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Prop Pitch Controller

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nuckolls.bob(at)aeroelect
Guest

Posted: Mon Apr 19, 2010 6:52 am Post subject: Prop Pitch Controller

I've sketched a current limiter circuit and posted it

at:



http://aeroelectric.com/Pictures/Schematics/PropPitchController.pdf



The parts are available from http://digikey.com and the bill of

materials is about $10.



Under normal operations, the big N-fet transistor is turned on

hard (under 3 milliohms). Combined wit the 0.1 ohm resistor

in series will contribute less than 1/2 volt drop to normal

motor operating voltage. Gate drive comes though the LED/2.2K

resistor string from +14v.



When the motor hits the stop and stalls, current will rise

to the point where the 2N3904 gets turned on by voltage drop

across the .1 ohm resistor. The transistor pulls down on a

2.2K resistor causing the LED to light up and simultaneously

starving the N-fet for gate drive. The circuit will limit

total current to some value on the order of 6A thus preventing

a trip of the circuit breaker.



The N-fet needs to be on a heat sink suited for up to say

5 seconds of delayed turn-off after the motor hits the

stop. This needn't be much. You could build this assembly

on say 6 sq inches of .062 or thicker aluminum. It might

have a right-angle bend along one edge for a mounting

flange. The fet needs to be isolated from the heatsink

with a suitable insulating washer.

During stall-time, the N-fet is dumping about 14v x 6a

or 80 watts of heat. Okay on small heat sink for a few

seconds. There is no protective feature that would prevent the

n-fet from getting smoked if someone holds the switch

too long while the motor is jammed against the stop. With

a few more components, we can probably add time delay

feature that deprives the n-fet of gate drive a few

seconds after the stop is reached. I suspect you won't

need it.



   Bob . . .

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etienne.phillips(at)gmail
Guest

Posted: Mon Apr 19, 2010 9:47 am Post subject: Prop Pitch Controller

On 19 Apr 2010, at 4:50 PM, Robert L. Nuckolls, III wrote:



 	  Quote: 	
		   

 

 I've sketched a current limiter circuit and posted it

 at:

 

 http://aeroelectric.com/Pictures/Schematics/PropPitchController.pdf

 

 The parts are available from http://digikey.com and the bill of

 materials is about $10.


	


Hi Bob, and everyone else following this thread...



I have an even simpler solution, possibly costing significantly less, using the hotly debated poly-fuse. Simply put a 6A or 7A one in series with the motor, with an LED+resistor across it. When the fuse trips, the LED is energised... In it's normal state, all the current travels through the low impedance fuse, and 'none' through the LED+resistor path. No heatsink required! However, there is the problem of securing everything and making sure vibration doesn't age the soldered joints prematurely.



Comments?



Thanks

Etienne

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dennis.glaeser(at)hp.com
Guest

Posted: Mon Apr 19, 2010 12:26 pm Post subject: Prop Pitch Controller

Thanks Bob! 



Now a few questions:



Does the current limiting characteristic you mention literally limit the current to the pitch motor?  My guess is it does (at least somewhat), which is why the N-fet becomes a cabin heater.  

I'm thinking that the electrical paths from the switch to the N-fet, and path from the N-fet to the ALSRJ resistor to ground need to be capable of 10A, but the other wiring will see much less.



How would I 'adjust for desired limit' on the ALSRJ resistor to go up to say 8A?  How did you calculate or estimate that value?  How sensitive would you anticipate that to be? 

The reason I ask is that the load on the motor does increase as the pitch on the blades approach the high (and low) pitch limits, before the mechanical stop is reached. The mechanism is literally twisting composite blades, they don't rotate in bearings like 'regular' adjustable blades. I may have to adjust the current limit to be sure I can get the blades close enough to the pitch limits for my purposes.  My rough measurement of current (reading a clamp on meter) indicates a pretty good knee in the curve at around 8A.  6A may be close enough, only testing will tell.



Finally, how would you compare this circuit to the reed switch approach that Joe Gores proposed, from a standpoint of repeatability of sensing the target amperage, and the ability to make adjustments to get to the desired target (number of winds and wire size selection)?



As usual, I'm awed by, and grateful for, your generosity of time and talent.



Dennis



-------------------------------------------------------------------

From: Robert L. Nuckolls, III (nuckolls.bob(at)aeroelectric.com)

Date: Mon Apr 19 - 7:52 AM



I've sketched a current limiter circuit and posted it

at:



http://aeroelectric.com/Pictures/Schematics/PropPitchController.pdf

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

raymondj(at)frontiernet.n
Guest

Posted: Mon Apr 19, 2010 12:43 pm Post subject: Prop Pitch Controller

Hi Bob,



	Related question. How is the amount of heat sink area calculated? I 

assume it is based on the amount of energy converted to heat.  Where can 

I find the info to determine what size heat sink is required. Does the 

electrically insulating material also thermally insulate the component 

and affect the design of the heat sink?



I'm not in any hurry for this info. Just came to mind when I read the email.



Thanks,

Raymond Julian

Kettle River, MN.

do not archive



Robert L. Nuckolls, III wrote:

 	  Quote: 	
		   

 <nuckolls.bob(at)aeroelectric.com>

 

 I've sketched a current limiter circuit and posted it

 at:

 

 http://aeroelectric.com/Pictures/Schematics/PropPitchController.pdf

 

 The parts are available from http://digikey.com and the bill of

 materials is about $10.

 

 Under normal operations, the big N-fet transistor is turned on

 hard (under 3 milliohms). Combined wit the 0.1 ohm resistor

 in series will contribute less than 1/2 volt drop to normal

 motor operating voltage. Gate drive comes though the LED/2.2K

 resistor string from +14v.

 

 When the motor hits the stop and stalls, current will rise

 to the point where the 2N3904 gets turned on by voltage drop

 across the .1 ohm resistor. The transistor pulls down on a

 2.2K resistor causing the LED to light up and simultaneously

 starving the N-fet for gate drive. The circuit will limit

 total current to some value on the order of 6A thus preventing

 a trip of the circuit breaker.

 

 The N-fet needs to be on a heat sink suited for up to say

 5 seconds of delayed turn-off after the motor hits the

 stop. This needn't be much. You could build this assembly

 on say 6 sq inches of .062 or thicker aluminum. It might

 have a right-angle bend along one edge for a mounting

 flange. The fet needs to be isolated from the heatsink

 with a suitable insulating washer.

 

 

 During stall-time, the N-fet is dumping about 14v x 6a

 or 80 watts of heat. Okay on small heat sink for a few

 seconds. There is no protective feature that would prevent the

 n-fet from getting smoked if someone holds the switch

 too long while the motor is jammed against the stop. With

 a few more components, we can probably add time delay

 feature that deprives the n-fet of gate drive a few

 seconds after the stop is reached. I suspect you won't

 need it.

 

   Bob . . .

- The Matronics AeroElectric-List Email Forum -

Use the List Feature Navigator to browse the many List utilities available such as the Email Subscriptions page, Archive Search & Download, 7-Day Browse, Chat, FAQ, Photoshare, and much more:

http://www.matronics.com/Navigator?AeroElectric-List

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