BabyO 168 voltage hooking up a servo w/ resistor

I will need to know the best way of hooking up a servo: pololu.com/catalog/product/501
to be controled by the babyO168. I am using a 7.2volt (@3500 mil amps) supply to the babyO. I have been advised to not use the VCC on the babyO as it would be the wrong amps for the servo. I am thinking of tapping in to the 7.2 (@3500 mil amps) volt in parrell with the babyO but i will need a resistor to cut down the amps/volts.

Can someone advise as to the specs of the resistor to be used with this servo pololu.com/catalog/product/501 and a 7.2v 3500mah source?

Please advise

Hello,

A resistor is definitely not the way to go. You should use a lower-voltage battery or get a voltage regulator to get your 5V. You can look into voltage regulators and build a circuit yourself (make sure it can handle at least half an amp per micro servo) or get commercial products, which are often called BEC (battery eliminator circuits) in the radio control world. For example, here are some units from Novak (which I have no experience with).

- Jan

Jan

My receiver (RX) has a BEC and a spare channel, purhaps i could use the rx power, but the babyO’s signal.

I’m pretty skeptical about that. A BEC claim might just mean that the receiver can get powered from a connected speed control. Does your receiver have any place you would connect a larger battery?

- Jan

Jan,

I have the Trex connected to the receiver via servo connections and the TREX’s jumpers are set to BEC. This is how the
receiver is powered, and also provides power to all other servos connected to it. The TREX is powered by a 7.2volt 3500mah battery.

dave

Dave,

You should not use the TReX to power servos! Please see the prominent warning in the jumper section of the TReX user’s guide.

- Ben

Ben

I am actually using the Trex and Trex Jr(BEC on this one) which is hooked directly to a Futaba Receiver. The Receiver controls both the TREX for Mobility and TREX JR for turret, gun evevaltion, and weapons control (1/16 TIGER I tamiya tank). I am NOT connecting servos directly to either of the TREX’s.

My idea was to connect a single servo ( with no signal connection ) to the futaba receiver, the signal wire
would be connected to the BabyO. The BabyO 168 would then control the servo postions, while the receiver would be providing the correct voltage and amps to the servo.

dave

If the TReX is powering your receiver via the BEC jumper, and you then power a servo via the receiver, you are using the TReX as the power source for your servo, and this will potentially break your TReX. The problem doesn’t come from connecting servos “directly” to the TReX, it comes from trying to pull large current from the TReX’s 5V power bus (as a servo will do if it’s connected to a receiver that is powered by the TReX). Am I misunderstanding what you’re trying to do?

- Ben

Ben

Yes this is exactly what I had in mind, you are understanding correctly.

So this means I would have to disable the BEC on the TREX and power my Receiver via a separate battery source in order to power the servo from the RX.

I am using 2 - 7.2 volt batteries 3500mah each. one is connected to TREX the other to TREX JR. I suppose the other method is I could tap in to the source parallel. Jan said if I ran parallel to the servo & TREX I would need a BEC to regulate the voltage and amps to the servo properly. do you know Where could I find such a circuit?

Did you look at the products Jan linked you to? That would be one place. Alternately, you could use a lower-voltage battery to power your servo, or you could build your own regulator circuit. Regulators are fairly common devices in electronics, so it’s not hard to find some examples if you google around. As Jan said, just make sure you use a regulator that can provide enough current for a servo (say 0.5A per micro servo).

- Ben

Ben

Ok , many thanks

Ben

I am using the mico GWS servo, will this BEC suffice?
r2hobbies.com/proddetail.php … ps81706_ub

Ben

If I were to disable all BEC on the TREXes by removing the BEC jumper, then connect a separate battery to the receiver, would I then be able to safely use my receiver as a power source for servos?

dave

Yes. It looks like the BEC you linked will be sufficient and can be powered from one of your 7.2V batteries.

- Ben