EEVblog 1476 How does the resistance multiplier work?

[ballsystemlord]

Hello,
I recently watched Dave's tutorial and teardown of this thing: https://www.youtu.be/JDXKrXJloSw
And I'm really confused as to how that one knob changes the resistance by an order of a magnitude or more. Dave didn't go into details on it either.

For example, if I have a switches with 1s, 10s, and 100s of ohms resistors, then the maximum resistance I can create is 999 ohms. If I have another switch that has a 1Kohm, 10Kohw, and 100Kohm, resistors in it, than switching it from a series short to a 1Kohm range would give me a maximum resistance of 1,999 ohms. Not 9,990 ohms.

How does the resistance multiplier work?

Thanks!

[AVGresponding]

Hello,
I recently watched Dave's tutorial and teardown of this thing: https://www.youtu.be/JDXKrXJloSw
And I'm really confused as to how that one knob changes the resistance by an order of a magnitude or more. Dave didn't go into details on it either.

For example, if I have a switches with 1s, 10s, and 100s of ohms resistors, then the maximum resistance I can create is 999 ohms. If I have another switch that has a 1Kohm, 10Kohw, and 100Kohm, resistors in it, than switching it from a series short to a 1Kohm range would give me a maximum resistance of 1,999 ohms. Not 9,990 ohms.

How does the resistance multiplier work?

Thanks!

It doesn't multiply the resistance; it changes the resistance ratios on opposite sides of the bridge. I recommend reading the manual, it can explain it better than I can. Drawing out a simplified version of the circuit for each multiplier range would probably help too.

It's interesting to note that it also doesn't use "true" (ten) resistor decades, but BCD decades, which use only 4 resistors to achieve each of the 10 values per decade.