Voltage spikes

[cscotthendry]

Jab7252:

 

That's a pretty detailed and correct description of what happens. But there's a missing part to the explanation. Looking at the circuit, you might say "but if the button is released, how does the spike from the solenoid get back into the system?" And that's where switch contact bounce comes in.

 

The contacts in switches obviously have to be made of metal. Anyone who has dropped a ball bearing on a concrete floor knows that steel balls bounce much better than the bounciest rubber balls. So, when a switch is opened or closed, the contacts don't make a single connection, they make a series of opens and closes until the contacts finally settle. The duration of all this on-ing and off-ing varies with different switches and also varies depending on how the switch is operated. In any case this random switch bouncing manages to couple some or all of the spike from the solenoid (and generate a few more smaller spikes) until the whole mess finally settles down in about 20-30 milliseconds. The timing of the bounces with regard to the collapsing magnetic field and the generated back EMF (spikes) accounts for part of the seemingly random nature of the destruction of other sensitive circuit elements.

 

Now you have a pretty complete picture of the mechanism that destroys radios and other sensitive circuitry and the fact that it occurs during Starting, Not during shutdown. By now most people are rolling their eyes and groaning "too much information". But the important point is that you should have an avionics master switch as Mark said, as a minimum and better still also a snubber diode across the starter solenoid, which will improve the life of the start button/switch by preventing arcing occurring across the contacts when the button or switch is released.

 

 

[Kyle Communications]

I love the difference between countries...cookies = biscuits , soft drink = soda , solder = sodder , and.....snubber diode = freewheeling diode

 

Just thought I would add that

 

 

[Old Koreelah]

The French (spit) defend their language with legislation. Perhaps we should do more to stop the creeping Americanisation of our lingo. ANZAC cookie, anyone?

 

 

[Blueadventures]

JakeI like the rotax manual it says we recommend a 22,000 uf cap put there but they dont supply it. I have seen quite a few rotax installs without it...it also protects the regulator if you turn off the master before the mags...apparently you can take out the regulator easily if you dont have the cap there

These can be purchased from 'RS online'

 

Cheers

 

Mike

 

 

[jakej]

These can be purchased from 'RS online'Cheers

Mike

Hi Mike

Where are you in Mackay, I'm there at moment at Snap On doing a wiring job - maybe we can 'catch up' ?

 

 

[Blueadventures]

Hi MikeWhere are you in Mackay, I'm there at moment at Snap On doing a wiring job - maybe we can 'catch up' ?

Hi my contact 0438800196 cheers Mike