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Soldering servos to external power source

Grifflyer

WWII fanatic
#1
Hi all, I'm working on an FPV tank that uses 360 servos to turn the tracks, and I want to increase torque and speed. I was looking at the servo's spec sheet and the speed and torque of the servo increases with voltage. So instead of using power supplied from the receiver could I solder the positive and negative wires to a 5v source (yes the servos are rated to handle that) and then solder the signal wire to the receiver?

It seems like this would work, but I thought I'd ask first just to make sure.
 

DamoRC

Well-known member
Mentor
#2
Yep - that should be fine. Just make sure the ground wires from your external servo supply and receiver supply are tied together somewhere to ensure that the signal low reference is the same. It probably will work without the tie, but it's better practice to tie them

Edit: Is your receiver running on 5 volts?
 

FDS

Well-known member
#3
Why not just buy a few cheap reduction gearboxes from a robot shop? Most of those have brushed motors in that could easily be controlled by an appropriate esc and you can can get varying gear ratios as well as metal gears. The basic ones are cheaper than servos.
 

Grifflyer

WWII fanatic
#4
Why not just buy a few cheap reduction gearboxes from a robot shop? Most of those have brushed motors in that could easily be controlled by an appropriate esc and you can can get varying gear ratios as well as metal gears. The basic ones are cheaper than servos.
That might work, but there's not a lot of room and the tank is designed around the servos.
P1280247.JPG

Plus they're the cheap $2 servos anyway
 

Grifflyer

WWII fanatic
#5
Yep - that should be fine. Just make sure the ground wires from your external servo supply and receiver supply are tied together somewhere to ensure that the signal low reference is the same. It probably will work without the tie, but it's better practice to tie them

Edit: Is your receiver running on 5 volts?
Nice I'll make sure I wire them together, the receiver runs anywhere from 3 to 6 volts.
 

DamoRC

Well-known member
Mentor
#6
Nice I'll make sure I wire them together, the receiver runs anywhere from 3 to 6 volts.
Cool. The only thing I am not 100% sure about is if you run the receiver at 3V (so the signals / pulses from the receiver range from 0 to 3V) and your servo is powered with 5V, whether the 3V pulses will be sufficient to properly control the servo. I think they will but this is something you will can check. If you are running the receiver at 5V this won't be an issue.
 

Grifflyer

WWII fanatic
#8
If you can run 6v in the receiver, you get more power and speed from the servo and they should be rated up to 6
(Copied from other website) Spektrum (and probably all 2.4GHz receivers) are regulated internally to 3.3V. Because the servo outputs come from an IC powered by that regulated 3.3V their output is around 2.7-3.0V regardless of the voltage supplied to the receiver.

Is this incorrect?
 

Craftydan

Hostage Taker of Quads
Staff member
Moderator
Mentor
#9
Internal voltage and external voltage are two different things.

If you apply 5v to the external power rail on the RX input (pins sticking out the back), all of those connectors will be at 5v . . . or 6v . . . . or 3v (whatever you supply), and the RX will have a regulator tapped off of that to regulate *down* to the needed internal voltage. Unless you're poking around with a multimeter, you'll never see it, as "internal" stays inside. The servos will receive the unregulated (it is usually regulated, but not by the RX) power from the RX's external rail. Feed the RX 6v at the plug, it gets 3v inside, and the servos/lights/??? get 6v from your feed.

That being said, not all RX's can stand 6v, but many(most?) can . . . but looks like yours can. keep an eye on that in future projects/upgrades where you might be tempted to pick a different RX.
 

Grifflyer

WWII fanatic
#10
Internal voltage and external voltage are two different things.

If you apply 5v to the external power rail on the RX input (pins sticking out the back), all of those connectors will be at 5v . . . or 6v . . . . or 3v (whatever you supply), and the RX will have a regulator tapped off of that to regulate *down* to the needed internal voltage. Unless you're poking around with a multimeter, you'll never see it, as "internal" stays inside. The servos will receive the unregulated (it is usually regulated, but not by the RX) power from the RX's external rail. Feed the RX 6v at the plug, it gets 3v inside, and the servos/lights/??? get 6v from your feed.

That being said, not all RX's can stand 6v, but many(most?) can . . . but looks like yours can. keep an eye on that in future projects/upgrades where you might be tempted to pick a different RX.
Cool, I didn't know that.

Quick check to the manual says the RX has a voltage range of 3.5 to 9.6 I'm going to be running a 2s battery, so I could wire up a battery connector to the receiver and plug in a 2s battery without worry, correct?
My servos are rated for 3~7 should I worry about frying them, or since they'll never have much torque opposing their movement could I run them on 8.4?
 

DamoRC

Well-known member
Mentor
#11
Cool, I didn't know that.

Quick check to the manual says the RX has a voltage range of 3.5 to 9.6 I'm going to be running a 2s battery, so I could wire up a battery connector to the receiver and plug in a 2s battery without worry, correct?
My servos are rated for 3~7 should I worry about frying them, or since they'll never have much torque opposing their movement could I run them on 8.4?
The rating on the servo might be okay if its only referring to the motor, you could probably afford to overdrive it a little, but its possible that higher than 7 volts might fry some of the internal logic circuitry in the servo. Only way to find out is to test it I guess.

I don't disagree with anything @Craftydan said, but my concern was the output voltage on the signal pin, whether it was the"external" input voltage or the lower "internal" regulated voltage. Quick check on one of my setups (Orange receiver) using a cheap-and-cheerfui DIY scope (DSO150) confirms that the signal pin output ranges from 0-3.3V when the input voltage to the Rx was 5..5 volts
 

Grifflyer

WWII fanatic
#12
The rating on the servo might be okay if its only referring to the motor, you could probably afford to overdrive it a little, but its possible that higher than 7 volts might fry some of the internal logic circuitry in the servo. Only way to find out is to test it I guess.

I don't disagree with anything @Craftydan said, but my concern was the output voltage on the signal pin, whether it was the"external" input voltage or the lower "internal" regulated voltage. Quick check on one of my setups (Orange receiver) using a cheap-and-cheerfui DIY scope (DSO150) confirms that the signal pin output ranges from 0-3.3V when the input voltage to the Rx was 5..5 volts
By signal pin do you mean the pin where the signal wire plugs in to or the whole servo port?
 

Craftydan

Hostage Taker of Quads
Staff member
Moderator
Mentor
#17
The rating on the servo might be okay if its only referring to the motor, you could probably afford to overdrive it a little, but its possible that higher than 7 volts might fry some of the internal logic circuitry in the servo. Only way to find out is to test it I guess.

I don't disagree with anything @Craftydan said, but my concern was the output voltage on the signal pin, whether it was the"external" input voltage or the lower "internal" regulated voltage. Quick check on one of my setups (Orange receiver) using a cheap-and-cheerfui DIY scope (DSO150) confirms that the signal pin output ranges from 0-3.3V when the input voltage to the Rx was 5..5 volts
Signal outputs are typically unbuffered -- the processor spits out a high-low pattern directly to the output with nothing inbetween. Since the processor only gets 3ish volts, the signal output will be high at something slightly less than 3ish, and low at ground.

Servos and other devices usually have their high/low threshold set pretty low on the inputs to tollerate 3v logic -- most times I've seen it between 1.25-2v so anything above that is enough.

That being said, the 7v limit may not be for the "it won't fry" limit, but instead for the "it stops responding reliably" limit. Ive not seen this in rc gear before, but I have seen when a device running too hot stops listening to devices running too cold.
 

Grifflyer

WWII fanatic
#18
Signal outputs are typically unbuffered -- the processor spits out a high-low pattern directly to the output with nothing inbetween. Since the processor only gets 3ish volts, the signal output will be high at something slightly less than 3ish, and low at ground.

Servos and other devices usually have their high/low threshold set pretty low on the inputs to tollerate 3v logic -- most times I've seen it between 1.25-2v so anything above that is enough.

That being said, the 7v limit may not be for the "it won't fry" limit, but instead for the "it stops responding reliably" limit. Ive not seen this in rc gear before, but I have seen when a device running too hot stops listening to devices running too cold.
Okay so it sounds like I should stay below 7.

I drew up a quick wiring diagram, does this look good to all of you?
Diagram.jpg