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 Post subject: Meter Protection Diodes Re-Re-Visited
PostPosted: Sep Sun 20, 2009 1:00 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 519
Location: Peru,IL USA
I originally posted a photo of a meter with a pair of cross-connected 1N4007 diodes across the meter terminals as a reminder to protect precious meters used on tube testers and other gear.

Then I did some preliminary tests and found that a 1N4007 diode has a forward voltage drop starting at about .4 volts and increasing as current thru it rises. Using a 1 1/2 volt battery I noted that the drop rose to about .9 volts. Since this .9 volts is applied to the meter coil it forces over four times the rated current thru an Eico 666 meter for example. The Eico meter is .0002 amp, 1000 ohm, and .2 volts full scale.

I feared that the excess current would bend the needle and possibly burn out the coil. Suggestions were made to use Schottky diodes. I went to the Mouser catalog and web site but did not find anything suitable and economical.

I searched in some of my books for meter protection. I found this article:

Image

The second from last paragraph indicates that a 50uamp(.00005amp) meter could withstand a one milliamp surge. That is 20 times more that its rated 50uamp full scale rating.

This made me wonder if I was fretting needlessly about protecting less sensitive meter such as the 200uamp on the Eico 666. I decided to test a few different meters by subjecting them to over voltage/overcurrent surges. Here is a photo of the meters I tested.

Image

Specs of each meter:

A. Large TVM Devry Meter: .001 amps(1000uamps) full scale, 101.7 ohms, .102 volts FS

B. VU/DB meter: .000180 amps(180 uamps) full scale, 650 ohms, .117 volts FS

C.Tiny round meter: .000094 (94 uamps), 1202 ohms, .113 volts FS

D.Small sideways reading VU meter: .0002 amps(200 uamps) full scale, 1190 ohms, .238 volts FS

E.Midland Zero Center Meter: .00005 amp (50 uamp), 590 ohms, .0295 volts FS

I cross-connected 1N4007 diodes to each meters terminals. Then I applied 1 1/2 volts from a "AA" cell to the meter terminals. The measured voltage across the terminals was from about .83 volts to .932 volts for each meter. The meter needles pegged severly. I applied the 1 1/2 volts a dozen times, but the meters were undamaged. There were no bent needles as they returned perfectly to zero after the abuse. I also left the 1 1/2 volts applied for aan extended period to the point where the forward conducting diode was noticably heating to the touch. Still no discernable damage to the meters. The Devry meter's coil is partially visible and I could not detect any darkening of the coil wire as might be expected if too much current is flowing.

I next repeated the experiment with 150 volts from my old Heath PS-4 power supply. The milliamp meter on the Heath pegged thus exceeding its 150 milliamp full scale marking. The volt meter on the power supply dropped nearly to zero due to the large overload. I applied the 150 volts another dozen times. All three test meters survived with no apparent damage. The milliamp meter on the power supply also withstood the abuse although I do not know the extent of its overcurrent.

It would seem that my fears of meter damage due to the diode forward voltage drop were unfounded. Four meters were getting a dose of current about 8 or 9 times full scale rating without damage. The 50 uamp Midland meter received a whopping 29 times its rated current and voltage.

I do not plan to test any of my actual tube tester meters this way, but I now feel secure that the 1N4007 diodes are adequately protecting them.

I plan to purchase some 1N5819 Schottky diodes that reportedly have a starting forward drop of about .3 volts and perhaps retest the Midland 50 uamp meter in the future. I am curious at what voltage they will clamp at with the same test voltages applied. Lower? Same? Higher?

Ron

PS: Alan's book serves as a nice backdrop and propped up the Devry meter, too.


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 Post subject:
PostPosted: Sep Sun 20, 2009 1:20 am 
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Joined: Mar Mon 17, 2008 5:05 am
Posts: 5599
Location: Ashhurst, New Zealand
Poor little diodes....

Hope you don't try the same treatment to the relatively expensive Schottky diodes. A shorted "AA" cell will put out about 2Amps for a while. Your forward biased diode had to pass that current. Fortunately for you the diode held together - if it had gone open circuit the meter needle would have gone round a few times before it had had enough..... :( :( :(

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 Post subject:
PostPosted: Sep Sun 20, 2009 1:45 am 
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Joined: Jul Fri 10, 2009 1:49 am
Posts: 154
Location: Location: Location: West Virginia
Mouser 1N5819---9 cents each, 7.3 cents each for 100

http://www.mouser.com/Semiconductors/Di ... ord=1N5819





Dave

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 Post subject:
PostPosted: Sep Sun 20, 2009 1:59 am 
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Joined: Oct Sat 20, 2007 3:36 am
Posts: 11481
Location: Southern NH, 03076
The Heathkit SB-200 & 220 series ham amps use a 200ua meter, I forget the resistance but its decent quality unit. Long out of production, almost unobtanium with Ebay bringing in the $75 range.

The meter goes into orbit when there is an internal tube arc and the meter is set to read grid current. The arc is with around 2500V B+. With a pair of 1N4007's the meters survive the arcs and the diode failure mode is a short.

Carl


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 Post subject:
PostPosted: Sep Sun 20, 2009 3:27 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 25381
Location: Pocasset, Cape Cod, MA
Nice to see some actual experiments. The Schottky's may work better, but could also conduct enough at the lower voltage to affect the meter readings. Don't forget that diodes have no "knee" when they "start conducting". They conduct a little at any forward voltage.


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