'Voltage Droppers'

[sabre2]

Hi

There's an obscure corner of the RC market where 'voltage droppers' live. These seem to be typically used between a 6V BEC and a tail rotor pitch servo which inconveniently wants 4.8V (or certainly less than 6V).

They're cheap things - £5 or so, but the one I want is sold out everywhere, plus I have a servo extension lead and a variety of diodes to hand. I'm thinking that a 'power' diode will drop the voltage by 0.7V or so, bringing the voltage into the range of 4 freshly charged nicads.

Only thing is, I don't know how much current a servo is likely to draw. Average current 1A? More?

Cheers
John

[mem]

It depends on the loads and the servos but you are unlikely to draw anywhere near an amp under normal conditions.

Some example test measurements:
HS-55 draws around 250 ma when loaded at 500gr/cm
HS-81mg draws 500ma with a 1000gr/cm load
Futaba 9252 digital servos draw 500ma at 2000gr/cm loads.

Real world usage should be a lot less (except when stalled).

[sabre2]

Thanks for that. 1N4001 it is then!

John

[mem]

They should handle the current, but bear in mind if your are driving the servos from NiCads then your voltage will drop at least a volt as the cells discharge. You may want to consider using a 1A low dropout 5v regulator so you don’t lose some tail authority at the end of your flights.

[sabre2]

It's an 11.xV LiPo driving a 6V BEC, so it should remain the same throughout. It's just that (apparently) these digital servos don't like 6V.

Cheers
John

[mem]

Then you should be good to go with a couple of the diodes

[stoopy]

I need to do this - drop from 6.0v to 4.8v for my digital tail servo (heli). Does anyone have a drawing to show how to implement the diode(s)? I don't have any experience with this sorta thing!

[mem]

You will need two low power silicon diodes, something like the popular 1N4001 or similar which can handle the servo current of up to 1 amp. Connect them in series with the anodes (sometimes marked with a +) connected towards the 6 volt line. The cathodes (usually marked with a dark line around the body of the diode) connected towards the servo.

[Discharger]

So, how would 2 or 3 diodes in series with a 2 cell lipo be for a simple rx power supply near the 6v mark? Could this cause instability problems? Perhaps some capacitor bypassing across the diodes and maybe after?

[mem]

Discharger, it may be easier buying one of the low cost commercial BECs but if you want to build something yourself then you will be much better off using a 6v regulator, you can find a number of different circuits here in the DIY threads.

The biggest problem with diodes in your application is that there is no voltage regulation and the voltage to the receiver and servos will drop as the battery voltage drops. In the case of 2S LiPos, you could see a two volt variation as the battery discharges under load.

[Discharger]

Thanks for your reply mem. I tried the 2 diodes on the bench with a load dragging about 350 mA and a supply voltage of around 8v. The diodes got quite hot and not to my liking however the variable voltage with normal battery discharge should not be a problem as the voltage is still on the plenty side. I have tried a ldo 5v regulator with a diode on the common pin for 5.7v and was quite good with a small heat sink. Probably the way to go.