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Servo voltage regulator

Discussion about Tx, Rx, Servo's, Batteries, Chargers, and all the other things we like to talk about..
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FrankS
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Joined: 18 Mar 2015, 14:29
Location: Gloucestershire

Servo voltage regulator

Post by FrankS »

Just read the thread on diodes, very useful. Another question for the electrical experts, friend is building a model using a model using HV servos with 2s Lipos, we'd like to use a couple of regular servos for ancillary functions, I was just going to put a small UBEC in the feed to each low voltage servo. Is there a voltage regulator we could buy to do the job instead that would drop the voltage to between 5 and 6v.
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Peter Balcombe
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Re: Servo voltage regulator

Post by Peter Balcombe »

Frank,
What you need for a DIY solution is a Low Dropout Voltage (LDO) regulator, either fixed voltage 5V or adjustable using 2 resistors.
Note that you need to use a LDO regulator to ensure the volt drop is only about 1.5v, otherwise normal regulators will drop more like 3V. Thus with a 2S LiPo pack, you have plenty of headroom for a 5V LDO regulator.

If you google 5V 3A LDO regulator (or 5A if you need more), it will throw up some alternatives like LD1085V50 for a fixed 5v output 3A device. The LM1085 series is the equivalent from another manufacturer.
The LM1084 is the 5A version.
Check the datasheet for connections & specs. but these devices are all 3 pin (input, 0V & output) TO220 type package & just need a small electrolytic capacitor on the input & output for stability. Note that as they are linear regulators, could get quite warm (Multiply input-output voltage difference by average current to get power dissipated).
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mjcp
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Re: Servo voltage regulator

Post by mjcp »

m̶j̶c̶p̶ Marc

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Some (now) pristine models that are un-flown for a year.
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Peter Balcombe
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Re: Servo voltage regulator

Post by Peter Balcombe »

Frank,
Just thought it worth mentioning that if you are putting a regulator between receiver power & each servo, then you probably also need to limit the servo signal to no higher than one diode drop (0.7v) above the servo supply voltage. This could be either a potential divider, or a series resistor in the signal line (to protect the receiver output) & a diode between servo signal connection & it’s supply. The latter stops the servo signal going more than a diode drop voltage above the servo supply as commonly used to limit input signal levels.
Barry_Cole

Re: Servo voltage regulator

Post by Barry_Cole »

Buy a couple more HV servos, much easier.....

8-) 8-) 8-) 8-)

BC
FrankS
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Re: Servo voltage regulator

Post by FrankS »

It would be, but we want to use a sail winch servo and i don't think they come in HV versions. I should have said regular voltage servos not just regular servos in my first post.

Multiplex do these https://www.modelmaniacs.co.uk/product/ ... regulator/ but I wondered if it was possible to easily make one up.
FrankS
Posts: 275
Joined: 18 Mar 2015, 14:29
Location: Gloucestershire

Re: Servo voltage regulator

Post by FrankS »

Peter Balcombe wrote:
> Frank,
> What you need for a DIY solution is a Low Dropout Voltage (LDO) regulator,
> either fixed voltage 5V or adjustable using 2 resistors.
> Note that you need to use a LDO regulator to ensure the volt drop is only
> about 1.5v, otherwise normal regulators will drop more like 3V. Thus with a
> 2S LiPo pack, you have plenty of headroom for a 5V LDO regulator.
>
> If you google 5V 3A LDO regulator (or 5A if you need more), it will throw
> up some alternatives like LD1085V50 for a fixed 5v output 3A device. The
> LM1085 series is the equivalent from another manufacturer.
> The LM1084 is the 5A version.
> Check the datasheet for connections & specs. but these devices are all
> 3 pin (input, 0V & output) TO220 type package & just need a small
> electrolytic capacitor on the input & output for stability. Note that
> as they are linear regulators, could get quite warm (Multiply input-output
> voltage difference by average current to get power dissipated).

Peter, Thanks the LM1085 series look the best bet, the LD1085V50 seem to need 8 volts min, OK with a fully charged Lipo but maybe not after Lipo has partially discharged. Application is to use a sail winch to open and close a canopy, so it will only be used intermittently and so heat dissipation shouldn't be any issue. I'll get a couple of LM1085s and test them with a cheap servo.
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Peter Balcombe
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Re: Servo voltage regulator

Post by Peter Balcombe »

> Peter, Thanks the LM1085 series look the best bet, the LD1085V50 seem to need 8 volts
> min, OK with a fully charged Lipo but maybe not after Lipo has partially discharged.
> Application is to use a sail winch to open and close a canopy, so it will only be
> used intermittently and so heat dissipation shouldn't be any issue. I'll get a couple
> of LM1085s and test them with a cheap servo.

Frank, where did you see that they need minimum of 8V?
Cannot see a minimum voltage on the Spec. Sheet as far as I can see. Max volts of 25v.
With 5.0v output & 1.5v Max drop-out voltage at 3A, then should work down to 6.5v.
Both the LD & LM series are the same I think.
Peter
FrankS
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Re: Servo voltage regulator

Post by FrankS »

Peter, on the product information on this page https://uk.farnell.com/stmicroelectroni ... EN-SKU-MDC it says input voltage minimum - 8v
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Peter Balcombe
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Re: Servo voltage regulator

Post by Peter Balcombe »

Frank,
I see what you mean! - the spec sheets identify a minimum input voltage which is 3v above the output voltage for each of the range of fixed voltage regulators, including 4.8v for the 1.8v regulator!!
However, this doesn’t seem to tie up with the stated max. dropout voltage of 1.5v and the standard definition of regulator drop out voltage:

“What is LDO dropout voltage?
Dropout voltage (VDROPOUT) is the input-to-output voltage difference at which the LDO is no longer able to regulate against further decreases in the input voltage. In the dropout region, the pass element acts like a resistor with a value equal to the drain-to-source on resistance (RDSON).”

Hence my belief that with a regulated voltage of 5v & a maximum dropout voltage of 1.5v @ 3A, the 5v regulator should provide a regulated output until the input drops below 6.5v. Very strange!!
If you are going to try one of the devices, it will be interesting to see what you get.
Peter
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