VDR(
Voltage Dependent Resistor/Varistor).
The VDR is a
resistor whose
resistance falls
as the
voltage over it rises towards a spesific point.
Most
modern VDRs are made from
granulated zinc-
oxide
that is
doped with
bismuth,
manganese,
antimony
and other
metals and then
fused together. The
contact surfaces of these grains
act as a
semiconductor with a
voltage drop of about 3V.
The total
voltage drop of the
component depends on the thicknes of the
VDR and the size of the grains. At
voltages below this, the
VDR has a high
resistance, but as the
voltage pass this the
resistance falls rapidly.
Because of this property VDRs can be used to
protect against
transient high
voltages(made by switching off
inductive loads
or by
lightning) by connecting them like this:
_____
~ --|_____|-----*------------
line 1 F1 | |
| | VDR 1 ..|..
|_| 150V : | :
| : | | :
GND --------------*---------: | | : load
| : | | :
| | VDR 2 :..|..:
|_| 150V |
line 2 _____ | |
~ --|_____|-----*------------
F2
If the
voltage on line 1 gets higher than 150V, VDR1s
resistance falls. This shorts line 1 to
ground and blows the
fuse F1
thereby protecting the load(your
computer,
TV,
stereo or some other important
device).