Servo Headers and Power Supplies

FooWho

I have a question regarding the servo headers on the Board of Education that hopefully someone can help me with.· In Chapter 2 of the "Robotics with the Boe-Bot" book, it says to set the jumper for the servo headers to V_in if you are using the 6V battery pack that came with the Boe-Bot kit.· Chapter 4 of the "What is a Microcontroller?" book says either V_in or V_dd setting is acceptable if you are using the 6V battery pack.· I was looking at the documentation for the PING))) bracket kit, and it notes that you must use the V_dd setting for the servo supplied with it.· The instructions in the "What is a Microcontroller?" book and the instructions withe PING))) bracket kit add-on module appear to disagree with the instructions in the "Robotics with the Boe-Bot" book.· Could someone explain this to me?

Martin_H

The jumper controls if the middle pin of the servo headers gets the battery voltage (Vin) or the 5 volt regulated voltage (Vdd). Now it really depends if the servo can handle the full voltage or only five volts. The servo included with the kit is the Parallax Standard Servo http://www.parallax.com/tabid/768/ProductID/101/Default.aspx which claims to be able to handle 6 volts, so that shouldn't be the issue.

But in step 10 the instructions have you plug the Ping into port 15. The Ping is a TTL device and requires regulated 5 volt power. If you have the jumper set to Vin you'll damage it. My take is that you should follow the bread board instructions on page 5 for the Ping. This will allow you to keep your drive servos at Vin so they have more power, but give the Ping the Vdd it needs. I think you're safe plugging the servo into the servo header with Vin, but I haven't tried that servo with 6 volts.

I'm using 5 volts for the servo for a different reason. The battery packs I'm using either have 7.5 or 9 volts depending upon the robot and I don't want to put that much juice into a servo labeled for 6 volts.

FooWho

Ah, thank you very much!· So the Boe-Bot kit says to use V_in because the servos driving the wheels can use the whole 6V available from the battery pack.· It would probably still work correctly if connected to the V_dd jumper, though the wheels·would have slightly less power available.· The PING))) device needs precisely 5V and could potentially be damaged by the higher voltage.· That makes sense.

Martin_H

FooWho, you've got it.

Zoot

said...
though the wheels would have slightly less power available.

A larger issue with running motors off of Vdd is that the power regulator can only handle about 1amp comfortably -- so if you are running the Stamp, LEDs, peripheral devices (like '595 shift registers, Pings, IR sensors, etc) the total power requirement can add up quickly. Then add the 250ma-750ma that the servos may consume, and you will find that the regulator gets hot quickly. That isn't to say you *shouldn't* run everything off the regulator; it really depends on total load. I certainly would not use the "convenience" of the 4 servo headers as a reason to decide what will run from Vdd and what will run from Vin; rather take into account actual power requirements.

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When the going gets weird, the weird turn pro. -- HST

1uffakind.com/robots/povBitMapBuilder.php
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silverperson

I read that a solution for all this Vin vs Vdd debate is to put a 7305 +5V regulator (cheap!) in series with the BOE Bot power supply, and that way, Vin handles all the circuitry (stamp and peripherals) and you don't have to use the stamp's internal regulator (with the potential of damaging the stamp), and pin 24 (Vin) is the only one needed, not pin 21 (Vdd). any thoughts on this solution?

Zoot

Silverperson -- not sure what you mean by "in series", but yes, you can set up a 7805 or similar regulated 5v power supply for peripherals, etc., and whatever feeds into that regulator from the main supply can also go to the Stamp Vin -- in which case the Stamp only will use it's on-board 5v power regulator.

Personally, when an external power supply is necessary, I generally prefer to hook the 5v out from the external supply to the Stamp Vdd pin and leave Vin disconnected. The on-board Stamp regulator really is not very powerful, and this way the Stamp can take advantage of whatever power filtering, short-circuit, over-temp, over-voltage and over-current protections the main external supply has.

A lot of this is really dependent on what you are running -- if it's just one LED and few 3ma peripherals, the on-board regulator is probably fine. If it's a bunch of LEDs, or lots of external chips, etc., the Stamp on-board regulator is not really meant for handling those kinds of loads.

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When the going gets weird, the weird turn pro. -- HST

1uffakind.com/robots/povBitMapBuilder.php
1uffakind.com/robots/resistorLadder.php

silverperson

thank you, Zoot.
you have answered my question as to overloading the on-board regulator (I do in the future intend to add a lot of loads).
I have a question, however, as to why you would hook up the external supply to Vdd (rather than Vin)... what advantage does that provide? isn't the Vdd pin on the stamp actually an output

Zoot

It's only a 5v output if Vin is supplied with power, if there is no voltage on Vin, then the Stamp circuitry expects a 5v regulated input on Vdd. The real caveat is: one or the other but not both.

The advantage is basically cutting the rather weak on-board regulator out of the equation -- in some of my projects I have 2A or 3A main supplies which feed the Stamp and all the peripheral circuitry. It's also a way of protecting the Stamp (as I mentioned).

▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
When the going gets weird, the weird turn pro. -- HST

1uffakind.com/robots/povBitMapBuilder.php
1uffakind.com/robots/resistorLadder.php

silverperson

thanks again: a good explanation, zoot.