USE MULLARD TO KEEP VOLTAGES STEADY, EDDY!

We have had a run on stabiliser devices recently, so I thought it was perhaps time to write a blog article about how these work.   Interestingly, they are not a valve in the true sense as they do not amplify but rather find use in maintaining steady DC voltages in power supplies where mains supply variations occur or within circuits where a reliable reference voltage is required for operation.

The simplest form of voltage stabiliser consists of a rod anode surrounded by a cylindrical cathode which are mounted within a glass bulb which has an atmosphere containing a single or multiple low pressure noble gas mixture with neon, helium or argon commonly being used - more on this later..  As a DC potential flows, the cathode will strike with a visible glow and a burning voltage flows from anode to cathode.  This potential has the useful property of only minimally being varied by changes in current as you can see in the following picture: -

The voltage at which a stabiliser strikes and burns at is determined by the gas or gas mixture used within the envelope with the supply voltage typically being higher than the operating voltage required to ensure strike and burn occurs.  Mullard made a variety of these devices, the parameters for which are listed below: -

This phenomenon has two effects - if the input voltage changes, then the burning current, I, and resistance, R,  will also change BUT the voltage across the stabiliser will remain constant.  The only deviation that can occur is when the ratings of the stabiliser are exceeded at which point, the stabiliser will rapidly fail - if one were to perchance reverse the anode and cathode connections then again, the stabiliser would (spectacularly & rapidly) fail.

So, there we have it, a quick treatise on these fascinating devices, Made by Mullard amongst others but remember, they are not a valve!!!