VDR's

[TowerRadio]

Whilst looking for a Brimistor I came across a tin marked VDR's, some of the things inside looked like PCB mounting thermistors but with a green dot on one end. Others were like an axial resistor with just a green band or a blue and black band at one end. Where were these used? Sorry,no pic I've mislaid my camera.Les

[MotorBikeLes]

Back with their hybrid TVs, Grundig used a few NTCs, VDRs etc in them. The spares description was something like "With two black dots". No idea now what that all meant, though I MAY have a few "pulls" somewhere.
Les.

[Horris]

They were used in the line and frame stages of some tv’s. Brc 950 series were a good example.

Dave

['LIVEWIRE?']

Radiomobile also used them in the input stages of car radios in the 1970s. In those the purpose must have been protection from the effects of lightning striking the car's aerial.

[Herald1360]

I doubt they'd help much with an actual strike though they would help mitigate the effect of a nearby flash.

[Wendymott]

VDR's are like bi polar zeners if you like..they conduct or breakdown at a preset voltage.
WE used them to limit the back emf pulse of compressor motors across the relay contacts in ICE bank controllers.

[Maarten]

The meaning of the colour band can be found in the relevant Mullard/Philips databooks. It doesn't really intuitively translate to a value, I think.

[Radio Wrangler]

Also known as MOVs Metal Oxide Varistors. They are breakdown devices. Also used in phone equipment to protect from lightning induced transients on the line. Very fast, they can absorb dramatic levels of power for a very short time.

Protection of lower voltage things usually uses TVS diodes (Transient Voltage Suppressor)

Both types are frequently used in aircraft electronics. Aircraft do get struck by lightning. The arc attaches at some point, flows through the metal skin and leaves somewhere else. Massive currents flow. Deliberate points are provided to try to get it to pick places other than antennae, but sometimes antennae are blasted off, coax burned up and the radio at the end of it destroyed. other wiring gets dramatic surges even if the lightning doesn't hit anything tender. typically, all connections to electronic gear has to survive 250v ringing and 125v 125A pulses. MOVs are star performers in achieving this. If you do get struck by lightning, you might lose one radio, but you should have another. You want the second one to survive. You probably also have an opinio on the importance of the robustness of engine controllers, navigation gear etc

Off topic, taking lightning currents in the fuselage is a hot topic in the design of composite aircraft. Carbon fibre is resistive and gets blasted apart by the power it dissipates. Alternate pathways have to be provided.


Several firms make VDRs/MOVs, and have their own colour codes. To identify them, you have to know who made them.

You'd need something rather sporty to measure them!

David