in which I discuss the two types of rotary switch
My name is Akiyama Kazuo, and I will be your “time-travelling” guide for today.
My field of expertise is events of 30 years ago.
Today’s article concerns rotary switches, also known as rotary selector switches─those old-fashioned dials that you turn with your fingers. While increasingly uncommon in today’s digital age, rotary switches one featured in a wide range of electrical appliances.
A typical example of a rotary switch is the dial that used to be on television sets and gave us the phrase, “don’t touch that dial.” In Japan, it was common for siblings to argue over TV channels and end up trying to force the dial, only to end up breaking it and get into trouble with their parents. In my day, this was a common occurrence. While rotary dials have become less common on consumer appliances, they still have many applications today and their convenience means they remain popular for controlling machine tools and in experimental circuits.
Once, early in my career, I used a rotary switch to construct a selector circuit for testing purposes. However, while this initially seemed like a good idea, it did not turn out to plan. In this article I will explain what went wrong.
Our Multimeter Shortage
As usual, I will be taking you 30 years back in time.
I was a member of a team that designed, engineered and supplied direct-current regulated power supplies to client specification. In those days, the trend was towards multi-output supplies, and we found ourselves building power supplies with five, or even seven, outputs. Once an OEM power supply had been built and was ready for testing, we would procure one digital multimeter for each of the output voltages to be measured.
Before testing, we would place the device to be tested on the bench, and connect it as necessary. To the input side of the OEM power supply we would connect an AC regulated power supply, and to each output we would connect an electronic load and digital multimeter. It was quite a spectacle, as we had to use one multimeter for each output. We would also set up oscilloscopes.
In those days, OEM power supply development at Kikusui was handled by two teams, with each team working on the development of one or two models of power supply simultaneously. It wasn’t long before we ran into problems, however, when engineers wanting to test their OEM power supplies found there were not enough digital multimeters to go around. The shortage gave rise to a “first in first served” attitude to multimeters, and engineers would hold onto the meters they had, planning tests in a way that allowed them to continue using the meters they had on hand. We had not counted on this issue: if a senior colleague told you there were no more meters to be had, you just had to accept that fact.
A flash of inspiration
While I thought of asking my employer to buy more digital multimeters, I didn’t know how long we would be building power supplies with multiple outputs, and felt that it would be a waste to buy more meters if multi-output power supplies turned out to be a transitory trend. It was then that I had a flash of inspiration. I used to play the electric guitar in my spare time, and happened to be practicing Masayoshi Takanaka’s Jumping Take Off at the time. Halfway through that song, you have to change the timbre of the guitar by flipping the pickup selector switch from the front pickup to the rear pickup. It was when doing this that I got my flash of inspiration. (No, I’m not talking about being electrocuted!) I had the idea of attaching the multiple power supply outputs from the power supply to a selector switch so that we would not need to use as many multimeters when testing.
The following Monday, after our morning meeting, I walked around the office searching for rotary selector switches. I found a rotary switch in the lab used by the oscilloscope development team, and decided to borrow it. I forget what that switch was used for, but it had six contacts and the OEM power supply we were working on had five outputs, in addition to the common line. I quickly soldered the output wires to the switch, and used a multi-tester to check continuity. With that, my multi-output power supply voltage selector was ready for testing.
An alarm sounds
I set about connecting my circuit to the OEM power supply, and turned on the power. I had intended to use the rotary switch to enable me to switch between different output voltages while taking measurements. However, every time I turned the dial, the power supply would emit a beep that sounded very similar to the one you got when there was a short circuit.
“Surely not…”, I thought, and tried connecting an oscilloscope so I could monitor the voltage waveform while I turned the dial. I found that whenever I turned the dial, the contacts were shorting with the neighboring terminal.
I then tried to find out more about the rotary switch. In those days, we didn’t even have the Internet, let alone Google. I went to my company’s library and consulted a catalogue. It turned out that the problem was due to the type of switch I had used. The rotary switch I had borrowed was of the “shorting”, or “make-before-break”, type. This meant that whenever the switch was rotated, it momentarily shorted two adjacent contacts, causing the power supply to emit a warning sound.
Up until then, I had no idea that rotary switches came in both shorting and non-shorting varieties. (See figure.) However, the experience taught me the important lesson that shorting/non-shorting properties were an important part of rotary switch function and depended on the switches’ intended application.
Undeterred, I went looking for another selector switch. This time I managed to find one of the non-shorting variety, which I used to make my second circuit, and that served me well for about three years. I was pleased that my employer had not rushed out and bought more digital multimeters.
A little ingenuity can change a setback into valuable lesson
My experience fitted a classic learning pattern: a problem arises, you think of a solution, your solution is unsuccessful, but you still learn from the experience.
Life is a series of such problems. What is important is your ability to be creative and think up solutions to them. If a problem arises but you do not have a strategy for solving it and things still end in failure, you have nothing to show for the experience other than regret or trauma.
In an earlier article, I advised readers to “try anything and everything” when solving problems, but I could have just as well have told them to be “ingenious”. When encountering a problem, ask yourself, is there an alternative to simply pursuing the conventional solution?” (In the case described here, buying more multimeters.) This is important. I am not saying that there aren’t times when creative solutions actually end up making a situation even worse, but as an old-timer, I urge the young engineers reading this to remain positive when tackling life’s challenges.
