Fun with Relays ??

[Musicmanager]

Hi
Can someone enlighten me please ?
I'm very much a novice but I managed to build a Precision timer with a circuit diagram from Pic Projects and some help from others on here. I was positively jubilant when I turned it on for the first time and it worked ! I'm using it to control a machine in my workshop which had mechanical timing which tripped a relay for on or off. Over the years this relay has been very vulnerable and has failed a number of times - I think because of the surge when the motor kicks in. For this reason I decided to add another relay on a PCB mounted socket so that replacing it was a 'plug and play' operation rather than possible damage to my pride and joy and then some soldering to repair it.
However, this relay played all kinds of 'silly tricks' with the microcontroller, false starts, stops, varying times etc.. and then I noticed that the relay on the kit had a 1N4148 diode across the coil terminals.. .. so I fitted one across my relay and all the 'silly tricks' disappeared !
As delighted as I am that this works, I haven't a clue why so .. .. can somebody explain please ?
Thanks

 

[Wp100]

Its called a Flyback diode and its designed to stop the very problem you describe.

Basic details in this link https://www.physics.unlv.edu/~bill/PHYS483/relay.pdf

If your relay contacts are burning out too quickly, then why not consider a relay with higher current contacts ?

 

[Musicmanager]

Hi Wp100
Thanks for the info, all seems perfectly simple now I've read the details ! Huh!
The machine is driven by a 1/2hp motor which normally draws about 1.5 amps but there is a current surge on start. I've never been brave enough to get close enough to measure the surge but I've known it trip a 5 amp fuse before now.
The relay I've installed is rated at 10amps to allow for this, but it occurs to me that there has never been any flyback diode in place before so maybe that was causing a problem since the original control was mechanical so a problem would not have been so obvious.
Anyway, thanks again for your help, Much appreciated

 

[Wp100]

Is your motor DC or mains AC ?

It is also possible to fit suppressors across the relay contacts to reduce the sparking and so lengthen the relays life.

Another option where heavy currents are in play is to 'slave' your relay with a much bigger heavier current relay off the pcb.

Have not seen your pcb but that sounds like quiet a heavy current for a pcb, was it intended /designed for such heavy loads ?

You can get anti surge fuses which help prevent blowing during the motors start up .

 

[JimB]

A practical demonstration.

See the scope traces.
In the first one, 12v is switched onto a 12v relay. The "fuzz" on the trace is due to contact bounce in the switch.
In the second one, there is a great big -350 volt spike as the switch is opened.
This spike destroys transitors and causes all sorts of odd effects.
As already mentioned, a diode in parallel with the coil kills the spike.
The third trace shows the effect of connecting a diode in parallel with the coil.
The left hand side shows the effect on the same scale as the first two traces (50V per division), on the right hand side the scaling is changed to 5V per division.

JimB

 

[Musicmanager]

Morning Guys

Thank you so much.
JimB - thanks for going to so much trouble - I must think about acquiring a scope so that I can explore this kind of problem myself. I'm learning such a lot from you guys though, much appreciated.
Wp100 - the motor is 240ac 1/2hp, quite old, american origin. I guess I have in effect put in a 'slave' relay - there is a 12v miniature relay on the pcb which is part of the original schematic. What I have done is to put 12v into the switch contacts which then operates a second relay(12v coil/ 240v 10amp) on a separate pcb mounted on a socket for ease of changing. I did consider the load implications for this pcb - I have connected the switched contacts to a soldered screw terminal rated at 10 amps and replaced the pcb track between the contacts with 1.5mm sq mains cable. During the course of the day I'll dismantle some of the box and take some pics for you to see.
Question ? - The suppressor across the relay contacts - Is that sometimes called a 'snubber circuit ?'

Thanks again Guys

 

[tunedwolf]

Question ? - The suppressor across the relay contacts - Is that sometimes called a 'snubber circuit ?'

Yes, often called a snubber network, or just snubber. It usually comprises of a small value metalised polyester capacitor and a resistor in series, fitted inside a small 2 lead package. Common component values are 0.1uF and 100 Ohms. It is not polarity sensitive and can be fitted either way round across your contacts

 

[Wp100]

Sounds good in the way you have added / beefed up that slave relay, though have your fitted a flyback diodes to both both relay coils ?

Yes, suppressor /snubber the same thing, basically a cap and resistor, that you really need to fit to the slave relay/motor.

Many pages on the web about suppressors, here is just one.
http://www.bpesolutions.com/bpemanuals/Snubber.pdf

You should be able to buy one at your local electric /electronics shop, eg maplin in the uk, but its critical that you do buy a genuine mains voltage suppressor where its internal capacitors are whats called Class X or Class Y with means they are properly rated for mains, otherwise they could burn or explode.