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I am an electronics novice. :)

I opened a working Casio vintage desk calculator (likely from the late 1980s/early 1990s). It has a 220 volt step-down transformer with six secondary wires. The two mains wires go into the transformer at the bottom. The 6 wires come out from the sides (three on each side). Following are the related images:

enter image description here enter image description here enter image description here

I found out continuity among the secondary wires as follows:

Purple-Grey-Purple: Continuity (side one)

Orange-Red-Brown: Continuity (side two)

However, the wires are wired to six separate contacts on the circuit board.

Are there differences in terms of voltage among each set of wires coming from the same side? If each carries a different voltage, how can I test this with my multimeter? I would appreciate guidance for using my multimeter safely/correctly.

If there are no differences among each set of wires exhibiting continuity, why were they wired separately?

Also, do the markings on the transformer mean anything? There's also another, smaller marking (not visible in the images) that says "E41-2L11/2".

Update: The calculator has an integrated printer, and therefore a motor. (Thank you TimWescott.)

Update: Calculator display:

enter image description here

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    \$\begingroup\$ What colour is the calculator's display? If it is green or orange, it may be a gas discharge display driven by voltages in the 50-150V region (at relatively low current). Then one of those secondaries may be e.g. 50-0-50V AC. Trace it on the PCB through (probably 2 diodes, possibly a bridge) to a capacitor - and note the rated V on the capacitor... \$\endgroup\$
    – user16324
    Commented Jan 4, 2020 at 19:20
  • \$\begingroup\$ @BrianDrummond The display is behind a green transparent plastic screen; I don't think it's gas-discharge. I updated the question with the related image. Thank you. \$\endgroup\$
    – Sabuncu
    Commented Jan 4, 2020 at 19:37
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    \$\begingroup\$ It's gas discharge, and it'll be pale green without the window.. Note the glass "pip" at the centre bottom, where it was sealed after pumping the air out. I may have been a bit pessimistic about the actual voltages but take some care until you know. \$\endgroup\$
    – user16324
    Commented Jan 4, 2020 at 20:03
  • \$\begingroup\$ @BrianDrummond OK, thank you, I stand corrected. Also looked it up and learned that they're called VFDs (Vacuum Fluorescent Display), but I didn't know they were gas discharge until you pointed it out. \$\endgroup\$
    – Sabuncu
    Commented Jan 4, 2020 at 20:05
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    \$\begingroup\$ OK I stand corrected too, I've always thought they operated more like Neon but apparently it's the electron beam hitting a phosphor - in a vacuum rather than a low pressure gas - that makes the light. Wiki says "about 50V" so at the low end of my range (would need something like 35V AC from the transformer.). Not dangerous but a bit of a surprise if you were expecting 5V... \$\endgroup\$
    – user16324
    Commented Jan 4, 2020 at 20:12

2 Answers 2

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Are there differences in terms of voltage among each set of wires coming from the same side? If each carries a different voltage, how can I test this with my multimeter? I would appreciate guidance for using my multimeter safely/correctly.

No continuity between the two sides means two windings that are isolated from each other. The separation between the two windings at the top and the winding at the bottom indicates an additional level of physical and electrical isolation between the primary and secondary.

The purple-grey-purple winding is likely a center-tapped winding. Yon can expect the voltage between grey and each purple to be half of the voltage between purple and purple.

The brown-red-orange winding likely has a tap somewhere other than the center of the winding. The highest voltage will be the sum of the other two, but the two lower voltages might not be equal.

Since the calculator is working, there does not seem to be any reason to check winding resistances. If the condition of the transformer had been unknown, prior to applying voltage, you would check the resistance from each lead to every other lead and the frame of the transformer.

The best way to use the multimeter would be to set it to an AC range that is higher than 220 volts. Apply 220 volts to the primary and measure all of the secondary voltage combinations. They will presumably be considerably less than 220 volts. Once you know the approximate voltages, you can use a lower voltage setting.

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  • \$\begingroup\$ Will run the tests and post back. Thank you. \$\endgroup\$
    – Sabuncu
    Commented Jan 4, 2020 at 19:38
  • \$\begingroup\$ Where would I place the multimeter's black probe on the board to determine the board ground? Thanks. \$\endgroup\$
    – Sabuncu
    Commented Jan 4, 2020 at 19:42
  • \$\begingroup\$ @Sabuncu: In this case, you place the two meter probes on two transformer leads to measure the voltage between those leads. You don't care about the board "ground" for this measurement. \$\endgroup\$ Commented Jan 4, 2020 at 19:59
  • \$\begingroup\$ @PeterBennett Thank you. I'll be using the AC range at higher than 220 volts, as directed by Charles Cowie above, and as such, I would not be shorting out anything, correct? \$\endgroup\$
    – Sabuncu
    Commented Jan 4, 2020 at 20:08
  • \$\begingroup\$ If you select a voltage range, you will not be shorting anything. Even if the range is too low, the circuit under test would be very unlikely to be shorted even though the meter might be damaged. Selecting a current range instead of a voltage range does short circuit the circuit under test and can damage both the circuit and the meter. \$\endgroup\$
    – user80875
    Commented Jan 4, 2020 at 21:09
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They're probably going to two separate power supply circuits. I thought I saw a motor casing in the corner of one of your pictures -- if there's a motor and possibly other power actuators, it would be sensible to power that separately from the (relatively sensitive) digital electronics.

Transformer manufacture doesn't have a huge benefit from mass production, so it's not at all uncommon even for devices with moderate production volumes to have their own custom winding. Chances are the number on the transformer is specific to that calculator, and wouldn't mean anything without the original documentation.

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  • \$\begingroup\$ Yes, there is a motor for the printer mechanism, good catch. Maybe a silly question, but are there different currents in the two sets of wires? Or is this not possible coming from the same transformer? Thank you. \$\endgroup\$
    – Sabuncu
    Commented Jan 4, 2020 at 18:30
  • \$\begingroup\$ There certainly can be two different currents taken from each of the two different secondaries. If you drive a transformer from a constant AC voltage, then as a first approximation you can treat each secondary output as a constant AC voltage -- that means that the current can be different, depending on the load. \$\endgroup\$
    – TimWescott
    Commented Jan 4, 2020 at 20:10
  • \$\begingroup\$ Just to be clear: Do you mean that if more current is drawn from side A, the current available at side B will be reduced proportionally, because the driving AC voltage is constant and there is a fixed amount of current? I am grasping at straws here because your response is mostly over my head. :) Thanks. \$\endgroup\$
    – Sabuncu
    Commented Jan 4, 2020 at 20:20
  • \$\begingroup\$ Yes and no. No, because the transformer has no mechanism to limit current, nor, until a circuit breaker trips, does the power from the wall. Yes, because there's probably a fuse. But until the transformer is drawing enough current to blow the fuse, the current draw from secondary A is basically independent of the current draw from secondary B. \$\endgroup\$
    – TimWescott
    Commented Jan 4, 2020 at 20:44
  • \$\begingroup\$ I actually understood this, thank you so much. \$\endgroup\$
    – Sabuncu
    Commented Jan 4, 2020 at 20:46

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