Servo Specifications

I work with a University Junior Design program, and many of the students I work with use your servos for their projects. But they frequently need to have more detailed information than your site gives. Specifically they need to know the stall currents of various servos to determine whether or not they are suitable for their designs. With only 4 AA alkaline batteries as a source, their robots are severely power-limited. This semester several of them are looking to use a 150mg (#1057) for a high torque application, but since they have no current specs available to them, I can only advise them to look elsewhere. It does no good to have servo capable of delivering their required torque if it requires more current than they can supply to actually produce it. Can you supply me with the stall current for these servos? And it would help in the future, and lead to more sales from my students, if you had this information for all of your servos, and not just a select few.


I have been writing about servos on my blog lately; I recommend you read over those articles to better understand why we don’t have specs for those kinds of things.

Here are some key points:

  1. The manufacturers usually do not specify currents, so even if we measured them, we cannot know how representative those results would be.

  2. If you draw a lot of current, your battery voltage will drop, and the current will go down; at lower mechanical loads, the servos should still work. In other words, the high-torque servos should still work at lower loads (see next point if you actually need the higher torque). We strongly recommend using NiMH cells, though.

  3. Servos work at basically the same voltage, so for the same servo speed, torque is going to be approximately proportional to current. If you actually need some amount of torque, the power for that has to come from somewhere, and given constant voltage, that somewhere is current.

- Jan

I appreciate the response, but maybe I should have clarified the specifics of my situation a little better. When I stated that I work with University Juniors, I meant that I work as a design mentor to Juniors in the BS Mechanical Engineering School at a major University. I personally am a PhD student at that University, I have many years experience in manufacturing, control systems, and design, and I have an associates degree in electronic engineering as well. So I thoroughly understand the technical aspects of torque, power, force, and servo function.

What I am looking for to help out my students is some basic information to perform engineering-based calculations as part of their design project. Part of their task is to mathematically verify that their design is feasible, including electrical supply and consumption. Some replies to your specific comments:

You could request/demand that information as a condition of selling their product. You could also measure them yourselves, and state the conditions and limitations as to the accuracy of your values.

I get the technical points, but 4 alkaline AA’s as a power source is a design limitation as put forth in their problem statement. If they had unlimited power they would not have as much of a challenge in their design process. Anybody can do it if it is easy.

This I get as well. But in the case of my students they don’t have unlimited current to work with. At this point in their engineering education they understand (or at least they had better understand) that the power that can be produced is limited by the power that can be supplied. In the case of this semesters project, they have to produce 1 ft-lb of torque on a bolt. The 1501mg claims to be able to deliver this directly, but with no current specifications the torque claims are completely meaningless to my students. They don’t necessarily need to know an exact number and should never depend on a manufacturers claim to be 100% accurate. But they need to know some type of ballpark reference. If the 1501mg can produce that torque at a claimed 6V and 900ma, then it is a design direction worth looking at. If it requires 3900ma, then it is clearly not going to do the job for them.

My intent was not to point you to technical aspects or to question your technical background. What you seem not to understand is the basic market realities of what these servos are and why we get the specs we get. Your suggestion of our demanding the information is akin to suggesting going to McDonald’s and demanding a steak as a condition of your dining there. If you look at something like our straight gearmotors, you’ll see that we have torques and currents listed.

As for measuring some of the characteristics ourselves, I’m not talking about the accuracy of our tests: if the manufacturer is not specifying these things, they are free to change them, and we won’t get any notice. That said, measuring some of the specs is on our to-do list, but it’s not a small undertaking, and it’s not a high priority. But, if we hear more from customers like you, the priority will go up.

On the third point, you still seem to be missing the point. For a given voltage and speed, all servos that give you a given torque will basically need the same current to get that torque; if one requires 3900 mA, you’re unlikely to find another that does it at 900 mA. Your best bet is looking for the slowest servos that can give the torque you need since the slowness is an indication of the gear ratio being high.

- Jan