Load bank switch selection

[newbiEE]

I have a load selector which basically connects resistances in series and parallel. I want to make this automated, preferably an Excel Sheet. Any suggestions??
The switches can be in two positions for each resistor, in series (0), in parallel (1) or disconnected (2). Each resistor is 1.6 Ohms.
Thanks

 

[gar]

121026-32008 EDT

I not sure what is your question.

Do you want to know how to design the logic circuit, the size and rating of the components, how to get data from an Excel sheet to control relays, or whatever else?

To switch two resistors in parallel or series and to be able to disconnect them from two terminals requires something with three state capability. If the control is from binary data, then there has to be a two bit binary word for control. This has four possible states and three of these would be used.

You can accomplish this with two relays. One has a SPDT contact, and the other a SPST contact. You could use three SPST relays.

You have to worry about sequencing order, and how quickly you can change the configuration.

More information is needed.

.

 

[Jraef]

Waaaaaay back in the day, the very first PLC program I was told to write in a class was for a "drum controller", essentially similar to what gar is referring to. Taking your spreadsheet target chart and converting it to logic statements with an Output Word in a small PLC will do that nicely. Then just connect those PLC Outputs to appropriately sized contactors or relays sized for the voltage and current of your load bank.

 

[zbang]

Ah, the humble drum controller. Back when, I operated a carousel that had a drum controller running a 10hp wound-rotor motor. It's still there and in use (original controller replaced with a new one in 1976).

Don't forget about switching time (contactors don't move instantaneously) and whether to make-before-break or break-before-make.

 

[gar]

121026-2336 EDT

newbiEE:

See if you can draw the circuit using your two resistors, one relay with a break-before-make SPDT contact, call its coil A, and one relay with a SPST NO contact, call its coil B.

Connect the contacts of relay A so the resistors are paralleled when A is de-energized.

Connect contact B from the midpoint of the series-ed resistors to the hot supply.

When both relays are de-energized the resistors are disconnected from the voltage source. So the load has infinite resistance.

When A is energized and B is de-energized the load resistance is 1.6 + 1.6 = 3.2 ohms.

With A energized, and if B is now energized, then the resistance is 1.6 ohms.

De-energizing A while B is energized will produce a resistance of 0.8 ohms.

So we can actually have four useful states of infinite, 3.2, 1.6, and 0.8 ohms.

This logic will remove the load with a loss of power to the relay coils, but it does not provide a linear mapping of the binary input number to the change of resistance.

B, A, = output
0, 0 = infinite
0, 1 = 3.2 ohms
1, 0 = 0.8 ohms
1, 1 = 1.6 ohms

Negative logic results in

B, A, = output
0, 0 = 1.6 ohms
0, 1 = 0.8 ohms
1, 0 = 3.2 ohms
1, 1 = infinite
.

 

[gar]

121027-0608 EDT

Note: In my last post there is no input state combination that will cause a short circuit across the load terminals. This is prevented by the SPDT break-before-make contact. With three SPST relays the load terminals could be shorted.

.