C-Pap power usage

DrewE wrote:

pnichols wrote:

MEXICOWANDERER wrote:
Again stick your finger onto the starter motor power terminal crank the engine release the key then scrape yourself off the wall.

It first - takes voltage (higher than 12 volts), then second - adequate current driven by that voltage for a person to get plastered to the wall.

I understand that the necessary current is available, but where is high voltage coming from during the engine starting procedure? The alternator of course is a source of relatively high AC voltage when it's spinning, but it's hardly doing that during engine start.

Induction in the starter motor or solenoid coils when the circuit is broken. Recall that an inductor is the complement of a capacitor; it tends to prevent changes to current (rather than voltage) by supplying or sinking voltage (rather than current). Open the circuit, and the current immediately goes from many amps to zero, and the response of the inductance of the coils causes the voltage there to spike. For this reason, coils on relays etc. often have snubber diodes or resistors to provide a path for these transients to discharge.

That particular voltage spike oughtn't make it back into the 12V system since it's on the disconnected side of the circuit, but there are other sources of probably lesser voltage fluctuations when the vehicle is started. Car and truck electrical systems are not particularly clean or stable supplies.

yup. Put a scope on many of the power lines in a car and you may be suprised what kind of spikes are there as relays switch etc. and the starter motor being a DC motor with brushes and comutator is particularly bad for putting big spikes on the 12V lines.

I've found several aftermarket accy for a car that would die pretty quick and prematurely. In all the cases, I found they had no snubber diodes on the supplies to prevent voltage spikes from getting into the circuit. And when your looking at logic rated at 20 or 30V destruct limits, it isn't very hard at all to kill stuff in short order with the voltage spikes running around in the car.

voltage across a capacitor can't change instantaniously, current through an inductor can't change instantaneously. So when you interupt these circuits you get big voltage or current spikes.

A good example is the arc you likely have seen when disconnecting somthing like a hair dryer while running from an outlet. that voltage spike is related to the inductance and interupting the supply quickly. The result is a big voltage spike.

Note that disconnecting a resistive load doesn't give any noticeable arc.