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eHam Forums => Elmers => Topic started by: W4FBI on August 21, 2013, 11:03:13 AM



Title: What is a Varistor?
Post by: W4FBI on August 21, 2013, 11:03:13 AM
My problem "varistor" shows as a normal diode in the schematic. Not sure what it is, tho. I know what a diode is, but this varistor tests infinity in both directions with an ohm meter.

I also have a diode tester in my DVM - it shows infinity in one direction. In the other direction (and just for a millisecond) it shows some voltage, then goes to infinity.

This is not to be confused with a varactor (capacitance varies with applied voltage). I have tested some of these and they test like a regular diode: infinity in one direction, and 700 millivolt drop in the other.

Kenwood calls it MV203


Title: RE: What is a Varistor?
Post by: KH6AQ on August 21, 2013, 11:09:48 AM
The failure mode for a Varistor is a short. If you don't measure a short - and you don't - the Varistor is good.


Title: RE: What is a Varistor?
Post by: WN2C on August 21, 2013, 11:10:30 AM
From Wikipedia:

A varistor is an electronic component with a "diode-like" nonlinear current–voltage characteristic. The name is a portmanteau of variable resistor. Varistors are often used to protect circuits against excessive transient voltages by incorporating them into the circuit in such a way that, when triggered, they will shunt the current created by the high voltage away from sensitive components. A varistor is also known as Voltage Dependent Resistor or VDR. A varistor’s function is to conduct significantly increased current when voltage is excessive.

Wikipedia is the first hit on Google.  Remember, Google is your friend!

Rick  WN2C


Title: RE: What is a Varistor?
Post by: KE4JOY on August 21, 2013, 11:18:12 AM
name is a portmanteau

Rick  WN2C

I knew what a varistor was but I learned a new word!!!  ;D


Title: RE: What is a Varistor?
Post by: W4FBI on August 21, 2013, 11:33:13 AM
OK, forgot about Wiki  :-[

Anyway, I just read that my varistor in question (long obsolete) my be replaced with two common 1N4148 diodes in series.

Anyone agree with that?


Title: RE: What is a Varistor?
Post by: KE4JOY on August 21, 2013, 11:47:09 AM
Probably but you will need to double check the voltage drop.

BTW varistors are still available and you should be able to find one that is reasonably close. However from your description above it does not seem that it needs to be 'fixed'. Varistors are commonly used as ESD suppression devices.

http://www.mouser.com/Search/Refine.aspx?Keyword=varistors



Title: RE: What is a Varistor?
Post by: K7KBN on August 21, 2013, 01:34:03 PM
name is a portmanteau

Rick  WN2C

I knew what a varistor was but I learned a new word!!!  ;D

Now, if you also have a STARBOARDMANTEAU, you have a balanced system... ;)


Title: RE: What is a Varistor?
Post by: W4FBI on August 21, 2013, 02:12:56 PM
I am only familiar with the AFTMANTEAU  ;D

I have (sort of) answered my own question. Here is the datasheet info on the Varistor in question (MV203):
---------------------------------------------------------------------------------------------------------------------
MV203 - Varactor Diode (interesting that Mitsubishi calls it a Varactor)
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
VF: 1.88 - 1.98 V (3mA)

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

------------------------------------------------------------------------------------------------------------

So, I look up the "suitable substitutes" and low and behold, they are STABISTORS. I do know what they are! It's just a very low voltage zener diode. We use to make those (Motorola Semiconductor) using two chips in the forward direction (1.7 - 2.0V) depending on current. Typical Zener diodes start at 3.3V and go to about 200V (in the reverse direction).

Opinions?



Title: RE: What is a Varistor?
Post by: N4NYY on August 21, 2013, 03:02:58 PM
Check this out: http://forums.qrz.com/archive/index.php/t-255680.html (http://forums.qrz.com/archive/index.php/t-255680.html)

I was unable to find the replacement (MV27). You might want to call Kenwood and see if they have a current cross reference.


Title: RE: What is a Varistor?
Post by: KE3WD on August 21, 2013, 03:07:42 PM
...In the other direction (and just for a millisecond) it shows some voltage, then goes to infinity.

Is that an in circuit or out of circuit reading?


Title: RE: What is a Varistor?
Post by: W4FBI on August 21, 2013, 04:14:43 PM
That is "out of circuit" measurement.

Personally, I think my particular device is "open" (infinite resistance in both directions)

I'm about to try a stabistor replacement (two 1N4148 in series). This would measure 1.7 - 2V in the forward direction (infinite in the reverse). The schematic says 2 volts at this point (anode)

This is what the "replacement" (substitute) calls for...


Title: RE: What is a Varistor?
Post by: KE4JOY on August 21, 2013, 04:22:54 PM
Is this in a meter circuit by chance?


Title: RE: What is a Varistor?
Post by: AA4PB on August 21, 2013, 04:35:42 PM
A varistor should be "open" in both directions until the applied voltage exceeds the breakdown voltage. Your ohmmeter probably doesn't apply enough voltage to make it conduct. Once the breakdown voltage is exceeded then it will conduct in either direction. Take a look at the conduction current curves on the Wikipedia page.


Title: RE: What is a Varistor?
Post by: W4FBI on August 21, 2013, 04:40:27 PM
No, this is in a VCO circuit (specifically, VCO-5, TS440)


Is this in a meter circuit by chance?


Title: RE: What is a Varistor?
Post by: W4FBI on August 21, 2013, 04:53:45 PM
Correct, but the device in question is a silicon device. There is no mention of silicon in Wiki (Varistor). I think the device is incorrectly named (Varistor) in the service manual. The substitiute for a MV203 is a Stabistor. And in the schematic it is looking for 2 Volts at the anode (cathode is grounded) - just right for a stabistor. And it is a silicon device.


A varistor should be "open" in both directions until the applied voltage exceeds the breakdown voltage. Your ohmmeter probably doesn't apply enough voltage to make it conduct. Once the breakdown voltage is exceeded then it will conduct in either direction. Take a look at the conduction current curves on the Wikipedia page.



Title: RE: What is a Varistor?
Post by: KE3WD 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


Title: RE: What is a Varistor?
Post by: KA4POL 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  ;D


Title: RE: What is a Varistor?
Post by: W4FBI 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  ;D


Title: RE: What is a Varistor?
Post by: KE3WD 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


Title: RE: What is a Varistor?
Post by: KA4POL 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.


Title: RE: What is a Varistor?
Post by: W4FBI 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


Title: RE: What is a Varistor?
Post by: KE3WD on August 23, 2013, 11:17:55 AM
There ya go. 


73


Title: RE: What is a Varistor?
Post by: W6EM 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.


Title: RE: What is a Varistor?
Post by: KA4POL 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.


Title: RE: What is a Varistor?
Post by: KE3WD on August 24, 2013, 06:04:39 AM

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


73


Title: RE: What is a Varistor?
Post by: W6EM 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


Title: RE: What is a Varistor?
Post by: KA4POL 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.


Title: RE: What is a Varistor?
Post by: W6EM 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.


Title: RE: What is a Varistor?
Post by: KA4POL 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.


Title: RE: What is a Varistor?
Post by: W4FBI 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.


Title: RE: What is a Varistor?
Post by: W6EM on August 25, 2013, 11:40:46 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.
The way you describe its connection is not the way a varistor/varicap would be connected.  If the anode is to the emitter and the cathode to ground, it's clearly forward-biased.  And, the voltage spec'ed appears to make it a series of several diodes that are forward-biased.  Puzzling about the posted specs, though.

Sorry for the confusion.

73,

Lee