Voltage Step down needed

[Fixer]

I’m going to run my heli on 6v using a 7.4 lipo and a regulator, I need to find a step down for the tail servo, does anyone stock these in the UK?
I’m told you can make one using a short servo lead and a 1n5400 silicon 3a diode but I don’t think I would trust it on a £90 servo.

[Smoothound]

Get the one that comes with the Trex 600

[Ade_Law]

be careful what reg you use there are some on the market that are dangerous... if you drop the voltage to 5.8v there will be no need to drop the voltage twice.

Ade

[Fixer]

Ade

5.8v I’m confused. I thought the 9251 only worked on 4.8v, I’ve already fried one connecting it to 6v.
I’ve just bought a regulator on eBay but you’ve put me off now,
http://cgi.ebay.co.uk/ws/eBayISAPI.d...MEWN%3AIT&rd=1
What do you think to the 6v Duralite Regulator with the step down?

Smoothound

Any more details?

[Ade_Law]

the whole voltage thing is a little confusing. there are 2 things we talk about nominal voltage (the voltage of the pack dictated by the cell count) and actual supply voltage.

when people talk about running a heli on 4.8v what they mean is a 4cell pack. however the actual voltage of the pack can be anywhere between 3.8 and 6v depending on the state of charge, condition and load on the pack.

a 6v pack is 5 cells which is capable of putting out between 5.8 and 7.5v

we generally run in the first half of the discharge or the peak currents can drop the battery voltage to dangerous levels. however most of the time a 4 cell pack will be putting out 5.3v nimhs will be a little higher.

a 6v pack will generally be up towards 7v.

its not actually 6v that kills servos its the voltage a "6v" battery is capable of putting out. even at high loads with a half discharged pack your going to be seeing over 6v.

this is why with regulator we are save to run at 5.8v yes your motors will wear out a little quicker as the extra voltage means more current which means the motors in the servos are running at higher powers. i have 9252s and 9251s that have run at that voltage for almost 3 years with many many gallons on them. As yet only 1 9252 has worn a motor out and that was very recently.

its been a long day, hope this makes sense!

Ade

[Ade_Law]

That reg is the J perkins one that retails at 6.50, I drastically reduces the range (around 10ft with aerial down) i know of one case that is actually going to court after it crashed a helicopter. this device is not fit for purpose.

its bad enough shops stock them but for a modeler to sell this to somebody for almost double retail is just insane. The worst bit is that this guy is a member of our club!!

Ade

[Smoothound]

as the extra voltage means more current which means the motors in the servos are running at higher powers.

As a rule upping the voltage reduces current. For example a T-Rex 600 on standard setup of 6s flight pack can pull a current of 80+ Amps but bung on a 10s pack and a 10s motor and you can easily use a 55A ESC

[ChrisB]

. The worst bit is that this guy is a member of our club!Ade

[Ade_Law]

that assumes same load. however the motor impedance stays the same so when the volts goes up the current goes up too.

good old ohms law...

V/R = I

V goes up, R stays the same = I goes up

Ade

[ChrisB]

Im bloody confused now

[Ade_Law]

lol

Ade

[Fixer]

Thanks for the explanation, I’m a little wiser now, as for the regulator on eBay your right it looks like a pile of crap.

http://www.jperkinsdistribution.co.u...s%20and%20accs

Guess I should have known better at that price, I’ve asked the seller for a refund or I will just put a claim in with pay pal.

[Smoothound]

I don't think you really have a case as it was an auction and you were the highest bidder...

[Smoothound]

that assumes same load. however the motor impedance stays the same so when the volts goes up the current goes up too.

good old ohms law...

V/R = I

V goes up, R stays the same = I goes up

Ade

Not having a degree in electronics I don't know all the rules all I know is what generally happens in the real world and my 4s Trex max current drops like a stone compared to running it on 3s packs as is the case with the 600 going from 6s to 10s . I don't really see how 5 servos at 4.8v or 6.0v would differ


Also as an aside I was told not to run my 9256 on the 5.8v output of the 600 regulator for love nor money!

[Ade_Law]

Neither do I!! Just an ONC... not even A level!

with a heli the load in the hover stays the same (assuming the weight doesnt go up) so the power requirement is the same.

So....

P = VI (power = Volts * Amps)

transpose to...

I = V/P

So... power stays the same Volts goes up, amps goes down

due to the weight increase of the extra cell the power will go up a little bit but not so much as to keep the amps the same or increase.

a servo is a bit different its working at 100% a lot more often so its down to impedance (resistance) of the servos motor. so...

V = IR (volts = amps * resistance)

transpose to:

I = V/R

R stays the same Volts go up, amps go up.

on the trex you can get into this situation too once the motor maxes out.

Ade