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Resistance box overkill [Permalink]

When it comes to electronics projects, I like overkill.

I wanted a decent but small resistance box that could do really big resistances and fairly small resistances, to a precision of about one ohm.

I happened to have some 2mm PCB test connectors lying around and thought they'd be ideal for this project as I wanted to keep it compact yet configurable. To make it fully configurable I had to have a switch for each resistor as the simplest way to make a resistance box is to just have a big line of resistors in series and then have a switch across each one that will short it out when closed. The idea is that with all switches closed, the resistance is 0, with all open, the resistance is the sum of all resistors in the box.

So, what resistors to use? Well it's easy enough to get your hands on resistors that are 1, 10, 100, 1k, 10k, 100k, 1M and 10M and so if I'm going for overkill then I should get ten of each. Easy.

Now, how to arrange eighty resistors and eighty switches, cheaply and quickly? I decided to go really small for the switches. I got ten sets of 8 dip switches. These were easy to mount on stripboard and had just enough space to solder resistors on the back between the pins of each switch.

Now it was overkill time...

The soldering was easy, once I got into the rhythm. It was just a case of getting a strip of ten resistors, cutting the legs on one side while they are still on the strip so that they are small-enough to fit on the back in their place, using the larger leg to hold it still and soldering the small leg in place. The large leg could then be trimmed and soldered on to the other side of the switch with the small leg of the next resistor. This method had the added advantage of keeping all of the resistors in the same orientation. I only started doing this after the first wer resistors however and so there is exactly one resistor with the tolerance band facing down instead of up. I don't have OCD so I'm fine with this.

The columns of resistors were joined with solder blobs in the correct places at the top and bottom of the many rows of switches. This made a large snaking route up and down the board with almost infinitley configurable resistance. The contact resistance of one switch is about 50 milliOhms, this is usually fine, but when you chain 80 of these together you get about 4 ohms residual resistance. It's alright. I would have liked a little less but I'm not really going to need it often enough. Even when I do, I likely need more wattage than this thing can do and so have to find alternatives anyway. For trying lots of diferent resistances however, it's great. Exactly how many is "Lots of different resistances"? Glad you asked, we have 80 switches and so theoretically there would be 2^80 combinations of these switches, that's about 1.2 septillion resistances. The highest of which is 111,111,110 ohms or 111MegOhms You could argue however that most of these overlap. I can have 10 lots of 10 ohms or I can have 1 x100 ohms etc. There is a 5% tollerance on each resistor however and so this ammount of redundancy allows you to get close to the perfect resistance.

Like I say, I like overkill. I think I'm good at overkill.

It comes in handy often, here's some pictures, click for bigger...

Front of the box

This is the front of the resistance box showing all 80 switches.


Back of box, showing resistors

This is the back of the box. This is where all 80 resistors are mounted.


The back again

Time for an artistic shot?



This photo shows the connectors and the pair of switch banks that ended up with a gap between them. Shame.


Connector angles

Connectors can go in from two different angles, I had to drill holes for the mounting position that goes straight through the board.


One bad resistor

This photo shows the one and only resistor that was put in backwards. Look closely. Third from left.

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