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G8MNY  > TECH     04.12.23 07:52z 93 Lines 3808 Bytes #26 (0) @ WW
BID : 61669_GB7CIP
Subj: Voltage Dependent Resistors
Path: SP7YDD<SR1BSZ<IW0QNL<ON0AR<GB7CIP
Sent: 231204/0744Z @:GB7CIP.#32.GBR.EURO #:61669 [Caterham Surrey GBR]
From: G8MNY@GB7CIP.#32.GBR.EURO
To  : TECH@WW

By G8MNY                                  (New Dec 07)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)

These transient suppressing devices also called Varistors (carbon) or Metal
Oxide Varistors (MOVs), they are used to protect modern electronics such as
mains SMPSUs from excessive line voltages. Unlike a normal zener they offer
symmetrical clamping on AC & VERY high pulse power capacity. For low voltage DC
there are special protection zeners that fail short at high current, that can
offer better protection to sensitive electronics than VDRs.

                              +V            VDR
                                      ___...----~~     Clamping
                                 _.-~~                      Range
                                /.---- Fail!           _
                                    Zener                   Max Use
                               
Amps 10k 1k 100 10 1 .1 .01.001/
   -I  +I
             Fail~~~~~~~~~~~~~/.001.01 .1 1 10 100 1k 10k Amps
                   Zener      
                              
                             / 
                VDR     __.-~  
       __.....----~~~       
                              -V

Clamping voltages start as low as 10V up to 600V DC. But the action is not as
defined as a zener & there are several voltages in the specs.

Disk      Voltage    Voltage   Max peak   Power
Size     AC     DC    @ 1mA    Current   Joules
SMD       4    5.5      7       150 A       0.4
SMD      10     14     16.4     150 A       0.4
 7mm    250    320    390       400 A       8.2
 9mm     11     14     18       250 A       0.8
 9mm    115    150    180      1200 A       8.4
 9mm    230    300    360      1200 A      17
16mm    250    320    390      4500 A      65
22mm    250    320    390      8000 A     140

As you can see from the above examples these devices can really handle some
peak current. But of course only for a short time like uS-1mS, so as not to
exceed the transient dissipation rating quoted in Joules. (Watt x Seconds)
                           ...
                  __      / /~~\
SMD  small       //~\         
 []   //\  Disk            
      \\/  Types \\_/     \_\__/
                      /_/\_\
                 Wires  Thick tabs

         Fuse
    L o-o>          VDRs should always
MAINS or      VDR  Protected   be used with a fuse
DC SUPPLY           Kit       in the supply line!
    N >

However as with a lot of safety items there can be a reliability cost. VDRs are
known to fail on there own when there are no peaks! So yes the kit involved is
better protected, but like adding a fuse there will be unexplained failures.

EXPLODING VDRs
As these devices are used in power circuits, if the dissipation is exceeded or
there is physical (e.g. wet) or electrical (cumulative) damage, the VDR is
likely to explode. In doing so it can do electrical & physical damage, from its
own EMP current pulse, as well as plating the surroundings with conductive soot
from the flash arcing when it failed.

Lo-o>
 ~~~~~~~~~~|  ______      Voltage
      ===  VDR nearby     Protected
           |        kit
           |  Zapped IC 
N>
 ~~~~~~~~~~~~Current spike

So when mounting them is worth considering this & leave room for a reasonably
safe discharge without recking the kit.

N.B. putting longer wires on one is not a good idea, as pulses "ring up" to
higher voltages along unterminated transmission lines.


See my buls on "Mains Power Protection" & "12V System Surge Protection".

  
Why Don't U send an interesting bul?

73 De John G8MNY @ GB7CIP


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