EEVblog > EEVblog Specific

EEVblog #1409 - The HUGE Trap of Inductor Back EMF

(1/4) > >>

A practical demonstration of Lenz's law and back EMF in an inductive relay coil and how to solve it using a Freewheeling/Flywheel/Flyback/Snubber/Clamp diode. Also the downsides of clamping diodes, and switch arcing supression.
This is a follow-on from the DC Transients Fundamentals video:
Also a look at an AMAZING potential phenomenon you probably haven't seen before!
Actually, two rather cool things you probably haven't seen before.
Along with transistor ratings, transistor storage current, and Collector-Emitter breakdown voltage, there is a lot to unpack in this video.

00:00 - Recap of Relays, Inductors, Faraday & Lenz's Laws
02:30 - Relay Back EMF Explained
07:09 - The Flywheel analogy of Inductors
08:30 - Relay circuit demonstration
12:35 - 700V Back EMF!
14:43 - BJT Transistor Storage Time
17:03 - Back EMF Diode clamp demonstrated
19:06 - An AMAZING demonstration!
24:43 - Trap for young players
25:23 - DOWNSIDES of Back EMF Diodes
28:38 - BONUS cool effect of Back EMF diode DEMONSTRATED

Nice work editing. Pretty slick without any wasted time; just under the 30 minute mark.

The extras are interesting too, never thought the spark gap type oscillation possible with a relay coil and a BJT. I wonder how much EMI it actually kicks out and if it's comparable to the other switching EMI in such a system.

The oscillation may radiate quite a bit - it is quite some energy, not just from the coil, but also new energy when the transistor turn on again. It is kind of lucky to turn off after some 1ms.

In the old days before diodes got cheap, they had RC elements in parallel to relays. There where ready made RC components allready in the 1920s for this.

Relay contact arcing is a big problem for noobs, "muh why did my Arduino/Raspberry Pi crash?". The radiated EMI generally causes havoc, especially WiFi mains remote stuff ESP8266 sitting an inch away.
Back in the day we used snubbers, when there was room and the cost was minor. I always put them in but the parts are huge and frowned upon. You don't see them in appliances like ovens, dishwashers, furnace, A/C etc. they just let the relays arc and have a short life.

I find any back-EMF diode will experience maximum whatever the inductor current was, and whatever the supply voltage is.
After all, you can't get more current from an inductor than what was flowing in the first place, unless you are Nikola Tesla  ;)   If the relay coil current was say 100mA then even a 1N4148 is adequate.
But the back-EMF diode significantly extends the time for the B-field to decay (relay contacts to open) so a poor choice when you need speed.
Automotive will also just put say a 680R resistor across the coil, to pass the reverse-battery requirement yet have tame spikes.

edit: the relay coil-to-contact capacitance is another path for contact's arcing EMI to get to VDD on your MCU.

Love this series.  This is what brought me to EEVblog in the first place.

One on different snubbers please.  In particular I'm interested in using a relay to switch a 240v pump motor.


[0] Message Index

[#] Next page

There was an error while thanking
Go to full version