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Author Topic: What is a Varistor?  (Read 8809 times)
KE3WD
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« Reply #15 on: August 21, 2013, 08:24:58 PM »

So put two volts or slightly more to it in series with about a 1K zitter and use your DMM across it to monitor the volt drop.  

Two AA cells, three volts, less voltage with worn cells, should do it.


73
« Last Edit: August 21, 2013, 08:29:40 PM by KE3WD » Logged
KA4POL
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« Reply #16 on: August 21, 2013, 10:03:39 PM »

MV-203 is a 2.1 V zener diode. I guess you know how to check it  Grin
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W7EJT
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« Reply #17 on: August 22, 2013, 07:48:11 AM »

Ah Ha! Someone agrees with me!!! Yes I know how to check it.

Yes, it has Zener function, however there are no "zeners" manufactured below 3.3v. So, two chips are used in the forward direction Vf (about .8 to .9 volts each). These two chips make a device called a Stabistor! You can make one by putting two (or more) regular diodes in series - no problem.

I maintain that the TS-440s Service Manual calling M203 a Varistor is an error, it should state Stabistor.

Perhaps in the 80's different terminology was used.

My .02


MV-203 is a 2.1 V zener diode. I guess you know how to check it  Grin
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KE3WD
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« Reply #18 on: August 22, 2013, 08:00:28 AM »

Many diodes sold as Zeners today are actually built up from using as many single diode junctions in series as will yield the voltage, the diodes are actually installed backwards in them, or another way to look at it is that the cathode band on the case is painted on the "wrong" side. 

If you are looking at the available voltages for a given subset of "Zeners" - and those voltages available are in multiples of 0.6V, odds are good that is what they really are...

0.6V, 1.2V. 1.8V, etc. 

I've even noticed some few imports that are now using 0.7V as the increments in a few cases, I avoid those.

There are still "real" Zeners available, that use the Avalanch method proper, and the available voltages do not fall in these increments, which is the clue when searching for such.  Most times we don't really need these, the noted exception being certain older circuits in need of repair that require the Zener to nail a voltage that falls outside of the forward voltage drop increments. 

They really shouldn't call those Zeners, IMO, but it is what it is.   

Two 1N4148s or two equivalent in series and connected backwards as to the Zener cathode marking can function as a 1.2V "or so" Zener. 

When stacking available diodes in this fashion and the need is to get closer to a specified voltage that falls outside of those volt-drop increments, one can use Schottky diodes and their 0.2V drop to get closer to the originally specified voltage. 


73
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KA4POL
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« Reply #19 on: August 22, 2013, 12:22:48 PM »

NTE Electronics offers the NTE5000A 2.4 V, 5001A 2.5 V, and so on 2.7 V, 2.8 V, 3.0 V.
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W7EJT
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« Reply #20 on: August 23, 2013, 10:59:59 AM »

Just did that - (device removed) it WORKS as advertised! It conducts around 2V...



So put two volts or slightly more to it in series with about a 1K zitter and use your DMM across it to monitor the volt drop.  

Two AA cells, three volts, less voltage with worn cells, should do it.


73
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KE3WD
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« Reply #21 on: August 23, 2013, 11:17:55 AM »

There ya go. 


73
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W6EM
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« Reply #22 on: August 23, 2013, 01:11:20 PM »

The MV device is a varactor diode.  Varactors are reverse biased and as the reverse bias is varied over a given range, the capacitance across the junction varies as electrons are forced away from the junction.  Essentially, a bias-controlled variable capacitor.  Primarily used in voltage-controlled oscillator circuits of phase-locked loops.  Or, for TXO or RXO circuits.

As for varistors, they behave like back to back zeners and avalanche at bipolar voltage points.  The voltage is known as the MCOV or minimum cutover voltage.  They are not like a "hard" implementation of back to back zeners (called TVSSs) in that they are guaranteed to pass a given energy (amp-squared seconds) and not burn open.  Think of them as a group of back to back zeners.   Varistors usually, over time, fail to do their job eventually by burning open slowly.  Opening up the parallel junctions.  A TVSS, however is a one-time, fast short circuit that quickly presents a permanent short across the applied voltage bus, to operate fuses or breakers.
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KA4POL
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« Reply #23 on: August 23, 2013, 10:14:02 PM »

The MV device is a varactor diode. 

No, this is not correct. For example this diode is used in a TS-440 from the emitter of a 2SC2459 NPN transistor to ground to provide a 2 V level for the emitter. Nothing else is connected there. By the way Kenwood recommends a MA27T-B as a replacement. If you check out datasheets you'll find this again listed as Zener diode.
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KE3WD
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« Reply #24 on: August 24, 2013, 06:04:39 AM »


Sometimes the terminology in older Japanese manuals suffers from translation/transliteration issues. 


73
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W6EM
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« Reply #25 on: August 24, 2013, 05:51:03 PM »

The MV device is a varactor diode. 

No, this is not correct. For example this diode is used in a TS-440 from the emitter of a 2SC2459 NPN transistor to ground to provide a 2 V level for the emitter. Nothing else is connected there. By the way Kenwood recommends a MA27T-B as a replacement. If you check out datasheets you'll find this again listed as Zener diode.
Per W9GB from a 4/11 QRZ post:MV203 - Varactor Diode
Mitsubishi Electric Semiconductor

V(RRM) Rep. Pk. Rev. Voltage = 30 V
Ct{Cj} Nom. Junction Cap. = 11 pF
@V(T){V(J)} (Test Condition) = 3.0 V
C1/C2 Min. Capacitance Ratio = 4.3
Q Factor Minimum = 300
@Freq. (Test Condition) = 50 MHz
P(D) Maximum Power Dissipation = 250 mW
Semiconductor Material = Silicon
Status = Discontinued
Package = DO-35
Military = N
1.88 - 1.98 V (3mA)

These are listed as suitable substitutes: BZ102/2V1; BZX75/C2V1; BZV86/2V0; ZTE 2
====

73,
Lee
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KA4POL
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« Reply #26 on: August 24, 2013, 09:48:55 PM »

Did you see the substitute diodes? All are Zener types (BZxx, BZXxx, BZVxx). With limitations you could use a regular diode as Varicap.
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W6EM
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« Reply #27 on: August 25, 2013, 10:00:39 AM »

Did you see the substitute diodes? All are Zener types (BZxx, BZXxx, BZVxx). With limitations you could use a regular diode as Varicap.
Every diode has an avalanche knee voltage.  So, each and every diode is a zener at some point.  If you exceed its PIV.

However, you won't find the specs as noted in the reference (like c1/c1 ratio and cT at a given volltage) for anything but designated varactors or varicaps.

Yes, even a 1N4148 can be a varactor or a zener.  Depends how you apply voltage.

Just curious, I don't have the TS-440 schematics in front of me, but you said something about it (MV-203) developing emitter bias.  Never, frankly, heard of that.  Especially if there's a coupling cap beteen the varicap and the emitter, as is usually the case in VCOs.  The voltage is, instead, applied to the varicap to achieve the desired capacitance.
« Last Edit: August 25, 2013, 10:05:00 AM by W6EM » Logged
KA4POL
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« Reply #28 on: August 25, 2013, 10:56:14 AM »

As I said, the diode is connected from emitter to ground, cathode at ground. No cap in between. The voltage chart says 2 V at emitter. I think we do not know which rig we are talking about here. Probably a similar circuit.
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W7EJT
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« Reply #29 on: August 25, 2013, 11:04:22 AM »

correct. vco-5 on the ts-440. Q35 npn transistor has it's emitter to this 2 volt "zener" to ground.
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