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DIY Low Thermal EMF Switch/Scanner for Comparisons of Voltage and Res. Standard

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1. Why Switch/Scanner
Comparison of standard(voltage/resistor) is precise, repeatable, low Thermal EMF.
Comparison can be easily made automatic.
Multi-reading results make statistics/averaging at ease and achieve high confident level.
Standards to be compared may be relatively permanently connected thus inconsistency due to contact variation is greatly reduced.
May compare multi-standard(ex. up to 16 or greater) for a long time.
Derived use i.e. measurement of temp-co.

2. Why DIY powerless Switch/Scanner
Very simple to use. No programming, no GPIB.
Powerless, and therefore no direct interference from the mains.
Powerless, and therefore very little heat generated and less Thermal EMF.
Compact, low cost.
No power switch, consume virtually no power when idle.

3. Features and Principles of the DIY switch
Provide 2*2(DPDT) or 4*2(4PDT) switch, or 2*4, 4*4, 2*8, 4*8, 2*16, 4*16 switch.
MC control(Multi-meter Measurement Complete Signal).
MC power supply, consume about only 0.3mA current.
Use 74HC CMOS and related low power IC.
Use Latching relay to achieve low power consumption.
Manual switch and free running mode provided.
Operates in loop manner only.

MC or VMC signal is usually provided by precision multimeters as a standard feature.
It usually stay logic high when idle or during measurement, and goes to logic low briefly when a measurement is complete.

When measurement by the multimeter is complete, it's the perfect time for switch/scanner to operate.

Newer multimeters use CMOS IC for VM output thus the high level is nearly 5V. Old ones such as my Agilent 3458A (made in 2006) uses TTL IC which gives only 3.65V when drawing 0.5mA current. However, this is enough for the switch to operate properly.

Although is mechanical, the relay will complete it's switch over in less than 5ms, plus settle down of the measured V or R, which normally within the default delay time of a typical multimeter. For 3458A 10V range NPLC=50, default delay is 10ms. This delay may be increased if needed.

4. Schematic of My first 4*2 switch for two standards(V or R) comparisons

The Latching relay, Omron G5AK-237P

Omron G5AK-237P, DPDT low signal double-winding latching relay.
Data sheet is here http://datasheet.octopart.com/G5A-237P-DC5-Omron-datasheet-109999.pdf

The Drive
Normally the set or reset coil operate at 5V individually.

However, the set and reset coils are identical and can be connect in series or in parallel. When in series, it also operate at 5V but consume half the current.

When paralleled. it will operate at 2.5V but the current will be doubled. This is the way I use.

Power supply

I use HT7136 low dropout CMOS low power regulator. Connect a 200 Ohm resistor and a Schottky diode to VM, and output is 3.5V minimum through the use of a 470uF filter capacitor.
DS of HT7136 is here: http://www.holtek.com/pdf/consumer/71xx_1v170.pdf

The Principle is straight forward, every time a measurement is complete, VM goes low briefly toggle the J-K flip-flop and hence the relay.

Optional trigger is also provided for manual momentarily switch K or by activating the free running astable vibrator.

5. Implementation and testing
Breadboarding first

Then assemble the device on universal board

Cat. 5 twisted pair, pure copper and screened, good for this purpose.

Test is performed by making measurements of two inputs alternatively

Sampling is done by another DIY interface device:

One end of the device is connect to multimeter GPIB, another end is connect to PC's USB port by a RS232-USB converter(early version) or direct USB connection(later version). This device is designed by llycom and is widely used among Chinese volt-nuts.

By running the dedicated program, one can send any GPIB command to the multimeter.

It also provides a precision temperature sensor(DS18B20).

The final graph is the test result when two inputs shorted. Ideally the dark blue line and purple line should be equally flat to zero but it's not owing to multimeter's noise and offset. However, the average of the two lines are quite the same with less than 5nV difference, well below the noise level of 3458A.

6. Next version of the 2*4 switch

First 8 photos show the implementation of PCB version by Mytek, last 4 photos show the miniature version.


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