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G8MNY > TECH 31.08.10 06:52l 93 Lines 3805 Bytes #999 (0) @ WW
BID : 61015_GB7CIP
Read: GUEST
Subj: Voltage Dependent Resistors
Path: IZ3LSV<IK2XDE<DB0RES<ON0AR<UA6ADV<GB7CIP
Sent: 100830/2206Z @:GB7CIP.#32.GBR.EU #:61015 [Caterham] $:61015_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EU
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
³ ___...----ÄÄÄÄÄ~~ Û Clamping
³ _.-~~ Û Range
³ /.----ÄÄÄÄ Fail! _
³³ Zener Max Use
³³
Amps 10k 1k 100 10 1 .1 .01.001³/
-I ÄÁÄÄÁÄÄÁÄÄÁÄÄÁÄÄÁÄÄÁÄÄÁÄÄÅÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄÄÂÄ +I
Fail~~~~~~~~~~~~~/³.001.01 .1 1 10 100 1k 10k Amps
Zener ³³
³³
/ ³
VDR __.-~ ³
__.....----ÄÄÄ~~~ ³
-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 //~\ ±³ ³
[] //\ Disk ³³ ³ ±³ ³
\\/ Types \\_/ \_\__/
³ ³ ³ ³ /_/\_\
Wires Thick tabs
Fuse
L ÄÄÄo-oÄÄÄÂÄÄÄÄ> VDRs should always
MAINS or VDR Protected be used with a fuse
DC SUPPLY ³ Kit in the supply line!
N ÄÄÄÄÄÄÄÄÄÁÄÄÄÄ>
However as with a lot of safety items there can be a reliability cost. VDRs are
known to fail on there own when there is 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.
LÄÄo-oÄÂÄÄÄÄÂÄÄÄÄÄÄÄÄÄÄÄÄÄ>
~~~~~~³~~~~³| ______ Voltage
=== VDR ³nearby³ Protected
³ ³| ÀÄÄÄÄÄÄÙ kit
³ ³| Zapped IC
NÄÄÄÄÄÄÁÄÄÄÄÁÄÄÄÄÄÄÄÄÄÄÄÄÄ>
~~~~~~~~~~~~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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