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.
[Table of Contents]
People old and young
enjoy waxing nostalgic about and learning some of the history of early electronics. Electronics World was published
from May 1959 through December 1971.
As time permits, I will be glad to scan articles for you. All copyrights (if any) are hereby
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
See all the available
Electronics World articles.
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 be made.
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