WAM2.2 Servo Warning

MikeS

In the Whats a microcontroller student guide v2.2 it states:

WARNING
Use only a 9 V battery when your Parallax Standard Servo is connected to the Board
of Education Rev B. Do not use any kind of DC supply or “battery replacer” that
plugs into an AC outlet. Improper use of these devices can cause the activity not to
work, or even permanently damage the servo.

I had been using a 6v regulated walwart power supply with the BOE USB reb B board without any problems with the servo.
In my application I want to use the servo without having to use a 9v battery.

Why the warning? What does " improper use of these devices" mean?


Thanks

Andy Lindsay (Parallax)

Yeah, I wrote that when we only had serial BOEs. Now we have USB BOEs too, and the Rev A, B, C is getting all confused with the Serial BOE revisions.

For servos, follow the instructions for the BOE Rev C. Since you will be using a wall wart, just make sure to set the jumper as shown in Figure 4-5 on page 107. This will supply the servo with 5 V (Vdd), regulated by the BOE.

MikeS

Andy,

Thanks for the fast reply.

Mike

Andy Lindsay (Parallax)

You're welcome. Here is a more complete answer:

Functionally, what you have is the USB version of the Board of Education Rev C. This being the case, follow instructions for the Board of Education Rev C. This applies for the instructions in both What's a Microcontroller and Robotics with the Boe-Bot.

Since you will be using a wall wart, just make sure to set the jumper as shown in Figure 4-5 on page 107 (of What's a Microcontroller v2.2). The jumper makes it so that the servo is supplied with 5 V (Vdd), regulated by the Board of Education. So long as your 6 VDC supply is rated for 300 mA or more and has the correct plug, it should work just fine. Appendix D in What's a Microcontroller has more details along with some examples of proper vs. "improper" use.

There are two problems I was trying to prevent with that warning:

1. Many so-called "battery replacers" really cannot replace a battery because they only have a few mA worth of oomph. Quite a few of the circuits in What's a Microcontroller will draw way more current than most battery replacers can supply. Commonly referred to as brown-out, the excessive current draw causes the battery replacer's output voltage to drop significantly. When the supply voltage to the BASIC Stamp drops below 5.5 V, its brown-out detector shuts it down until the input voltage returns to acceptable levels. The resulting program halt/restart can repeat itself intermittently, or even rapidly when the battery replacer is seriously overloaded. Either way, it inflicts major confusion on the person trying to complete the activity.
2. Some wall warts have universal connectors so that you can purchase separate plugs for your various appliances. The problem is that many of these plugs allow you to reverse the polarity. The only consequence with the BASIC Stamp and/or Board of Education Rev C is that they will not work until the polarity reversal has been corrected. If the servo is connected to a servo port on a Board of Education Rev C (Or a USB BOE Rev , and the jumper is set to Vdd, it is also protected. However, if the servo is connected to a (non USB) Board of Education Rev A or B or a BASIC Stamp HomeWork Board, the result is entirely different. The servo makes a few ticking/crackling noises, and a few seconds later, it releases a small amount of blue smoke. Then it never works again.

Sorry about the confusion. I wrote those warnings when we only had the serial Board of Education (BOE). Now we have USB BOEs. With the inconsistent revision letters, I can see some "who's on first, what's on second" confusion on the horizon.

Post Edited (Andy Lindsay) : 1/6/2005 4:30:20 AM GMT