifier years ago, I used back-to-back 1N4001s. The B+ was 2.2 KV and would blow through (I mean shrapnel) a 1N34 without even tripping the breaker!

Rich


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 Post subject: Re: METER MOVEMENT PROTECTION DIODES
PostPosted: Feb Tue 19, 2013 12:38 am 
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Just be aware that all diodes, silicon or germanium, do start to conduct at zero forward voltage. They do not "clamp" at .6V, and any "knee" is an artifact of the linear voltage scale used for the plot. Whether they conduct enough to affect the meter reading, you would have to find by experiment: apply full-scale current, connect a diode and see if the meter reading changes.


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 Post subject: Re: METER MOVEMENT PROTECTION DIODES
PostPosted: Feb Tue 19, 2013 8:22 am 
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Infineon makes the BAT 63-02V H6327, rated 200mV Vf @ 10mA If. This device is marketed as a detector diode rather than a rectifier, but would work in this application. The package is reasonably convenient for the application also, as it could be mounted on a small PC board that mounts on the meter studs.

They're $0.41 each at Mouser http://www.mouser.com/ProductDetail/Inf ... qrQ6y80%3d

- Leigh

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 Post subject: Re: METER MOVEMENT PROTECTION DIODES
PostPosted: Feb Tue 19, 2013 4:09 pm 
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I'd opt for a power Shockley diode over a germanium. You'd want a protection device that will survive the overload, at worst short, rather than burn open. A small germanium signal diode isn't going to shunt much current before burning open, like a fuse.

pete

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 Post subject: Re: METER MOVEMENT PROTECTION DIODES
PostPosted: Feb Tue 19, 2013 5:36 pm 
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Peter,

I know you meant Schottky diode, not Schockley diode. Haven't seen a Schockley diode in over 35 years. They used to be used to trigger SCRs. Nearest "modern" equivalent is a DIAC.

Rich


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 Post subject: Re: METER MOVEMENT PROTECTION DIODES
PostPosted: Feb Tue 19, 2013 5:49 pm 
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Mea cupla... :oops: Thank you for catching that, Rich!

Pete

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 Post subject: Re: METER MOVEMENT PROTECTION DIODES
PostPosted: Feb Tue 19, 2013 11:51 pm 
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A few additional considerations, as noted the Vf is a usually function of current and can also change appreciably with temperature. The temperature effect on Vf could be an issue with Schottky diodes for meter protection in a tube tester that uses tubes such as the 83 and 5Y3 and left on for long periods of time. The one exception to some degree is Silicon Carbide diodes which do not conduct at very low currents. At 1 ua standard diodes and Schottky diodes conducted with no difference between applied and measured voltage drop Vf, Silicon Carbide diodes did not start conducting until 0.3-0.4V was applied. At 1 ma, the Vf for a standard diode was ~0.6-0.7V, Schottky diodes (3-5A 100V) were ~0.3-0.4V and Silicon Carbide diodes (2A and 5A 1200V) were 0.75-0.80V. I haven't tried the Silicon Carbide diodes for meter protection, probably to high a Vf for 30-50uA meters, but may work well with other meters with a slightly higher full scale voltage or situations where the diode could be exposed to high voltage in a protection mode. As mentioned it is always important to check any changes in full scale readings w/wo the diodes across the meter. Most of my other tube testers with 115-120 uA movements FS ~200mV, I have used two 2A 1000V conventional diodes across the meters with no ill effects.


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