TECHNICAL SUPPORT
Published 2026-03-09
One of the most annoying problems with aservois getting power to it. I believe many friends have encountered this: theservoeither has no power when connected to the line, or shakes non-stop, or even smokes and is scrapped. This is probably because the power supply is not handled properly, especially the critical current-limiting resistor. I don’t know how big it should be. Today we are going to talk about this topic and help you avoid these pitfalls.
Many people think that just adding a resistor will make everything fine. In fact, this is a completely wrong idea. When selecting a resistor, you must first pay attention to the peak current of theservo. Take a common 9g servo as an example. When the rotor is blocked, the current may reach several hundred milliamps or even 1 amp. If an ordinary 7805 voltage regulator chip is used, since it has an overheating protection function, if the resistor is selected too large, the voltage will be pulled down, causing the servo to stop working directly. Under normal circumstances, it is recommended that the current limiting resistor be selected between 0.1 and 1 ohm, and the power must be sufficient. It is best to use a cement resistor of more than 2 watts.
Sufficient power is crucial. Take the ordinary 7805 voltage regulator chip as an example. Since it has overheating protection, once the resistor is improperly selected, for example, if it is too large, the voltage will be pulled down and the servo will directly stop working. Like the common 9g servo, the current can reach hundreds of milliamps or even 1 amp when the rotor is blocked, so you need to consider carefully when selecting the resistor. Generally speaking, the current limiting resistor is suitable to be selected between 0.1 and 1 ohm. It is best to use a cement resistor of more than 2 watts to ensure the normal operation of the servo.
When performing actual wiring operations, the resistor needs to be connected in series between the positive terminal of the power supply and the red wire of the servo. If you are using an adjustable power supply, you can first adjust the voltage to the rated value of the servo without connecting the resistor, and then measure the maximum operating current of the servo. For example, if the measured peak current is 0.8A and you want to limit it to 0.5A, use Ohm's law to calculate. The resistance value is approximately (supply voltage minus servo operating voltage) divided by 0.5. This calculation method is simple and straightforward.
During the actual wiring process, it must be noted that the series connection position of the resistor is between the positive terminal of the power supply and the red wire of the servo. For the use of an adjustable power supply, it is a critical step to adjust the voltage to the rated value of the servo without connecting a resistor, and then to measure the maximum operating current. For example, the measured peak value is 0.8A. If you want to limit it to 0.5A, the resistance value calculated according to Ohm's law is approximately (power supply voltage minus steering gear operating voltage) divided by 0.5. This method is simple and easy to understand.
The current-limiting resistor is only the first step, and the parallel capacitor is equally critical. When the servo is started, the current surge is very large, and the power supply voltage will suddenly drop. Connecting a large capacitor in parallel behind the resistor, such as an electrolytic capacitor ranging from 470 microfarads to 1000 microfarads, can act as a buffer, like a small reservoir, instantly providing a large current to prevent the servo from twitching.
Choose a capacitor with a higher withstand voltage value, such as 16V or 25V. Don't reverse the positive and negative poles, otherwise it will explode. Some experts will add a 0.1 microfarad ceramic capacitor next to the capacitor to filter out high-frequency noise, so that the servo will run more smoothly and the positioning will be more accurate.
Many novice diagrams save trouble and directly use battery voltage to power the servo. For example, a 7.4V lithium battery is directly connected to a 5V servo. As a result, the servo becomes seriously heated and the control board is burned out. If the voltage is high, the motor inside the steering gear will rotate too fast, the potentiometer will also be easily worn, and its lifespan will be greatly shortened. What's even more dangerous is that if the servo gets stuck, the current will be very large and directly burn through the driver chip.
️ The correct approach is: use a voltage stabilizing module to reduce the voltage. For example, use this adjustable voltage step-down module to stabilize the voltage at the rated value of the servo. Don't expect the resistor to reduce the voltage, because when the current changes, the voltage across the resistor also changes, which is not stable at all, and the steering gear will work well and badly.
The symptoms of insufficient current are typical: the servo is normal when it is unloaded, but cannot move when it is loaded, or it crawls slowly. Because when the current is insufficient, the voltage will be pulled down, the control chip will be reset, and the servo will vibrate randomly. What's more serious is that if the power supply is overloaded for a long time, it will overheat and even catch fire.
1. Calculate the total current. For example, if you want to bring two servos, each with a peak value of 1A, the power supply must have at least 3A margin.
2. When choosing a power supply, look at the nominal current. It is best to buy a branded switching power supply. Do not trust those with false specifications.
3. If you have purchased a low-power power supply, you can use it in parallel, but you must pay attention to current sharing, or simply replace it with a larger one.
Some tutorials say that signal line resistance prevents interference, but it actually depends on the situation. If your servo is far away from the control board, such as more than 30 cm, the signal line is easily interfered by the motor or power line, causing jitter. At this time, you can string a 100 to 300 ohm resistor at the signal input end to absorb some reflected signals.
But if it is a short distance, such as directly plugged into the development board, adding a resistor may worsen the signal waveform. A more reliable method is to use twisted pairs for signal lines and keep them away from power lines, or use shielded wires. If that doesn't work, connecting a small capacitor between the servo signal pin and ground can also filter out burrs.
The servo keeps shaking, probably because the power supply ripple is too large. Use an oscilloscope to look at the power pins. If the waveform looks like a sawtooth, then the filtering is not done well. First check the matching of the current-limiting resistor and capacitor. If the resistor is too large, it will easily cause voltage fluctuations. If the resistor is too small, the current limit will be insufficient. It is recommended that the resistor be about 0.5 ohms and the capacitor be increased to 2200 microfarads.
Another trick is to supply power to the servo and the microcontroller separately. The steering gear uses a power supply, and the microcontroller uses a stabilized power supply. The two power supplies can share the same ground. In this way, no matter how hard the servo is, it will not affect the voltage of the controller, and the jitter will disappear naturally. If you are making a robot, this method is particularly practical and the stability is significantly improved.
What's the weirdest power failure you've ever encountered while debugging a servo? Welcome to share your experience in the comment area, like it and save it so you can look it up next time you encounter problems. If you find it useful, don’t forget to forward it to your friends who play with servos, so that everyone can avoid detours together!
Update Time：2026-03-09
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