Measuring relay contact switching time ? - Page 1
Forum section: electronics / projects

author: chris_leyson
author_rank: Super Contributor
timestamp: August 20, 2019, 07:30:37 pm
content: I have an small issue with a single pole single throw 250V 5A relay, namely measuring the switching switching time. The relay drives a small inductive load across 230V AC and when the contacts open a small arc is generated. The relay is a Hongfa HF46F and according to the datasheet the coil is not polarized. The switched current is only 50mA or so.If I energize the coil in one direction and then remove the drive I get an arc lasting typically 100us and if I energize the coil in the opposite direction and then remove the drive the arc lasts typically 1ms. There is a single diode across the coil for back emf protection and the time taken for the contacts to open after removing the drive is typically 4ms regardless of drive polarity.There is almost a 10:1 difference in contact opening speed depending on the coil drive polarity and there is also a small magnetic bias in the steel which I can't measure but that might explain the different switching speeds. I can measure the contact opening speed by looking for an arc but are there any other measurement methods that don't rely on high voltage ?

author: OM222O
author_rank: Frequent Contributor
timestamp: August 20, 2019, 07:41:46 pm
content: Quote from: chris_leyson on August 20, 2019, 07:30:37 pmcan you please explain how on earth you measured 100uS and 1mS just by "looking for an arc"?If you have the coil connected to a high voltage supply, then you can't magically do something to remove that high voltage in your measurements, unless you remove the load, which automatically removes the spark anyways.the diode across the "coil" is to reduce back emf to the MCU, not to suppress the arc created by the inductor. add a similar diode across the inductor as well, it should remove the arc issue. if that wasn't enough, add a snubber network across the relay switch contacts as well.

author: Seekonk
author_rank: Super Contributor
timestamp: August 20, 2019, 08:33:26 pm
content: Diodes definitely slow things down. Use a higher voltage switching device and just a minimal RC network or high voltage zener/mov. A method I use to see how fast they CLOSE is looking at coil current. You will definitely see a change in slope when armature gap closes.

author: woodchips
author_rank: Frequent Contributor
timestamp: August 21, 2019, 10:01:43 am
content: What you need is a Programma TM2 relay contact timer.From the manual it does exactly what you are requiring from 12/24V DC to 230V mains. It is so useful I have hung on to it, but never actually used it.

author: David Hess
author_rank: Super Contributor
timestamp: August 21, 2019, 02:22:42 pm
content: The diode across the coil slows down switching in one direction by clamping the back EMF voltage to a low level slowing the decay of the magnetic field.Sometimes an avalanche diode is used instead to clamp the back EMF to a higher level which still will not damage the transistor switch. Or an RC snubber across the coil can be used. Or the coil can be clamped to the supply voltage with a normal diode across the transistor instead of the coil although this introduces switching noise back into the supply voltage.