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Better undestanding the SumoBot BS2 Vdd 5v amps... — Parallax Forums

Better undestanding the SumoBot BS2 Vdd 5v amps...

cprcrackcprcrack Posts: 12
edited 2008-07-16 20:25 in Robotics
Hello. I have recently started in this world and I really liked it. I'm spanish, so please use simple words with me. Here is my fist question:

I have a SumoBot Rev D. I have 5 IR's emitter and detectors and 1 PING sensor, as well as a piezo speaker. I use 4 1,5v batteries so I provide 6v to Vin and then the BS2 gives me 5v·in Vdd. In the BS2 documentation I can read than each I/O can sink and source about 25 mA. My question is: What is the limit of mA when conecting things to Vdd when they are not conected to any I/O pin? All the IR receptor and the PING need Vdd (+5v) and Vss (ground 0v) justo to keep them working even if you dont need to read the sensor. Is there a limit of mA in this case?

Thanks you very much.

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2008-07-15 23:31
    Vdd is the output of a 5V Low Drop-out regulator.· The maximum amount of current it can supply depends on several things.· Most such regulators are rated around 1A.· This is how much they can supply before they'll shut themselves down to protect against a short circuit.· They also have temperature cutoffs.· Anything above 5V has to be dissipated as heat (6V-5V=1V) in the case of a 6V battery, more if you run it off a 7.5V or 9V AC supply.· The regulator doesn't have much of a heatsink and, if you use a 9V AC adapter, you may have trouble drawing the full 1A load due to overheating (9V-5V=4V x 1A =4W).· If you're running off batteries, you will also be limited by the maximum discharge current of the battery and the total capacity of the battery (current x time).

    The biggest load to be concerned about is the servo motors.· These can draw up to 1A each peak current when stalled.· Average drain when moving is about 250mA each.· These usually are not powered through the regulator, but connected directly to the battery (Vin).· The Stamp, PING))), IR emitters and sensors, and general I/O usually draws a fraction of the amount needed by a single moving servo (when idle, servos don't draw much, maybe as much as an LED).




    Post Edited (Mike Green) : 7/15/2008 11:37:48 PM GMT
  • cprcrackcprcrack Posts: 12
    edited 2008-07-16 14:52
    First of all I am sorry for putting the same post in two forums, I didnt know in which of them to put it, thank you for moving it to the correct one.

    About your answer, the servo motors are connected directly to the batteries (6v) as you say, so I think I dont have to worry because you say that I could even use at about 1A for all the stuff (not including the motors) and all those sensor don't even use 0.1A, is it correct? It is surprising that I can't find anything about the 5v regulator mA rating of the BS2 in the documentation...
  • Mike GreenMike Green Posts: 23,101
    edited 2008-07-16 15:18
    I've been misleading you. I'm sorry. I was thinking about the Board of Education used with the BoeBot, not the SumoBot which is designed differently. The SumoBot manual has the schematic for its controller board towards the back and shows that the regulator used is an LM2936. This has an official current limit of 50mA, but you can draw perhaps twice that before it will shut itself down. It will also shut down if it overheats, but this won't happen with a 6V input and 50mA current draw. All of this is in the datasheet for the LM2936 that's downloadable from the manufacturer (National Semiconductor).

    The microprocessor itself will draw about 7mA when active. IR emitters will draw an average of maybe 10mA each when active. Normally, only one is active at a time because of how they're driven by the Stamp. The IR detectors draw roughly 3mA each. The PING))) will draw roughly 30mA. I don't know how much this drops when it's idle.
  • cprcrackcprcrack Posts: 12
    edited 2008-07-16 15:38
    Ok, that is more similar to what I thought. I have meassured the PING))) when idle with a tester and it is 20 mA. So I have when all the robot is powered on but idle:

    -BS2: 7 mA
    -IR detector: 3 mA x 5
    -PING))): 20 mA

    What makes a total of 42 mA.

    As IR emitters and PING))) add 10 mA when they are active, I will NEVER activate them at the same time, so I think there won't be any problem with 52 mA, don't you think so? I want to remember the 220 ohm 5 resistances I also use for the IR emitters, in case it has to be taken into account. And finally, I also have a piezo speaker, the one that comes with the SumoBot, I suppose that it's power consumption is low...
    And one thing, whats the meaning of roughly? aproximatly?

    Thanks a lot!
  • cprcrackcprcrack Posts: 12
    edited 2008-07-16 15:49
    Oh my God I forgot the QTI line sensors that are apparently consuming 50 mA when active according to the documentation!!! Is my robot going to burst? You mention that "you can draw perhaps twice that (50mA x 2 = 100mA) before it will shut itself down". Will I have any problem if I read all 2 QTI + 5 IR + 1 PING = 8 sensors every 20 ms?

    If you are wondering how I was able to use so much sensors, I use a 8:1 MULTIPLEXOR 74LS151 (I suppose less than 1 mA) for the readings.

    And another thing, is it safe all this system. In case the LM2936 cant supply so much current, will it shut down WITHOUT damaging any device?
  • allanlane5allanlane5 Posts: 3,815
    edited 2008-07-16 20:25
    Yes, if you try to pull too much current out of the LM2936, it will first over-heat and shut itself down (short its output to zero).

    If it doesn't overheat, and you hit the 'crow-bar' current, again it will short its output to zero. In either case, it should not damage any connected devices.
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