Hobbs

[Anticept]

Hobbs meter would run backwards. They have diodes to prevent that.

 

Also CTLSi, rotax manual recommends tracking by hobbs... I think it's silly though!

[tennesseect]

Tom,   I believe I can answer your question about why the ALT light doesn't stay on when the rectifier switches the Hobbs into the circuit by drawing on my memory of basic electronics from the Air Force. 

 

The reason is because of the much lower resistance of the bulb compared to the Hobbs.  Direct current follows Ohms law (Volts = Current X Resistance), behaving like water flowing through a hose:  Water pressure is like voltage, the water flow rate is like amperage and pinched places in the hose are like load resistors.    Think of a hose with two pinched places - one just slightly pinched (the bulb) and the other very nearly choking off the hose (the Hobbs).  The water pressure drops drastically as it passes through the Hobbs but only very slightly through the bulb.  These drops in water pressure are analogous to the voltage drops across the bulb and Hobbs.  The same current passes through both the bulb and Hobbs and its value depends on the total circuit resistance and the applied voltage. 

 

If the Hobbs has ten times the resistance of the bulb then much less current flows when the Hobbs is added into the circuit and only about a tenth of the applied voltage is dropped across the bulb.  A 12v bulb that glows brightly when it's the only circuit resistance will fade out when the current drops precipitously.  So Flight Design picked a bulb with the right filament characteristics so that it glows when it's the only circuit element but doesn't get hot enough to glow when the Hobbs resistance is added into the circuit.

 

Does that make sense?

[FastEddieB]

Does anyone here know who Rube Goldberg was?

 

Seriously, I had no idea how the hour meter in my plane worked, so I looked.

 

I have a dedicated "Hourmeter Relay Board", so I'm not sure that's any better and/or simpler and/or more elegant.

 

Still, the CT way does seem a little convoluted, doesn't it?

[Anticept]

Actually the CT is extremely simple. It takes advantage of electrical engineering to create a super simplistic hobbs design.

 

Tennessect: i just realized what he was asking. He was asking why doesn't the bulb light if power can go from pin 6, throught the light, through the hobbs, and to ground.

 

Simple answer Tom: hobbs meters only uses a tiny tiny tiny amount of power to run, so only a fraction of the power will pass through the light. Can't force electricity through something that only draws a tiny amount without raising the voltage.

 

Remember, your water meters are also electrical hobbs meters, and you don't see them plugged in. They only need the power generated by the sensor, MAYBE a watch battery if you wanted to get really fancy.