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Old 19th May 2019, 11:28 am   #61
Karsten
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Default Re: AVO CT160. Protecting the meter.

Improved Analogue Meter Protection
I found that the original meter circuit (39,75 µA f.s. deflection) can be most efficiently dampened (swift needle motion, no overshoot) by a parallel resistor of approx. 900 Ω in place of the existing 10 KΩ. With an operational amplifier the tester can be made to still see the original 2453 Ω (97,5 mV / 39,75 µA).
This op amp circuit should also limit the current through the meter movement to between zero and +30 µA (= full scale deflection) in order to prevent the needle from being forced against its end stops by overcurrent.
This can be accomplished by full wave rectifying and limiting the meter current to a value slightly above 30 µA so in case of an overload the needle just "kisses" the right end stop. The rectification also makes the needle "dip" to zero which greatly facilitates precise nulling of anode current prior to mA/V measurement.

Circuit description
The circuit described here may also well serve to adopt third party meters as a replacement for the rather volatile original instrument.
Fig. 1 shows the circuit diagram. It operates from a single 5 V supply and uses a standard LM358 dual op amp. As the circuit has been designed for a "siliconized" tester, the supply voltage is derived from the now free and isolated 6.3 V filament winding on TR2. The AC is rectified, doubled and regulated to 5 V DC. Allow for a separate and isolated supply in a tester which uses rectifier valves.
One op amp is a low output impedance unity gain voltage follower providing a virtual ground between the V+ and Ground rails. The other op amp serves as a non inverting high input impedance amplifier.
The half wave rectified input voltage developed across Ri is smoothed by the 100 kΩ / 1.5 µF RC combination, limiting the AC component of the amplifier output to max. 20 mV pp.
If the rather odd value of Ri cannot be realized by a combination of fixed resistors simply use an appropriately adjusted 5 K trimpot. In my case Ri = 2516 Ω = 100 mV / 39,75 µA because input can also be measured by a digital panel meter which displays "100" (mV) at full scale ("100") of the meter. This small deviation from the original value (2516-2453=63 Ω) has no influence on the rest of the tester circuitry.
The 100 KΩ resistor on the + input together with the other 100 KΩ resistor at the - input also neutralizes the influence of the + and - input bias currents.
The amplifier feedback consists of the shunted meter and the meter rectifier. Because the rectifier is part of the feedback loop, the forward voltage of its conducting diode pair is entirely compensated by the amplifier's open loop gain. The 6.8 nF feedback capacitor prevents oscillations when the meter current approaches zero.
The meter rectifier is a standard component for power applications. The reverse currents of its two diode pairs will differ somewhat, resulting in slightly different meter readings on both input polarities. However, this is irrelevant as only the positive polarity readings are used.

Alignment
  • Set the 1 K trimpot to full resistance. Set the 500 Ω trimpot wiper closest to pin 1. Adjust the 5 K trimpot to 2.5 V on pin 7.
  • Apply +97.5 mV (max. value, +100 mV in my case) half rectified AC to the input and adjust the 1 K trimpot to full meter deflection ("100"). This is the final setting. Do not touch this pot any more.
  • Increase input to +150 mV and adjust the 500 Ω trimpot until the needle just begins to leave its right end stop ("kissing point").
  • Reverse input polarity to -150 mV and adjust the 5 K trimpot to a point where the deflection begins to decrease.
  • Alternately adjust the 500 Ω and 5 K pots with + and -150 mV until the needle "kisses" the right end point at +150 mV and stops at approx. 2/3 scale at -150 mV. This difference between max. positive and negative deflection aids in distinguishing the direction to which to adjust the controls when nulling the anode current.

Linearity and deflection limitation
Fig. 2 shows the measured transfer function. Linearity and limitation of deflection (meter movement voltage) are quite good for all measurement functions. Thanks go to Dekatron for pointing me to this possible issue.

Practical assembly
Fig. 3 is my implementation on a breadboard held by the meter mounting screws.
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Old 19th May 2019, 12:42 pm   #62
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Default Re: AVO CT160. Protecting the meter.

Really nice!

Could you please download a ZIP-archive with the photos (possibly some closeups too) and the schematics as they become very small on the forum.

Please also add some photos of the meter during the calibration explaining how the needle should be set during calibration - which direction it is moving, possibly marked with arrows of direction. This would help me a lot as I am more of a "visual" person than a "text reading" one.
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Old 19th May 2019, 2:10 pm   #63
Karsten
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Default Re: AVO CT160. Protecting the meter.

I have attached a ZIP Archive with the three photos. Additional detailed pictures of the realization will not really reveal more details. The circuit is straightforward and not critical in any aspect. My breadboard realization is just an example of how it might be done.

Pictures of needle motion will not really be helpful either. The mentioned right end stop is somewhat beyond the right end of the instrument scale as is normal with mechanical pointer type meters.

Alignment is as simple as described. It is in no way critical for the meter as the instrument is inherently protected by the limited output swing of the op amp. All that is needed for alignment is a standard DMM with DC millivolt range and a variable half rectified AC supply which can be easily hooked up with a transformer, some resistors, a potentiometer and a standard diode.
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Old 19th May 2019, 5:45 pm   #64
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Default Re: AVO CT160. Protecting the meter.

Is there any protection for latch-up of the LM358 which is a common problem with that op-amp if it sees negative input voltages - I'm not good enough at electronics to see if this can happen with this circuit but I've experienced it in other designs where the op-amp had to be replaced to circumvent this problem.

Should the protection diodes and protection/damping capacitor in your previous protection circuit be kept as before or should they be removed?
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Old 19th May 2019, 6:44 pm   #65
Karsten
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Default Re: AVO CT160. Protecting the meter.

Protection diodes against latch up: the two 100 K resistors should prevent latch up. Also, no destructive high currents can flow because the virtual ground provided by the first op amp output (pin 7) is part of this current path. Therefore I think that clamping diodes across Ri are not necessary.

Protection capacitor: the 100 uF capacitor in my previous design is for signal smoothing only so the input peaks are lowered and don't get cut off by the clamping diodes. Here this smoothing function is much more efficiently accomplished by the 100K - 1.5 uF low pass filter. Therefore, no capacitor across Ri.
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Old 19th May 2019, 7:11 pm   #66
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Default Re: AVO CT160. Protecting the meter.

Ok, thanks for explaining!
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Old 19th May 2019, 7:19 pm   #67
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Default Re: AVO CT160. Protecting the meter.

Many thanks to Karsten for sharing this . I've been looking at it with the idea that I could adopt the circuit for my VCM163 with its 50uA meters. One of them already has an op-amp circuit to drive it and correct for what appears to be shorted turns on the coil.

Noting Martin's suggestion a while ago that a good mod for the VCM163 is to add a separate transformer to run the 14kHz oscillator and amplifier, I could make one single job of re-configuring the PSU which I put in for the op-amp, and at the same time, replace the existing op-amp with Karsten's version. That would be neat!

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Old 21st May 2019, 3:34 pm   #68
Karsten
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Default Re: AVO CT160. Protecting the meter.

No Input protection diodes needed:

Max. input to Ri is max. voltage drop across R10 = 10 V mean DC. This can occur with selector controls in Test position and mA/V dial is accidentally turned to "SET ZERO" and anode current controls are turned fully clockwise with no valve inserted or anode current controls at zero and valve inserted with max. anode current (100 mA). In that case peak voltage = pi * 10 V = 31,4 V. I have applied + and -11 V mean DC to the input. No latch up occurs. Meter needle stays within about 2/3 of scale at -11 V and still "kisses" right end stop on +11 V. Virtual ground voltage does not change at all. Op amp remains functional of course.

Compare this to the original circuit - you would probably have to send the instrument to Herts meter for repair right after the first of these two brute tests.

Last edited by Station X; 21st May 2019 at 4:41 pm. Reason: At poster's request.
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Old 21st May 2019, 4:29 pm   #69
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Default Re: AVO CT160. Protecting the meter.

Did you apply this voltage as a half wave rectified voltage with a peak voltage of 31.4V - you say mean DC +/-11V?

What voltage does the LM358 see on its input pins then, compared to the virtual ground?

I am trying to understand why there is no latch-up as I've seen similar circuits that have had problems with the LM358 discussed in valve-tester circuits.
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Old 21st May 2019, 4:37 pm   #70
Karsten
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Default Re: AVO CT160. Protecting the meter.

@Bazz4CQJ:
Before you start determine best damping resistor for the meter: apply ca. 2/3 f.s. step voltage and observe needle movement. Vary parallel damping resistor until needle moves swiftly with no or only minimal overshoot. Be sure to use a voltage source with high inner resistance much larger than the damping resistance.
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Old 21st May 2019, 5:09 pm   #71
Karsten
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Default Re: AVO CT160. Protecting the meter.

@Dekatron / Martin:

+-11 V is arthmetic mean of half wave rectified AC of course. Therefore voltage peaks are in the order of 35 V.

At -11 V: Voltage on pin 3 = distorted sine 50 Hz. Min peak = -720 mV; max peak = -360 mV. Voltages are measured relative to pin 4 = ground because according to data sheet voltages negative to (physical) ground are critical.
It would be easy to add two protection diodes across the input of course. Whoever adopts the circuit may do so if he/she feels better with diodes. I have found them to be unnecessary, imho the reason being the 100 K resistor on pin 1 which prevents the current necessary for latch up.
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Old 21st May 2019, 6:54 pm   #72
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Default Re: AVO CT160. Protecting the meter.

Thanks for the explanation and measurements!

I think I'll have to hook up an LM358 to understand the latch-up conditions, and get it to enter latch-up, to understand this.
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Old 22nd May 2019, 10:34 am   #73
Karsten
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Default Re: AVO CT160. Protecting the meter.

Addendum:

I have inserted a 100 Ohm resistor between pin3 and the 100 K / 1.5 uF junction and used an oscilloscope to measure the current flowing out of pin 3 as the voltage drop across this resistor. Wth input = -11 V arithmetic mean of half wave rectified AC (as above) the oscillogram shows 20 ms periodical peaks with 28 mV = 280 uA minimum and 0 mV maximum. This currrent amplitude is iinsignificant.
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Old 1st Jul 2019, 7:22 pm   #74
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Default Re: AVO CT160. Protecting the meter.

Thanks to Karsten for the meter protection circuit. For anyone building the circuit (I am)
you may find the following useful for obtaining the 2516R or 2453R resistor values:-

1: A 2k7 in parallel with a 37k yields a value of 2516.37R
2: A 2k7 in parallel with a 27k yields a value of 2454.54R
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Old 4th Jul 2019, 4:04 pm   #75
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Default Re: AVO CT160. Protecting the meter.

An excellent in-depth discussion.
As it happens, coincidently, I've just got a CT160 in for a service & calibration.
There might be a few CT160 owners who aspire to service/calibrate &/or improve their testers, but aren't familiar with opening them up. Awkward & heavy, they are for their size. For in-depth work, one could de-solder the connectors from their tagboards. But woe betide getting the wires accidently crossed when re-attaching, or damaging the tagboards. For a quick shufti & a bit of work - provide fall-proof support beneath & a sky-hook. Crunch time is when removing the last of the big hex 2BA securing bolts & prising the whole bottom chassis away from the big rubber seal.
As many folk have said, AVO's 30uA meters are not known for their robustness & are hellish to replace. So they'll need to be delicately removed before attaching a new wee component board to the terminals.

Regards, David
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