Comments by Jan

  • Understanding battery capacity: Ah is not A

    Understanding battery capacity: Ah is not A

    - 14 April 2014


    I don't know what that 12A/5h notation indicates. My first guess is that it's a typo or other mistake; my second one is that it's 12Ah at a five hour discharge rate. If the batteries weigh about the same and have similar dimensions, their capacities will be similar.

    - Jan

  • Understanding battery capacity: Ah is not A

    Understanding battery capacity: Ah is not A

    - 12 March 2014

    "How many mAh a battery will last" is not a good question formulation because it sounds like you want to know how long a battery will last just based on its capacity. It's like asking how many days until you need to refuel your car that has a 12-gallon gas tank without saying anything about how much you drive per day.

    As I said in the 9V alkaline battery example, you're basically going to get about the same amount of energy per weight for any given battery chemistry, so since a 9V weighs about the same as a AA, it's going to have about the same amount of energy, and since energy is measured in Wh, which is Ah times V, a 9V battery will have about six times lower mAh rating than a 1.5V battery of the same weight. If you happen to know your application for the 9V draws the same current as a specific application for AA or AAA, then yeah, it will not last as long, but that kind of situation is rare since there aren't many products that can run from either 9V or 1.5V.

    - Jan

  • Introduction to Jan's blog

    Introduction to Jan's blog

    - 7 February 2014

    Hi, Raymond.

    I am happy to hear you have had good experiences with us. Lynxmotion has sentimental value for me since I played with some of their earliest arms and hexapods almost 20 years ago, but I have not really kept up with their more recent offerings. Is there something you see RobotShop actively doing to hurt Lynxmotion? My impression was that Lynxmotion was mostly the effort of its founder and that he retired; if that's the case, RobotShop would be saving it from disappearing completely rather than causing a problem. But I have no inside information and have not even paid attention to public information about it, so for all I know, all the original Lynxmotion folks could still be active in the company and butting heads with their new parent company.

    I would like to offer more of the servo-based parts and kits you are asking about, but there is nothing specific in the works, so it's unlikely that we would have anything new for at least the next few months. We might start with something basic like small grippers; for bigger, more complicated systems, it's not clear to me that general-purpose RC servos are the way to go. Something like the Robotis/Bioloid products, with specialized (and proprietary) servos offer a lot more for robotics. How do you think these newer systems compare to the Lynxmotion type of products?

    - Jan

  • Introduction to servos

    Introduction to servos

    - 7 February 2014


    Your question has nothing to do with servos, but briefly: cheap RC toys often have separate frequencies, like 27MHz and 49MHz, to avoid interference. More sophisticated RC systems use several dozen channels in the 72MHz and 75MHz frequencies, and you have to use different channels if you want to prevent interference between multiple systems operating at the same time and place. There are also rules about which frequencies can be used for surface models and which can be used for aircraft; these things can all vary by country. Newer 2.4GHz spread spectrum systems automatically jump around several channels, so multiple systems are not supposed to interfere with each other (you will have to do some kind of "binding" process so that a particular receiver knows which particular transmitter to listen to).

    - Jan

  • Force and torque

    Force and torque

    - 6 February 2014

    Hi, Inaam.

    The 9.81 in your calculations indicates you are getting thrown off by thinking about the force being in Newtons, but I think you are better off thinking of your 2kg as a force since stepper motor torques are commonly specified in kg-cm, anyway. So, if you have a 2.5cm radius pulley wheel directly on your motor and hang 2kg on it, you would need 5 kg-cm to hold it. Keep in mind that the holding torque that is usually specified is a stepper motor's strongest torque, so you will need a motor with a much higher holding torque if you want to be able to lift the 2kg instead of just hold it in place.

    The 0.5 factor you have in one of your alternatives makes me nervous that you are talking about some complicated system of pulleys instead of just the single wheel on the motor shaft (what I called a spool in the article). If that is your situation, you would have to say what that pulley arrangement is for anyone to be able to calculate the forces and torques involved.

    - Jan

  • Ten years in Las Vegas

    Ten years in Las Vegas

    - 31 January 2014


    You should come to the LVBots meeting next Thursday at 6:00. More info is at:

    If that works for you and you can come early, you should contact us in advance (call or email the general contact address) and someone can probably show you around.

    - Jan

  • Servo control interface in detail

    Servo control interface in detail

    - 31 January 2014


    I am happy my post was helpful to you. Unfortunately, I'm not sure where you're getting that conclusion, and most of what you are saying does not make sense. So, here are a bunch of quick facts that I hope help clear things up:

    1. Current coming from a supply should only depend on what the load (servo in this case) draws. If the power supply cannot deliver the current needed, get a better supply. Therefore we should only talk about what a servo draws, not what a supply sources.

    2. Servos do not draw constant current. You should see that from the screenshots in the post.

    3. The peak current is a function of the motor speed and the voltage applied.

    4. The average current depends on the mechanical load on the servo.

    5. There is no fundamental difference between analog and digital servos regarding the above points.

    6. The current depends on the the servo and load. For the specific cases in the article above, I say what the currents are and you can see them in the screenshots. In general, it will range from maybe 0.5 A for a small or weak servo to several amps for a high performance servo to 10 A for a really high performance servo.

    - Jan

  • Understanding battery capacity: Ah is not A

    Understanding battery capacity: Ah is not A

    - 10 January 2014

    Hi, Gerard.

    First, a little nitpick: a battery that can do 1A discharge for 50 hours does not mean it can get charged in 50 hours at 1A. But still, if you're getting a full charge at 1A in 10 hours and not doing anything special voltage-wise, you can be sure the capacity is nowhere near 50 Ah.

    Your supplier seems to acknowledge that the actual capacity is 12 Ah and gives you the basic math that a battery with 50 Ah would weigh over 10 pounds. I don't know what your actual thing weighs, but if it's a couple of pounds, then that would be another sanity check that your battery has nowhere near 50 Ah.

    The last part does start sounding like BS. Maybe he's just trying to say that the electronics is good enough to support a 50 Ah battery if you had one there. Perhaps this is some modular product where the battery part can be upgraded or retrofitted.

    - Jan

  • Understanding battery capacity: Ah is not A

    Understanding battery capacity: Ah is not A

    - 8 January 2014


    I think you should be able to figure this out if you read the post and other comments carefully (or your question is something more complicated that I am not following). I'm not clear on this audio amplifier vs. motor you're talking about, but on the most basic level, if you need to supply 22 A, a 25 Ah battery will give you about an hour tops, so you'd need at least six of those batteries for around 6 hours. If your 22 A spec is not continuous but something like just when you're accelerating or going uphill, you might get a lot longer battery life based on the actual load you put on the motor over the course of the 6 hours. In that case, it seems like you already have these parts so you could just see how long the batteries last in your typical scenario. If your two batteries last 4 hours, you'd need one more to last 6 hours, and so on.

    - Jan

  • Electrical characteristics of servos and introduction to the servo control interface

    Electrical characteristics of servos and introduction to the servo control interface

    - 26 December 2013


    Yeah, but your servos might cut out if you strain both servos at the same time. If you're also powering whatever is controlling the servos with the same source, you might have bigger problems with power dropping low enough to reset your device, in which case you should power that separately or just get a power supply that can supply more current. You should also measure your adapter output to make sure it's actually close to 5V at no load and with the servos doing something.

    - Jan

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