Wiring a servo to make it work at 7.2V

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andrea.taras
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Wiring a servo to make it work at 7.2V

Post by andrea.taras » Sat May 05, 2018 12:57 am

Hello to all!
I've been trying to find some lightweight NiMh battery packs but with no luck so far! Therefore I thought I'd be switching to LiPos, as they're far more common and lighter than some NiMh packs I've seen around the web. The issue is that obviously my receiver can only take up to 6V, so it will need a 5V BEC, but that also means that the servo will work at just 5V, since it feeds straight off the Rx, just like the motors' ESCs. Since it's a HV servo, it'd be a shame not to use the extra volts coming from the LiPo, so here's my question: how do I wire my bot so that the receiver works at 5V but the servo keeps working at 7.2V?
Thank you in advance and sorry for the dumb question :roll:
Andrea

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DieGracefullyRobotics
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Re: Wiring a servo to make it work at 7.2V

Post by DieGracefullyRobotics » Sat May 05, 2018 6:28 am

It's not a dumb question at all, it's a very good one.

A servo has three 3 wires going to its plug (usually black white and red or brown orange and red). One is positive, one negative and the third is the signal wire (I'm never 100% sure which but someone will be able to tell you definitively). So if you power your reciever with a 5v BEC, you can plug the signal wire from the servo into it as normal for operation but wire its positive and negative wires into the full battery circuit in the same way as the ESC.
Dave
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andrea.taras
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Re: Wiring a servo to make it work at 7.2V

Post by andrea.taras » Sat May 05, 2018 7:29 am

Thank you Dave :) I somehow didn't think of doing that... I'm obviously using the Turnigy HV servo like the vast majority of us who built a flipper ant and I'm 100% sure that the yellow wire is signal, whereas brown and red are the power wires. So if I do this and, say, use a 5V 1A BEC (like this one https://hobbyking.com/en_us/hobbykingtm ... 5v-1a.html) would I be having brownout issues? My layout is fairly simple, with one HV servo for the flipper and two 75:1 micro motors driven by two 45g Servo boards, all feeding off a Lite Orange Rx receiver. Would a 1A BEC be enough, knowing that it would have to feed the ESCs and the Rx only, or should I go for something slightly bigger? Sadly I've majored in translating and interpreting, so that makes me a total ignoramus when it comes to tinkering with anything electrical. And yes, the servo boards turned into ESCs are only operative at 4.8-6V, so they'd still be feeding off the Rx, therefore passing through the BEC.

Pardon the OT, but where do you guys find THESE BATTERY PACKS? every shop I checked says they're out of stock, and I'm apparently too blind to spot them on chinese shopping sites... Thanks again
Andrea

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andrea.taras
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Re: Wiring a servo to make it work at 7.2V

Post by andrea.taras » Sat May 05, 2018 5:35 pm

Note to self: read the description of each product thoroughly before you buy it!

I didn't realize that my Rx (the OrangeRx R610V2 Lite from Hobbyking) can actually handle 2S LiPos out of the box, therefore saving me a lot of head-scratching issues. So I should be safe wiring a 2S LiPo straight into it and wire my HV Turnigy for the flipper... DOH!

Just asking out of ignorance, the output from the Rx to the servos is the same as the battery voltage right? So if I feed my Rx 7.4V, the same voltage will reach the servos and ESC, correct?
Andrea

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MarkR
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Re: Wiring a servo to make it work at 7.2V

Post by MarkR » Sun May 06, 2018 10:08 pm

Possibly the pulse levels output from the RX is different from the battery voltage if it has an internal regulator. That should not matter, servos will generally work with lower pulse levels than their battery voltage (I've tested 3v3 pulses on a 5v supplied servo, that worked fine)

Nominally the pulses should be close to the power rail voltage of whatever chip is generating them, but if the RX is a 2S LiPo tolerant device, it might contain an internal regulator chip and operate internally at 5v (or even 3v3 perhaps).

In digital logic, "high" is usually quite close to, but not actually at, the power rail voltage.
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Shakey
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Re: Wiring a servo to make it work at 7.2V

Post by Shakey » Sun May 06, 2018 10:49 pm

The positive pins of an RX's outputs/inputs are all linked together normally. So whatever is put into the RX is fed directly back out the rest. I haven't seen an RX that regulates its own servo positive outputs.

Signal lines will always be a regulated logic level (can vary between 3.3-5v).
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Re: Wiring a servo to make it work at 7.2V

Post by andrea.taras » Mon May 07, 2018 5:56 pm

Thank you guys for chiming in!
Yeah the ORX is a very cheap receiver so I had my fair share of doubts about it being able to regulate the output voltage automatically, but to sort it out I bought myself a voltage tester (always handy to have) and I was able to confirm that the voltage output is indeed the same as the input voltage. Anyway, I decided to buy some cheap HV servos to gut and use as motor drivers anyway, just to be safe! Hopefully nothing will explode or burst into flames.
I'm hoping to share my work on this forum soon :)
Andrea

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