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How can this be? (picture)

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Tom45:

--- Quote from: billbyrd1945 on February 16, 2019, 12:47:49 pm ---I was released from high school on a plea bargain. Can you show me the math you just suggested. I know how to figure 5% of the displayed power supply values. You lost me on the "+ 2 to 3 digits". The Fluke is on auto-range and displays the same values as the PS. (Then I have a follow-up question regarding series vs parallel.)

0.005A + 0.00025(5%) = 0.00525A vs 0.00634A

--- End quote ---

The +- 2 or 3 digits is really 2 or 3 counts.

As you know, the 0.005 reading could be off by 5% either way. The 0.00025 you stated.

The +- counts says that the 0.005 reading could be as much as 0.008 or as little as 0.002. Pretty dreadful! But that is because the meter scale is amps. If you have a milliamp scale, it could indicate 5.000. Then the +- counts would give a range of 4.997 to 5.003 milliamps. A lot better.

OK, I just looked up the Fluke 15B and it does have a milliamp scale. Also a microamp scale but that wouldn't be appropriate here. See what the current reading is on the milliamp scale. Note that there is a separate input for the milli and micro amp scales.

Tom

billbyrd1945:
(Response to Tom's last) Oh, I see what you mean. I have another meter with more counts. I'll check it with that. But-- I actually only used the multimeter (one time) to see how it compared to the PS display. After learning that the PS doubles as an ammeter, I just check to see what it shows. And it is that figger (a little Churchill) that's too low. Might there be a trim pot in the PS for calibrating it to agree with the math? (And, I do understand that meters used in checking skew the very values they're measuring.)

billbyrd1945:
Thanks Tom! Learned a lot. Essentially splains everything! Build with math, then check with meters. If they agree, you're probably off a little bit. ;D

billbyrd1945:
It seems obvious in this simple circuit that R5 will be used as a whole number added to the resistance equivalents of R1-R5. In more cluttered circuits (for noobs like me) it isn't always so obvious. I was trying to come up with a rule. At first, I thought "Any component followed or proceeded by a node, would be in parallel". But then... that's probably not correct. Any suggestions?

ebastler:
I may be misunderstanding you; but I think you have your terminology mixed up, or have misread the circuit diagram. In your diagram,

* R1 and R2 are connected in series, and R3 and R4 are in series,
* the group (R1+R2) is in parallel with the group (R3+R4),
* R5 is in series with the group ((R1+R2) || (R3+R4)).Whenever two resistors are wired in series, their resistances add up:
Rtotal = Ra+Rb.

Whenever two resistors are wired in parallel, their conductivities (i.e. the inverse of the resistances) add up to the total conductivity:
1/Rtotal = 1/Ra + 1/Rb.

May I again recommend drawing the circuit as shown in reply #6 above? As I said back then, there is no "right" or "wrong" here. But, at least for me, that way of arranging the components makes the parallel vs. series connections more obvious.

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