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HomeAircraftHelicopterRadio - Futaba FASST › Minimum voltage for bls servos?
01-12-2009 08:12 PM  9 years agoPost 1
tchavei

rrProfessor

Portugal

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Hi

I'm trying to diagnose a possible voltage problem and I would need to know what the minimum operating voltage for a BLS 251 would be. At what voltage would the movement start to be erratic or actually stop?

Thx

Tony


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"Perfection and patience usually walk side by side..."

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01-12-2009 08:38 PM  9 years agoPost 2
Four Stroker

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Atlanta

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My BLS451's work down to about 3.2 Volts. I run a standard 4.8 volt pack.

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01-12-2009 08:40 PM  9 years agoPost 3
tchavei

rrProfessor

Portugal

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Thats bad... damm. I was hoping they would go nuts at 3.8V or something like that...

Thx
Tony


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01-13-2009 01:21 AM  9 years agoPost 4
tchavei

rrProfessor

Portugal

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Are you sure its 3.2V? I made a BLS 251 glitch at 3.6V just 10 minutes ago... maybe the BLS 251 needs higher voltage... hum

Tony


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01-13-2009 03:00 AM  9 years agoPost 5
lfalsetto

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COLORADO

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I understand they have a high current draw. do a search a friend had a data logger with the bls servo's and they over taxed the battery.

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01-13-2009 03:35 PM  9 years agoPost 6
tchavei

rrProfessor

Portugal

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Not by very much. I did those tests myself and my bls251 draws around 150mAh during normal flight with 1.5A peaks under heavy piro stuff with sudden reversals.

Maybe the normal BLS work at a lower voltage but at 3.6V the BLS251 starts to glitch occasionally.

Tony


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01-13-2009 03:59 PM  9 years agoPost 7
Four Stroker

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Atlanta

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Tony;

You may not be catching the low voltage spikes below 3.6 v. Where is this 3.6 volts coming from ? Do you have a scope that you can put across the 3.6 volt supply ?

I replaced three 9250's with BLS451's. They draw about the same AVERAGE current.

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01-13-2009 04:30 PM  9 years agoPost 8
GyroFreak

rrProfessor

Orlando Florida ...28N 81W

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Just because it seems to work at some low voltage , you need to load it so it pulls lots of current. Also keep in mind that a battery at it's low voltage discharged level has a much higher internal impedance which means the servo may seem to work until a sudden heavy load is imposed on it, causing a sudden negative going spike in voltage which may cause the servo and/or the receiver to do strange things.
As fourstr said, you should use a scope and the exact configuration that the original problem occurred.

I think about the hereafter. I go somewhere to get something, then wonder what I'm here after ?

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01-13-2009 05:16 PM  9 years agoPost 9
tchavei

rrProfessor

Portugal

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No. I just needed to confirm it was in fact happening and finding a different solution. My original problem was trying to find out why my tail started to dance under heavy loads when the lipo was at 35% but not with a freshly charged battery. After several experiments I found the problem to be in a particular regulator I'm using where the Vout-Vin diferential gets too low and will drop the main voltage. This will cause the 1.4V stepdown to drop from 5V to like 3.5V... I wanted to confirm that I would infact experience wierd stuff when the voltage dropped to 3.6V. Now since I was able to measure 3.6V, I suppose that under real load (the servo working) the voltage might drop even further but what matters is that I found the culprit and I'm in the process of changing the power supply configuration

Tony


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01-13-2009 07:00 PM  9 years agoPost 10
Four Stroker

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Atlanta

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That sounds about right! You need a low dropout voltage regulator. All regulators for RC are crap. Look at them on a scope. Generally speaking, they don't have enough passive components on them to survive a spike. There might be a few exceptions from Germany.

Also, what GyroFreak said about discharged batteries.

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01-13-2009 09:05 PM  9 years agoPost 11
tchavei

rrProfessor

Portugal

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Yeah. I now have a smartfly to supply 6.52V all around (tested it with an almost dead lipo and it was still giving 6.52V even under heavy load). Its also a drop out reg with a 0.5V reduction so even if the lipo drops to say 6V (I'll have more serious problems if it hits that mark) then it will still supply the electronics with 5.5V and the tail will still get 5.2V

For the tail I'm now using an emcotec mini 5.2V regulator with low voltage drop out (this one without V fall off). I should be good to go. Its expensive but has never failed me.

Tony


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01-14-2009 05:01 AM  9 years agoPost 12
tkilwein

rrNovice

boulder co

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This is a example of what Four Stroker is talking about.
The reg picture shown is for something else but shows the point.
This one does not dip to the extent I have seen some RC BEC's do.

EDIT: I have tested more than a few different BEC's and have measured over 2+v dips for short durations. Some caused problems some did not.

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HomeAircraftHelicopterRadio - Futaba FASST › Minimum voltage for bls servos?
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