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Old 28th Nov 2020, 10:44 am   #1
Superscope
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Default Avo Horse-shoe Movement Shunt

Can anybody explain why the Horse-Shoe Movement found in the old Avo Meters has a 12.5 K Shunt across the Coil ?
(That's the Big Brown thing with the Red and Blue ends)

Been wondering about the purpose of this part of the Movement for a while.

Not sure if they are always 12.5 K, I have only measured this one, as I had the Movement out.

It is only across the Coil, it is not across the Swamp Resistor.




Ian
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Old 28th Nov 2020, 11:11 am   #2
stevehertz
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Default Re: Avo Horse-shoe Movement Shunt

Maybe currents are induced in the magnet that could affect its strength and subsequent readings? The shunt serving the purpose of damping out the current.

When I worked at Foster rectifiers a call came in from an irate customer that the paint was literally falling off a newly made high power, DC rectifier cabinet. It seems the steel frame/chassis was acting like a shorted turn of a transformer, being driven by the massive local currents contained within the cubicle.
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Old 28th Nov 2020, 11:56 am   #3
pmmunro
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Default Re: Avo Horse-shoe Movement Shunt

Ian,

It shouldn't measure 12.5k because it's rectifier diode.

Its purpose is to ensure that there is deflection of the moving coil if an alternating current or voltage is applied with the meter set on a DC range. With a DC current or voltage, the diode is back biased and has no significant effect.

As the average deflection reesulting from of the AC waveform is zero, a very large current could be applied without the user realising, causing the moving coil to burn out. With the diode there acting as a half-wave shunt rectifier, the coil will deflect, and if the current in the moving coil is excessive, the coil frame will hit the bel crank levers and cause the cut-out mechanism to trip.

This arrangement is only used on instruments with mechanical cut-outs because early meters were protected by fuses which, of course, operate equally well on AC or DC.

PMM
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Old 28th Nov 2020, 12:57 pm   #4
Stevie342000
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Default Re: Avo Horse-shoe Movement Shunt

I thought it was a rectifier as well, is one of those ones that smells like rotten eggs when it goes to a new home in the sky?

If it needed to be replaced what should you replace it with?
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Old 28th Nov 2020, 1:44 pm   #5
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Default Re: Avo Horse-shoe Movement Shunt

Thanks Peter,

I did wonder if it might be the H2 Rectifier as shown on the Schematic,
but I thought that would be more like a conventional Rectifier, and I
expected that to be on much later Meters.

I have never dismantled one of these components, as I don't have any
defective Horse Shoe Movements to play with.
It's a very substantially constructed Rectifier, why did they build it like that I wonder?

Armed with that information, I have just done a Diode check and yes it measures a
forward Bias of 0.7 Volts.
Which is odd, as that suggests it's a Silicon Rectifier, which I wasn't expecting ?

Having double checked the resistance, it shows 12.5K forward and 59 k reversed.
That's what through me.

Would be interesting to see how this is constructed internally.


Ian
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Old 28th Nov 2020, 8:44 pm   #6
Ed_Dinning
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Default Re: Avo Horse-shoe Movement Shunt

Hi Folks, a thought on this style of magnet.
Would it be possible to use one of the new super strong modern magnets, with a pile of washers at each end (in lieu of proper pole pieces) across the legs of the magnet to boost its flux?
Then check meter sensitivity

Ed
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Old 28th Nov 2020, 9:47 pm   #7
pmmunro
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Default Re: Avo Horse-shoe Movement Shunt

When Avometers used horseshoe magnets, the choice of solid state rectifier technologies was quite limited; pricipally copper oxide and selenium.

The copper oxide type was a speciality of Westinghouse who produced a range of instrument rectifiers, from which the rectifier bridges in Avoneters were chosen.

Westinghouse and Dr. L O Grondahl, took out a US patent (1,640,335) in 1927 and in 1933 ACWEECO used one of these instrument rectifiers in the 20-range Universal Avometer. Prior to the introduction of this type of rectifier, a universal multimeter would have been much less practicable.

This form of protection was patented by ACWEECO, GB476681, (Application Date 12th May 1936, full specification accepted December 1937), and used in the Model 7, introduced in August 1936.

The particular diode used in the horseshoe magnet movements was obviously designed to fit across the magnet assembly and this packaging may have been made by, or specifically for, ACWEECO. I've never opened one up, but it may well have a standard Westinghouse copper oxide diode inside.

PMM
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Old 29th Nov 2020, 6:41 pm   #8
The Philpott
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Default Re: Avo Horse-shoe Movement Shunt

If i can find one of these rectifier diodes that is obviously sickly (in one of my dead meters) i will autopsy it.

Ed, i have only tried boosting flux on one of these horseshoe magnets once, it was successful but did involve having the Neodym. magnet on the concentrator, very close to the moving coil assy. I made no attempt to balance the field by using magnets on either side....just the one...hence i'm not sure why it worked; only that it did. I think it was a Japanese bloke who popularised this method.

I took photos which are on the forum.

Dave
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