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GM7HUD > HELP 24.08.07 01:18l 33 Lines 1980 Bytes #999 (0) @ WW
BID : 74777_GB7ESX
Read: DG5YM GUEST
Subj: re: Switch mode
Path: DB0FHN<DB0MRW<DB0ERF<DB0ABZ<DK0MAV<DB0FC<DB0CEL<DB0EWB<DB0PDF<DB0OSN<
DB0RES<ON0AR<ZL2BAU<GB7ESX
Sent: 070823/1259z 74777@GB7ESX.#31.GBR.EU $:74777_GB7ESX [Witham, Esx]NNA V3.1
A VDR is as you suspect a voltage dependant resistor. This is a kind of
semiconductor that you place in parallel with a voltage source. The resistance
of the VDR is normally many megohms and so very little current flows in it.
At some magic voltage, the resistance drops to almost zero and at this point,
a large current flows through the VDR. When the voltage drops belows the
magic voltage, the resistance goes back to megohms. What you get is a device
that can chop off voltage peaks from a supply. Because it is a resistance
effect you can use them on DC or AC supplies.
Only when the trip voltage is exceeded does the VDR do anything and it is ideal
as a spike supressor. There are 2 parameters used to spec the VDR, the voltage
that it conducts at and the energy rating it will absorb. You want a voltage
that is higher than anything you would normally see and an energy rating
that will protect you. You should note that if the energy spike is larger
than the device can cope with it, it will clamp for sometime and then may
fail open circuit. This makes it difficult to check as a working VDR and an
open circuit VDR both appear high resistance. If you subject the VDR to a
huge spike it might fail short circuit and cause the line cord fuse to fail.
So when you are dealing with VDRs that may have been exposed to a spike, it
is often considered expedient to replace them anyway.
I cant remember if there is a colour code or not. The ones Ihave used have been
red or black or blue. There will be many suppliers but if you check the online
Farnell catalogue, (http://uk.farnell.com), you can browse the range available.
If you select something physically similar in size, you'll probably get the
right device. As for voltage rating, well IIRC Eire has 230V mains which is
still probably 240V. I know you live in a rural environment and so your mains
might be a lot more variable than many cities. So you want to choose VDRS that
clamp at 275VAC.
73 de Andy GM7HUD
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