Many early computers were s. Like any other inductor, when the coil in a relay is shut off, the current through the coil continues to flow. This is a result of Faraday's Law of Induction and is called "flyback". If there is no other path for the current in the coil to dissipate, it will flow into the switch that tried to turn off the relay (often the contacts of another relay), eventually burning that switch out.

There several ways to deal with inductor flyback, some of which may not have even been available in that era. How did early (before 1950) relay computers deal with this problem?

  • Flyback diodes? (This is the preferred method today.) What kind of diodes (germanium, silicon, vacuum tube?!; regular vs. zener)?
  • RC snubbers?
  • Other devices (MOVs, TVSs, etc.)?
  • Nothing? (They just accepted the fact that relays eventually burn out.)

Researched answers please, not speculation.

Although not technically computers, I will also accept answers about relay-based telephone switching systems in that era (pre-1950), as it seems they were the source of inexpensive relays for computers.

  • 2
    Perhaps the induced high voltages were a feature to kill moths?
    – DrSheldon
    Jan 24, 2023 at 20:01
  • 1
    @Raffzahn: I am asking how it was done back then. If I ask in EE, the answers will be how they would do it today, using today's components, or they will tell me to go back to RC.
    – DrSheldon
    Jan 24, 2023 at 20:17
  • 1
    I would guess the simplest approach would be to run everything off of alternating current. Things which are switched at random line phases would still spark quite dramatically, but arc duration would be limited. If some relays were switched much more often than others, one could add spark suppression coils to them with attached spark gaps (similar to automotive ignition circuits), but quality contacts can operate many, many, times even without such measures.
    – supercat
    Jan 24, 2023 at 20:27
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    @Raffzahn: We've had many questions about the components of historic computers. We even have a component-failure tag. It's definitely on-topic.
    – DrSheldon
    Jan 24, 2023 at 20:59
  • 2
    Actually funny thing is that your assumption for a modern preferred method is a flyback diode. Sure it is best to suppress the kickback, at the expense of being also the worst if the target is to switch the relay fast, so unlikely the preferred method.
    – Justme
    Jan 24, 2023 at 23:19

1 Answer 1


From electronics point of view, the simplest part that can be used for this purpose is a resistor across the relay coil. The inductive voltage spike will then have a maximum voltage of I · R, where I is the operating current of the coil and R is the resistor in parallel.

Because relays as switches can tolerate much higher voltages than semiconductors, the resistance can be large and does not waste much energy. In addition, the large reverse voltage means that the current in the relay coil decays faster than it would with a modern flyback diode, resulting in faster switching of the relay.

The inductive spike voltage is also limited by parasitic capacitance in the circuit and in the relay coil. The result will be high-frequency oscillation in LC circuit until the energy dissipates in coil resistance. In some cases this could be an acceptable way of dealing with the inductive spike, if all components involved can tolerate the maximum peak voltage and there are no EMC issues from the oscillation.

To research what methods were actually used, I studied the US patent 2636672A which details the IBM SSEC electromechanical computer. This is a system that uses both relays, vacuum tubes and specialized electromechanical counters in its implementation.

The circuit diagram does include several series RC snubbers. The snubbers are placed across some relay switch contacts, but not all of them and not consistently.

In modern circuit diagrams it is typical to see the flyback protection drawn in parallel with the relay coil that generates the spike. Electrically it is equivalent to have a snubber across the switch contacts that it is protecting.

Figure 29 from patent

Later in section 11, the text of the patent refers to arc suppression:

Text about arc suppression

The Figure 36 referred in the text:

Figure 36 from patent

If I interpret the description correctly, the relays AM turns on delay relay AMD, which turns off later. This shunts the inductive spike to the parallel RC circuits.

The relays used in the machine are of double-coil type. In the AMD relay the second coil is short-circuited, which will result in slow turn-off due to the low resistance maintaining the flyback current for a longer time. This simultaneously dissipates the energy into coil resistance. In some diagrams, the second coil is connected to a resistor, which would also delay turn-off but by adjustable amount, and simultaneously work as arc suppression.

Examples of double-coil relay connections are seen in Figure 32b, where the coils are driven by vacuum tubes:

Figure 32b from patent

In conclusion, at least RC snubbers, auxiliary delay relays and double-coil relays were used for arc suppression in IBM SSEC computer. I found no evidence of any type of diode being used for the purpose in this machine.

  • The point is implicit: before relays were connected to semi-conductor driver circuits, diode protection was not required.
    – david
    Jan 26, 2023 at 8:35
  • @david Yeah - RC snubbers are probably a better match for mechanical switches, because their arc voltage limit increases gradually as the contacts separate. For semiconductor switches the maximum voltage is constant and relatively low. Though diodes are not always a great idea nowadays either.
    – jpa
    Jan 26, 2023 at 8:49

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