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Can somebody tell me why analog multimeters leads are reverse polarity in resistance range?
I read all around web and all says thats how the meteres are designed.
The meter movement deflects clockwise in proportion to the current through it. On the Ohms range(s), current is driven through the test resistor in series with the movement; hence the greater the resistance the less the current. That means the scale is marked with resistance values ranging from high at the left (anti-clockwise) end to low at the right (clockwise) end.
Long before the first DMM hit the shelves I was using my old Simpson 260 analog meter to test things like Diodes and Transistors, oh yes, also to look at capacitors and let's not forget SCRs and Triacs. I can clip my leads on a component and use that little polarity switch. The resistance ranges were used for a great deal more than just checking or measuring resistors where polarity matters not. That polarity reversal switch comes in real handy for testing semiconductor devices using those resistance ranges.
Analog Meters use positive on Red (+)
Digital meters use a current source referenced from the Vbat + to simplify the common mode range or the commutator rotary switches and hence is negative, ... I think.
But then they added Diode test mode which is positive V ~1mA current source.
I'm not quite sure if the original poster was asking about the "reverse" (maximum reading when fully anticlockwise) scale of analogue meters or the outputting of a "reverse" voltage in the Ohms range. Alec's post #2 answers the first question well.
With regard to the polarity of the voltage that's output in Ohms range (ie, whether the red terminal is positive or negative) I'd just advice you check it and don't assume that all instruments are the same. I seem to remember my father's old GPO meter used to output opposite polarity on one resistance range relative to the others.
As to why they where designed this way.... I'm afraid I can't help. Perhaps there is a service manual floating round the 'net that could shed some light on the matter?
As tomizett commented, It appears that the OP was questioning why the positive probe has a negative voltage on it and the negative probe has a positive voltage on it. The simplest way to explain is to refer to the attached simplified multimeter schematic. In order to have the meter deflect correctly, the battery polarity must be as shown, and hence you get the apparently wrong potentials on the test leads. The manufacturer could add in additional sets of contacts on the range switch in order to reverse the leads when in ohms mode, but it makes the circuit quite a bit more complicated, and not worth the effort on inexpensive meters. Also, since the ohms range is intended for measuring resistances which are generally not polarity sensitive, it ususally doesn't matter.
BobW describes the idea correctly and the diagram helps significantly.
All that is missing now is the equivalent diagram of a DMM set to the Ohms range.
BobW describes the idea correctly and the diagram helps significantly.
All that is missing now is the equivalent diagram of a DMM set to the Ohms range.
For the truely curious at heart this link will go to a .pdf file of many Simpson 260 Analog multimeter schematics, also a simple Google works well. The schematics in the link clearly show the operation of the meter and ranges.
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