April 1967 Electronics World
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
Sometimes, component datasheets can be pathetically lacking
in detail for parameters that you might think would be a minimum
amount of information needed in order to integrate the device
into a circuit or system. Or, it may be that you found a part
in a spare parts box and can't find a datasheet for it. When
those times come, it is necessary to characterize the part yourself.
Relay control really isn't rocket science when it comes to electrical
circuits, but there are a few rules of thumb that should always
be observed, the most important of which (other than not exceeding
voltage and current maximums) is to wire a diode reverse biased
across the coil terminals so that the voltage/current induced
due to the field collapse during de-energizing does not smoke
the driver circuit.
This article from the April 1967 edition of Electronic World
recommends a test setup for determining the cut-in and cut-out
speeds. It might be important to know the speed in case the
application has a timing requirement for synchronization with
another part of the circuit/system.
Here are links to the other relay articles:
Operate and Release Times of Relays,
Finding Relay Operate and Release Times,
Arc, Surge, and Noise Suppression
Finding Relay Operate and Release Times
By Donald Ludwig
When designing circuits using relays it is often necessary
to know the relay's operate and release times. Because of certain
circuit conditions, the times given by the manufacturer will
not always hold true. The method to be described will help establish
these parameters and requires only a regulated power supply,
a single-trace oscilloscope, and two simple circuits - as suggested
by the National Association of Relay Manufacturers. This article
illustrates the standard s.p.d.t. break, make Type-C relay under
test. This type of relay is frequently used and the circuits
employed can easily be converted to test other types of relays.
Operate time begins when the energizing voltage is applied
to the relay coil and ends when the wiper arm has reached the
energized (N.C.) position. See Fig. 1. Release time starts with
the removal of the energizing voltage from the coil and ends
when the wiper arm has returned to the de-energized (N.C.) position.
See Fig. 2.
Fig. 1. Circuit used to find operate time
of relay along with oscilloscope display that is produced with
The checks should be made under conditions closely approximating
actual operating conditions. These include temperature, mounting
position, use of arc suppressors, and any other circuit conditions
that may affect operate and release times. It should also be
remembered that the accuracy of the test is dependent on the
accuracy of the scope, if all other precautions are taken.
If the relay being tested has more than one set of contacts,
the operate and release times of each set being used should
be checked. The time required for each set of contacts to function
may differ from one set to another. An increase in coil temperature
due to repeated operations of the relay may cause the resistance
of the coil to change and affect the time elements of the relay.
Each set of contacts should be checked several times before
calling the test conclusive.
When determining relay operate and release times, the scope's
sweep speed should be set with a time base which will permit
viewing of the waveforms and still allow accurate readings to
This method requires no elaborate equipment to establish
the relay's operate and release times. Using an oscilloscope
also permits contact bounce to be examined.
Fig. 2. Circuit used to find release time
of relay. The values of CLR and Vc depend on particular