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What is best way to step down 6 volt to 5.3 V for tail servo?

OneHoof

Senior Heliman

Cold Spring, Kentucky

I'm having a new Benzin Acrobatic built after a crash, possibly caused by NiMh transmitter pack failure.

Anyhow, I've decided to go with an SR 3500 NiCd 5 cell receiver battery.

Initially, I didn't stop to realize the 5 cell at 1.2 volts per cell would end up being 6 volts.

I have Futaba BL451 high torque servos, which can handle 6 volts fine. I also think the Futaba GY401 gyro can also handle 6 volts too.

However, the Futaba 9254 tail servo can only handle 5.3 volts max according to the manufacturer.

So, my question is do I need a voltage regulator and what kind. Since the JR R921 and the gyro and other servos are fine with the 6 volts... I feel putting a step down, either between the gyro and tail servo... or between the receiver and gyro would suffice.

What is the best way to step down the 6 volts to 5.3 or less for the tail servo... and which step down is best for the job?

Best regards,

Lone Wolf

09-25-2010 Over year old.
Justin Stuart

rrMaster

Plano, Texas

Use two 1A diodes in parallel. That way if one fails, you have another. You can buy a pack of 3 1A diodes at Radio Shack for $1. Get a servo extension. Cut the red lead and solder the diodes in parallel into the red lead with the bands on the diodes towards the servo. It will drop 6.0V to 5.3V. Easy as that.

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09-25-2010 Over year old.
TrevJonez

Key Veteran

St.George, Utah - USA

i just had to put a step down on my 450 pro tail servo.

i put a outrage 760us tail servo on there and and ran into the same issue.

myself i used an align that said 5.1 step down but i checked the voltage and it came out to 5.3 as you are saying. it came with my 600CF kit with the 3 amp bec that puts out 5.8volts so i immagine that is why it said 5.1 on the step down.

worked like charm but i would have to agree that having the redundancy of a parallel diode would be great for piece of mind.

Who cares... its just money...

09-25-2010 Over year old.
Justin Stuart

rrMaster

Plano, Texas

A diode reaches a step-down voltage asymptotically, so if you test the voltage when it is not under load, it will read a higher voltage than you might expect. Therefore if you have a 6.0V source and you use an Align diode and a cheap voltage meter, it might read 5.8V because it is not under load. But you can figure that most regular diodes (not light emitting diodes or Schottky diodes) will drop the voltage by 0.7V under a typical servo load. If you have a good multimeter that will let you test the voltage under load, the Align diode and the Radio Shack diode will both read 0.7V lower than the input voltage.

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09-26-2010 Over year old.
Zaneman007

Elite Veteran

Texas - USA

A diode will drop the voltage by .7 volts as stated above, but....

Unless you are using a regulator, the voltage will vary and...

What is the voltage of the battery under a load?

A step down is effective for a regulated power supply, only.

I'd check the battery voltage under a 1.5 amp load, if it is 6 volts or less you are probably good.

Keep in mind that the battery voltage will be less with every flight

Old Guys Rule!

09-26-2010 Over year old.
JAYHELI

Senior Heliman

Northeast Pennsylvania

i use one of these between the receiver and the gyro /tail servo
http://www.horizonhobby.com/Product...rodID=SPMVR5203

as stated above do not use a stepdown unless with a regulated input

09-26-2010 Over year old.
rexxigpilot

Elite Veteran

Florida

A step down is effective for a regulated power supply, only.
Not trying to sound like an Enron guy, but WHY can't you use a diode with an unregulated pack? I use a diode to drop voltage to the tail servo from an unregulated 2S A123 pack on many of my nitro birds. The voltage under load from the pack is about 6.6V to 6.8V. Therefore, the servo (BLS251 in my case) will see 5.9-6.1V.

When a servo is rated for 4.8V, that's a nominal value based on a 4 cell NiXX pack. Fully charged, the pack is about 5.7V. That's close enough to the stepped down voltage for me. I've never had a tail servo fail yet. However, I've never used a 9254 for the tail - only 9256, BLS251 and 8900. Is the 9254 particularly voltage sensitive?

I will allow that a 5 cell NiXX has a fully charged voltage of 7.1V (2S A123 is 7.2V at full). The nickel packs drop very quickly to about 6.5V. So, again the tail voltage will be around 5.8V after the 0.70 drop from the diode.

If the high initial voltage is a concern, you can run two diodes in series for an additional 0.70V drop. My point is there is no reason an unregulated system can't use a diode or 2.

09-26-2010 Over year old.
Jeff polisena

Elite Veteran

westpalmbeachflorida usa

I run 6v on all my tail servos and haven't had one fail yet . I feel most failures come from setup issues (tracking,too high gain,etc).
duralite makes a step down as well as align ,the duralite you can get two different step down voltages.

https://duraliteflightsystems.com/a...?product_id=120

I stole it ,flew it and gave it back ;)

09-26-2010 Over year old.
Justin Stuart

rrMaster

Plano, Texas

That Duralite stepdown in the link is nothing more than a servo extension and a diode soldered in it. $17 is too much for that.

I prefer to make my own stepdown because then I can use 2 diodes in parallel so that if one fails, the other one will continue to work. The probability that a diode fails is very small, but if it is 1%, then by combining 2 in parallel I get .001% chance of failure. Besides, using a couple of diodes in parallel takes up hardly any space at all.

You don't have to test a 1A diode under 1.5A load to figure out it's voltage. More like 0.1A would be sufficient. The voltage drop on a diode hits it's asymptote fairly fast. With a 30A diode, you might need to test it at 1.5A, but for a 1A diode, a very small load will be sufficient.

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09-26-2010 Over year old.
Jeff polisena

Elite Veteran

westpalmbeachflorida usa

I agree but for me it's easier to buy duralite

I stole it ,flew it and gave it back ;)

09-26-2010 Over year old.
SandyEM16

Senior Heliman

Enid, OK

The Align step-down is called a 5.1v bidirectional regulator and obviously uses two diodes.. What's up with that?

Logo 600SE
TDR 1 and 2

10-11-2010 Over year old.
Dr.Ben

rrProfessor

Richmond, VA, USA

The benefit of building your own stepdown is that QC the soldering job under the heatshrink.

I've seen some HIDEOUS soldering work done on at least one brand of diode stepdown, truly a crash waiting to happen.

If you buy one premade, pull the heatshrink and check the work of whomever built the thing, because they won't replace the model that just made the smoking hole in the ground because of a crappy soldering job

Ben Minor

Team Thunder Tiger Team Futaba Team Kontronik USA
Esprit Models Progressive RC

10-11-2010 Over year old.
Justin Stuart

rrMaster

Plano, Texas

The Align step-down is called a 5.1v bidirectional regulator and obviously uses two diodes.. What's up with that?
If you take it apart, it's got a diode in both directions so it will work both ways.

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10-11-2010 Over year old.
007

Senior Heliman

WHEELING WV

601 GYRO

IM RUNNING A 601 GYRO WITH A 9251 TAIL SERVO IM ALSO RUNNING AN ALIGN TWO IN ONE, I PUT TWO 1 AMP DIODS IN PARRELLEL ON THE TAIL SERVO WITH THIS BE GOOD ALSO 9252S ON HEAD BUT THEY WILL TAKE 6 VOLTS I PRESUME rON

02-04-2011 Over year old.
rexxigpilot

Elite Veteran

Florida

Building your own parallel diode step down is definitely the best way to go.

Not only do you get redundancy, but you double the current handling capacity of the device too.

02-04-2011 Over year old.
Four Stroker

Elite Veteran

Atlanta

I would use two 3A diodes in parallel like Justin Stuart. The tail servo can average 1A. Or Robbe claims the BLS254 will handle 6 volts.

02-04-2011 Over year old.
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What is best way to step down 6 volt to 5.3 V for tail servo?

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