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Old 9th Nov 2017, 3:12 pm   #1
SeanMcK
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Default Can I create a circuit in an Avo 12? If so how?

To cut a long story short is there a way to create a 36A AC current range in an Avo Model 12?
The objective is to use the 36A DC resistor/shunt, R1, to pass the main current and take a 'signal' off R1 which is fed to the movement, via a route as yet unknown.

Making the new (additional) connection from the main switch to "R1" might be feasible and, for the purposes of this post, I am assuming it is but my query relates to obtaining and rectifying a signal 'off' R1 and how do I allow for the "average responding"? nature of a moving coil?

For what it is worth, as far as I can see there is one unused switch position, immediately anti clockwise of the 360V AC switch position, that might, somehow, be used to employ the existing half wave? rectifier.
Attached is the circuit diagram altered to show what I see in the meter I have here.
Thanks
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Old 9th Nov 2017, 7:12 pm   #2
Silicon
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Default Re: Can I create a circuit in an Avo 12? If so how?

The 36A DC passing through R1 will create a voltage of 0.1V which can be sent to the DC moving coil meter and give a full scale deflection.

An ideal rectifier (if it existed) could rectify the AC voltage and create 0.1V DC and send that to the moving coil meter.

However simple rectifiers will not rectify until the voltage gets to 0.2V (in the case of Germanium diodes).

There is no simple modification that I know of to give you a 36Amp AC range.
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Old 9th Nov 2017, 8:01 pm   #3
SeanMcK
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Default Re: Can I create a circuit in an Avo 12? If so how?

Thank for the reply. That's a pity.
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Old 9th Nov 2017, 8:29 pm   #4
RogerEvans
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Default Re: Can I create a circuit in an Avo 12? If so how?

36 Amp AC passing through R1 will give 0.1V RMS AC across R1 or across the meter polarity switch (which may be more accessible) . The mid range DMMs (£20 - £30) will have a 0.2V RMS AC Volts range but they don't usually measure AC current. So if you connect one across the meter polarity switch it will give you 72 Amp AC full scale and you should be careful not to exceed the rated 36 Amp. This is a bit messy and it depends how much you really need the 36 Amp AC current measurement. You can buy dedicated high current AC clamp meters and they are not too expensive.

Roger
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Old 9th Nov 2017, 9:27 pm   #5
Ed_Dinning
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Default Re: Can I create a circuit in an Avo 12? If so how?

Hi Sean, the Avo is pretty robust but using the switches that are designed for 10A at 36A may give problems.

Avo did a range of shunts that allowed the meter on an appropriate range to measure up to about 100A (by measuring volt drop as detailed above). They also did an AC current transformer that would extend the Ac amps range to 100+A.
Often seen on e-bay.

Ed
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Old 9th Nov 2017, 9:46 pm   #6
Herald1360
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Default Re: Can I create a circuit in an Avo 12? If so how?

Unless I'm mistaken, a suitable current transformer isn't difficult to make- ten turns round an old toroidal mains transformer core should do it. Pass the 100A carrying wire through the core with the ten turn winding to the meter on its 10A AC range.

It would be safer for the meter, though, to use 100 turns on the toroid and load them with a 10 ohm 10W resistor and read the voltage (10V=100A) across the resistor with the meter.

Less chance of a meter burnup due to incorrect range setting!

Current transformers are pretty nifty for dealing with rectifier voltage drops if the burden resistor is on the rectified side since the TX will act as a current source rather than a voltage source. This will drive the appropriate current through the burden more or less regardless of the rectifier forward drop.

At the end of the day, a moving coil meter is a current operated device anyway.
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Old 9th Nov 2017, 10:55 pm   #7
SeanMcK
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Default Re: Can I create a circuit in an Avo 12? If so how?

Roger and Herald I will look into your suggestions.
Ed, I initially had the same thought but then realised that the existing 36A DC circuit is probably switched. Whether or not the appropriate terminal is a 'special' I do not know. That portion of the switch ring is partially buried and not clearly visible, though I have had cause to dismantle and reassemble the switch's core and I do not recollect an abnormal radial 'tab' but I wasn't looking for that sort of thing at the time.
Anyway, thanks for the replies
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Old 21st Nov 2017, 4:42 pm   #8
SeanMcK
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Default Re: Can I create a circuit in an Avo 12? If so how?

Another question.
As far as I can see the AC voltage circuit simplifies to that is depicted in the attached jpg.
Why does the section D2 to the switch contact "a" exist?
Assuming my simplification is correct I am beginning to wonder if, with ab closed, it prevents an excessive reverse voltage arising across D1 when the meter's 'negative' input terminal is positive.
Thanks.
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Old 21st Nov 2017, 5:44 pm   #9
RogerEvans
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Default Re: Can I create a circuit in an Avo 12? If so how?

This is a guess, but the second diode means that both halves of the AC cycle will draw similar currents from the circuit under test. Without D2 the meter would load only one half cycle and this might have an effect on the circuit under test such as adding harmonic distortion into an amplifier. If that were the reason you might think that there would be a resistor in series with D2 to match the resistance of the meter movement so there may well be a completely different reason.

Roger
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Old 28th Nov 2017, 3:18 pm   #10
SeanMcK
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Default Re: Can I create a circuit in an Avo 12? If so how?

Ta Roger.
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