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Building a R2-R ladder load bank

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Long story short. I need to build a load bank of variable load. I was thinking R2-R ladder style with SIP SPST relays would do nicely. I even have (had) everything laid out on the PCB. Then I went to find the parts numbers for the resistors. Problem is 5% resistors have nominal values 10% apart. The first load is 100 ohms. Great they (actually) make that value. 50, 25, 12.5, etc. they don't. They don't have values that are 2x apart from each other. I considered just buying a bunch of 100 ohm resistors and paralleling them, 1, 2, 4 OK. 8, I supposed. 16 and 32, I don't think so.

Question to the lot, how would you go about making an R2-R ladder from standard resistor values? (1/2W resistors, avg 5% precision)
 

JimB

Super Moderator
Most Helpful Member
Don't make all your resistors the same value.

Code:
Resistor Value = 100 Ohm
Number in parallel  Overall resistance
              1                                100
              2                                  50
              4                                  25
              8                                  12.5

Resistor Value = 10 Ohm
Number in parallel  Overall resistance
              1                                10
              2                                  5
              4                                  2.5
              8                                  1.25
(I hope the code tags work OK)

JimB
 
The first part I get. The second part not so much. I can see that one 10 ohm in series with four 10 ohm in parallel also gives 12.5 ohms, so 5 resistors instead of 8.

I still need to get to 6.25 an 3.125 ohms.

It also occurred to me that 3, 47, and 22 are standard values, so a two resistor combination to get 50 and 25. Making 12.5 ohms from two sets of 22+3 in parallel. I'm knocking out the first 4 out with some easy combinations. it's the next two that are driving me nuts.
 

Les Jones

Well-Known Member
Most Helpful Member
I think JimB is choosing values so the values can be selected in decades rather than straight binary. You can get 100 and 200 ohm resistors in the E24 series of resistors.

Les.
 
OK. There are limited number of resistors that are in 2x increments.

I'm not sure how decade (or what I'm thinking of as decade), helps get the value options I need.
 

MikeMl

Well-Known Member
Most Helpful Member
A true ladder requires Form C relays.

If the digital input value is n, where 0<=n<=(2^b)-1, where b is the number of bits, do you want R=Ro*n or R=Ro/n?
 
Multiply or divide doesn't really matter. It is just based on where one defines Ro. In my case I am working with load and thus current. I know what the top resistor value is for the lowest current, 100 ohms. In this case you would list it as Ro/n. All of this is irrelevant. The question at hand is the best method for making a set of resistors that are 2x apart where the highest value is 100. So we are talking about 100, 50, 25, 12.5, 6.25, and 3.125 ohms.

I don't see why a form c (spdt) relay is required. I have the circuit perfectly laid out with form a spst relays, as I see in the textbook. (Using for reference, college was 20 years ago)
 
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