Electronics > Electronics
3.7 v lipoly powering micro servo? bad idea?
dmehling:
The servos I am thinking about using require somewhere between 100-200 ma. Would a 3.7 v battery be able to supply that much amperage?
dmehling:
I have also considered using a 7.4 v lithium polymer battery, and using voltage regulators to provide the right voltage for the microcontroller and servos. Does using a voltage regulator reduce the amount of amp hours that a battery can provide? And by how much would it be reduced?
A related question is, do I need separate voltage regulators for the microcontroller and the servos?
jamesd168:
is weight of the battery a key design consideration for you? if so, adding a voltage regulator will also add the weight to your device. if weight is not an issue, then your design options are wide open, you can use a booster for one cell, or a divider with two cell battery pack, etc.
bdeuell:
--- Quote from: dmehling on February 10, 2015, 05:18:15 PM ---Having said all that, I would like to know the potential problems. What would be the harm of lower than recommended voltage? Could it damage the battery or the servo? And what is the harm of reduced torque on the servo itself? Could that also be something that could damage the internal components?
--- End quote ---
I doubt you will cause any harm to the servo electronics. The low voltage could cause them to be underpowered resulting in higher currents when compared to the same torque at rated voltage. If run for a period of time this could overheat and burn out the motor. caution that servos are often powered even when not moving this allows them to maintain their position if a load is applied. basically at the lower voltage you still do not want to exceed the motors rated operating current for a given duty cycle.
Reduced torque should actually help the life of the mechanics inside the servo.
The battery life is often dependent on the charge discharge cycles you put it through. Typically deeper discharge = reduced life, faster discharge = reduced life, higher temperature = reduced life.
--- Quote from: dmehling on February 10, 2015, 05:18:15 PM ---With regards to using a voltage booster, the batteries I want to get actually have a voltage of 4.2 v when fully charged, and they go down to 3.7 v when fully depleted. What impact would that have on the ability of the voltage booster to output adequate voltage? Would that make any difference? Does 3.7 v provide enough voltage to be boosted to 4.8 v? And is there a danger of boosting it too high?
--- End quote ---
A boost converter is designed to give a constant output voltage for a range of input voltages, the allowable input range depends on the converter and would be given in the specs/datasheet.
--- Quote from: dmehling on February 11, 2015, 06:40:01 PM ---The servos I am thinking about using require somewhere between 100-200 ma. Would a 3.7 v battery be able to supply that much amperage?
--- End quote ---
This depends on the battery specs. No battery should be discharged at a rate greater than its specified maximum and lithium batteries are particularly sensitive and at risk to blow up if discharged too quickly, over discharged, or overcharged. youtube has plenty of videos if you want to see this i would recommend avoiding it yourself. ;D Most lithium batteries you find will have a built in protection for these conditions but i would not rely on these to protect the battery, you should design your own protections for these conditions into your system.
--- Quote from: dmehling on February 12, 2015, 03:57:12 PM ---I have also considered using a 7.4 v lithium polymer battery, and using voltage regulators to provide the right voltage for the microcontroller and servos. Does using a voltage regulator reduce the amount of amp hours that a battery can provide? And by how much would it be reduced?
--- End quote ---
It does not directly reduce the capacity of the battery but a regulator/booster will waste power and reduce the overall efficiency of your system, how much depends on your regulator type/design. However battery capacity is dependent on discharge rate so drawing more current from your battery will shorten its run time.
--- Quote from: dmehling on February 12, 2015, 03:57:12 PM ---A related question is, do I need separate voltage regulators for the microcontroller and the servos?
--- End quote ---
Depends on the voltage you supply for the motors, microcontrollers are usually not very flexible on their input voltage. However its not a bad idea having a separate linear regulator for the microcontroller can eliminate a lot of problems by isolating the power.
dmehling:
I have done some research since I posted my last message and I have found some lower voltage servos that will work with 3.7v lipoly batteries. However, I'm not sure about the best way of connecting the batteries to the microcontroller and servo. The particular microcontroller I want to use is a Moteino, which has a 3.3v voltage regulator and can provide up to 250 mA of output current. The servo I intend to use can receive input of 3.3v and has an idle current of 50 mA and no load running current of 90 mA. The stall current is 500 mA. I want to know if I can connect the battery to just the microcontroller and then connect that to the servo, without the need to separately connect them to the battery. Furthermore this will ensure that the servo gets 3.3v because of the onboard regulator on the mcu. I really don't think that my servo will ever reach stall current levels, so I am thinking that I should be able to do this without any problems.
Having said that, I would like to know how much resistance would be needed to really increase the current draw on the servo. The specs say that the servo has .3 kg-cm of torque. I really have no idea how much that would be. The point of the servo is to gently touch a surface and then return to its original position. So I would think that it would barely increase the amount of current it would draw. I know this may seem like a simple matter, but I am just trying to stay on the safe side with this first project of mine
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