RC Snubber Values across relay contacts when load is small (8W)

Thread Starter

Alan_uk

Joined Nov 15, 2016
6
Hi, stumbled across this forum when trying to research a solution to my problem, and got nagged by the pop-ups to register ;) Looks a useful and interesting community. Hopefully I can give something back in time.

I'm building a microprocessor (Raspberry Pi) controller to monitor humidity in a loft (we have a condensation problem) and then switch on extractor fans. The switch is an opto-coupler Songle 10A 250VAC SRD-05VDC-SL-C as used by many Raspberry Pi projects. http://www.ebay.co.uk/itm/5V-1-2-4-...no-Raspberry-Pi-ARM-AVR-DSP-PIC-/252051910091 Many example projects say a Snubber should be inserted across the contacts and invariably state one at 100nF and 100Ω but then say "dependent on load".

The extractor fan is Envirovent SILENT 100 running at 240V 50Hz 8W. No other information provided. These typically run off the lighting circuit. I calculate 8W as 0.03A but have read one should multiply by a factor of 10 for the start up load (back EMF?). But that is still only 0.3A.

I came across a previous thread on RC Snubber Values http://forum.allaboutcircuits.com/threads/rc-snubber-values.78661/#post-555959 and read the reference document and down loaded the spreadsheet:
http://www.hiquel.com/fileadmin/userfiles/AppNotes/Englisch/HIQUEL_Snubber_AppNote_EN_0100.pdf
http://www.hiquel.com/fileadmin/use.../HIQUEL_SnubberCalculator_AppNote_EN_0100.xls

I entered:
Line frequency [Hz] 50 Hz
Supply voltage [V RMS] 240 V
Nominal load current [A RMS] 0.03 A
Damping factor 0.23
Preset: 0.01
Maximum relay inrush current 0.3 A

The results are:
Capacitance Cs [nF] 5 nF

Resistance Rs [Ω] 33720 Ω

I set the chosen Capacitance value at 6.8nF (row 65) but the maximum chosen Resistance (row 70) allowed is 3300Ω - way below the above figure of 34KΩ. These values generate lots of warnings (orange) and one "error": Damping factor 0.022.

The reference document states: "If the snubber circuit is over-sized the leakage current would rise and the protection circuit would loose its efficiency".

Something doesn't feel right with these values. Most RC Snubbers (RC Networks) seem to be 100Ω or 120Ω. E.g. RC NETWORK, 250V, 0.01UF, 120R, PCB MPN: XE12001 ROXBURGH (wattage not stated).

Guidance much appreciated.

Many thanks
Alan
 

#12

Joined Nov 30, 2010
18,222
Imaginary problem. There is no point in worrying about the efficiency of an 8 watt load. You're getting, "error" warnings because you don't need a snubber for a load of .03 amps. That's like installing seat belts in a Tonka Toy.
 

Thread Starter

Alan_uk

Joined Nov 15, 2016
6
Many thanks #12 for a quick and reassuring reply. I'm being too cautious again ;). I like your analogy!

Actually I've just calculated that if I bought 2 low energy fans (4.3W each) I could run them for a year continuously for £9/$12 but I suspect that the bearing wouldn't last a year so there would be a high capital replacement cost.

Now to explore the project pages on this website.....

Alan
 

#12

Joined Nov 30, 2010
18,222
I suspect that the bearing wouldn't last a year
A lot of refrigerators use a 9 watt fan, and they last 20 to 30 years with bearings you can't lubricate.
I have a 10 watt, shaded pole fan with bronze bushings in my bathroom exhaust, and it has been in place for 38 years. I oil it every 10 years, whether it needs it or not.

Efficiency and longevity theory say: slow speed, big fan blades.
If it makes noise, it's wasting energy making turbulence.
 
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