Resistors for p-lead RPM pickoff

[RV10Rob]

How does too little and too much resistance in the p-lead pickoff line manifest itself?  I believe too little can result in erratic and higher than actual RPM indications.  Does too little resistance result in lower than actual?

Looking at the data log from my takeoff yesterday, I see an indication of 3250 RPM between two seconds of 2680 RPM (which is what it should be), so I'm guessing I need more resistance in the line.  I started with higher than the recommended amount (since I've heard there are problems with the IO-540), but forgot how much I used (dumb, I know).

Thanks...

-Rob

 

[dynonsupport]

Too much resistance can cause the voltage pulses that you're trying to read to not be read because the voltage gets cut down too low. This causes low rpm reading. Too little resistance can cause phantom pulses (electrical noise) to be mistakenly be read as the pulses you're trying to read, giving you rpm indications that are too high. 30K is good for most people, and if anything, folks normally need to go up. People have gone up to 150k, but just how high or low you'll have to go is hard to say.

 

[jdavidso]

First let me confess.  I am not an electrical engineer.  I have a Jabiru 3300A with the Dynon EMS 120 in my Zenith 601 (HD).  When I reduce power somewhat quickly, I get an tach overspeed warning.  I have read that the fix is to utilize resistors, first 60, then 100, then higher ohm rated resistors in the pickup from the alternator.  Researching resistors at Radio Shack, they come in different wattages.  Which do I use?
Jeff Davidson

 

[jakej]

Jeff

100k oHm should work fine - 1/4 or 1/2 watt is ok too.

Jake J

 

[jdavidso]

Thank you. Now I know what to buy. I'll give adding the resistor a try. This is a little distracting on the base leg of the pattern.
Jeff

 

[jdavidso]

I installed the resistor today (Sunday 5/1/11).  I did my best to recreate the problem and was unable.  Thank you for the help!
Jeff

 

[jakej]

Thanks for posting the result - good to know our advice helped.

Jake J

 

[RV10Rob]

OK, I'm giving up. I got a 100K pot in series with a 30K resistor (to give me 30-130k, adjustable in flight). Nothing works in all RPM ranges.

Do you know whether a sensor like this would work?
http://97.74.21.14/webstore/category-173/rpm-sensor-slick-red-af-2500-af-3400-af-3500-af-4500-.html

Thanks...

-Rob

 

[dynonsupport]

I don't know what that sensor's output is, but we do know that the tach generator (you don't need the tach, only the transducer) for the vans tachometers works:  

 

[jakej]

OK, I'm giving up.  I got a 100K pot in series with a 30K resistor (to give me 30-130k, adjustable in flight).  Nothing works in all RPM ranges.  

Do you know whether a sensor like this would work?
http://97.74.21.14/webstore/category-173/rpm-sensor-slick-red-af-2500-af-3400-af-3500-af-4500-.html

Thanks...

-Rob

Rob

I don't think you need do anything different from adding a resistor to the P lead (I do this at the switch end) as per the Dynon manual.  I've connected Dynons to many IO-540's without an issue so my guess is there is something else in the mix here.  Do you have the shielded wires going directly to an ACS switch ?

Jake J

 

[RV10Rob]

Thanks, guys... Jake, I have the shielded p-lead going to a toggle switch, and I have the line to the D120 (with the resistor) coming off the switch. The p-lead shield is grounded on the mag case only, and the ground on the mag switch goes to the common airframe ground. Is that how you do it? I've heard some people have used the shield as the actual ground path for the mag switch.

What's a typical resistance value that's worked for you?

-Rob