I found in a book a scheme representing a terminal connection used in making an ammeter shunt (with the purpose of extending its measuring domain). enter image description here

I can't understand what the drawing at b) represents. All those resistances want to model the contact resistances.

Then, the book says that in order to make the measurement more precise, we use a "double terminal", drawn here: enter image description here

What do these drawings represent in real life and why are the resistances arranged as in the first picture, at c) (the delta connection)?


Drawing b) represents a terminal cut in half. Let's say that the white rectangle where R3 and Rs are connected is a nut, then you have one ring terminal lug between R3 and R2, then another one between R2 and R1, and then you have the head of a screw at the other end of R1.

Here the contact between the screw and the nut is considered perfect (rather surprisingly) so R1 is directly connected to R3 and Rs.

The double terminal allows to eliminate some of the parasitic resistors. On current shunt you often find 2 big terminals for the circuit where you measure the current and 2 smaller terminals where you connect your voltmeter:


Likewise, on the SMD shunt resistors you'll find 4 pins, 2 for the main circuit and 2 for the measurement:

SMD shunt

  • \$\begingroup\$ Thank you for your answer. So the drawing is just a cross section of a screw on which lugs are connected. If so, what is the thing on the top? And why c) is drawn that way? R1,2,3 represent the contact resistances between the lugs and the screw, or resistances between the lugs themselves (caused by the contact with the screw)? \$\endgroup\$ – user42768 Oct 14 '14 at 6:42
  • \$\begingroup\$ Can you be more specific about "The thing on the top" please? Both, R2 is between the lugs and R1 and R3 are between the screw/nut and the lugs. \$\endgroup\$ – Biduleohm Oct 14 '14 at 16:57
  • \$\begingroup\$ I am sorry for my description. I thought that the more wide and "short" rectangles represented the lugs themselves, so I did not know what the topmost "wide and short" rectangle is. So R2 is the resistance between the lugs (caused only by the material from which the screw is made, not because of the imperfect contact?), R1 is the contact resistance between the lug linking the ammeter and the screw and R3 is the contact resistance between the lug linking the shunt and the screw? \$\endgroup\$ – user42768 Oct 14 '14 at 17:24
  • \$\begingroup\$ Or are the two lugs connected using the "screw" and also they are somehow touching? \$\endgroup\$ – user42768 Oct 14 '14 at 19:53
  • \$\begingroup\$ Ok, the top wide and short rectangle is the screw head. No, they are contact resistance. It's resistance caused by imperfect contact, surfaces, ... \$\endgroup\$ – Biduleohm Oct 14 '14 at 20:04

Cool, First I have no idea. But b, would look like c if the top of R1 was connected around to the bottom where Rs is. Perhaps this was from an era when the temperature coefficients of resistors weren't so good, and this was a method to reduce it.


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