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Can a servo be safely connected to Vin on HW board using recommended DC adapter? — Parallax Forums

Can a servo be safely connected to Vin on HW board using recommended DC adapter?

Kevin WoodKevin Wood Posts: 1,266
edited 2005-10-31 19:51 in BASIC Stamp
Using the recommended 6Vdc/800ma adapter from Radio Shack connected to a HomeWork board, is it possible to safely connect the WAM kit servo to Vin as shown in Fig. 4-10 of WAM v2.2? If not, can it be connected to Vdd with the adapter.

I looked at the servo docs, WAM Appendix D, etc., and don't see why it wouldn't be safe, provided everything is properly connected.

Thanks

Comments

  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-10-29 23:06
    Kevin,

    ·· If you're using a regulated 6V supply then by all means use the Vin.· Not only will you be able to run the servo at it's rated voltage, but you'll have more current available to it.· I would not recommend running it with any load on it from the Vdd.


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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • Kevin WoodKevin Wood Posts: 1,266
    edited 2005-10-30 03:51
    Chris,

    Would the servo exceed the max current rating of the voltage regulator if connected to Vdd? Thanks.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-10-30 03:52
    Kevin,

    ·· It could if it were stalled...Besides, 6V is the norm for servos.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • Clock LoopClock Loop Posts: 2,069
    edited 2005-10-30 04:17
    Kevin Wood said...
    Chris,

    Would the servo exceed the max current rating of the voltage regulator if connected to Vdd? Thanks.

    Most modern voltage regulators get REALLY HOT fast if they are at or near their current limitations.
    Also I recall reading that most modern regulators have thermal protection too. Too hot, it shuts down, let it cool, turn it back on. But obviously doing this continually will eventually kill the regulator.

    Do a current measurement on the servo when it stalls, then you will know if it exceeds the ma limit of your VDD regulator.
    Most 5v regulators have a ma limit of 1000ma or 1amp. (I have some at 1.5amp.)
    Personally I wouldn't run ANYTHING off my homework board(VDD) that required a high load. I use another breadboard and another power supply. (with merged grounds, VSS)

    Same thing if you were running the servo from VIN. But at least if you toast the regulator on the radioshack adapter(VIN), you can just buy a new adapter. Replacing the regulator on the homework board is more work. And if you toast the homework board regulator, who knows what kind of voltage it will send out when it is being toasted to hell.... It might take out your stamp.

    Be cautious when using VIN tho, always remember what kind of voltage is present there. The same voltage as your adapter.

    For clarification:

    VDD = HOMEWORK BOARD REGULATOR

    VIN = DIRECT FROM YOUR 6v RADIO SHACK POWER SUPPLY, SKIPPING YOUR HOMEWORK BOARDS REGULATOR. *recommended servo use*

    (if you look at your first post you mixed the two up)

    Post Edited (BPM) : 10/30/2005 4:21:55 AM GMT
  • Kevin WoodKevin Wood Posts: 1,266
    edited 2005-10-30 04:52
    Thanks for the replies. The warnings are pretty adamant about not using a power supply, only a battery, and I didn't see a reason the recommended 6Vdc/800ma supply wouldn't work.

    The reason I asked about connecting to Vdd when using the power supply is that on page 107 there is a note concerning the servo jumper for the BOE Rev C that says to use Vdd with a 9v battery or DC power supply. I figured that since you have to wire the servo header on the HW board, you should be able to connect the power to either Vin or Vdd, using Vdd if your DC supply exceeded the 6v max of the servo.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-10-30 04:55
    Kevin,

    ·· The BOE has a much beefier regulator than the HW Board.· That's one reason, and the other is that, as you pointed out, it refers to a 9V or so P/S, which will be too much for the Servos.· If you notice on the BOE-Bot the Vin can be used since you're using batteries.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • Kevin WoodKevin Wood Posts: 1,266
    edited 2005-10-30 16:16
    Chris,

    I'm rusty when it comes to electronics theory, but I'm not certain I see the difference with respect to Vin. What would be the difference if the 9Vdc comes from a battery or wall supply, or even a bench supply? Initially I assumed that the reason not to use Vdd on either board was that the servo would draw more current than the voltage regulator could handle.

    Also, could you give me the part numbers on the VRs for HW & BoE revC? I'll look up the ratings.

    Thanks.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-10-30 19:09
    Kevin,

    ·· I was not referring to Vin in general.· I was referring to the comparison you made to 9V or a Wall Adapter on the Vin of the BOE in relation to the 6V you wanted to use on the H/W Board.· You asked if you should use Vdd on your board and I said no, you should use Vin, for two reasons...One it's the correct voltage for the servo, and two because you will have more current available to the servo.· If you were using a BOE, it would depend on whether you were using that same Wall Adapter as to what you'd use.· Generally with a Wall Adapter we suggest Vdd, because most Wall Adapters are unregulated, having much higher voltages than what they are rated when they are not at their rated current load.· So you can expect a typical 9V adapter to be around 15V right off the connector.· In this case·you do not want that going into your servo.· So instead you use the BOE regulator and send it 5V @ around 1A available.

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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
  • Kevin WoodKevin Wood Posts: 1,266
    edited 2005-10-31 17:57
    Chris,

    Thanks for the help. I worked for several years in the IT industry as a technical writer, and I know the challenges of trying to make one document applicable to several different systems, in this case the BoE revs and HW board. There is always the possibility that something will get read out of context, and it is difficult to anticipate and account for this beforehand. For myselfI I have a tendency to wrestle with a problem until I understand it, so please let me ask one last question on this subject.

    The servo doc states that the maximum voltage for the servo is 6Vdc. I understand that using an unregulated power supply isn't recommended because the ps might apply more voltage to the servo than it can handle. So given that using a 9V battery with BoE and HW board (forget about adapters, battery only), and connecting the servo to Vin on either board, you should be connecting the servo directly to the 9Vdc of the battery. My understanding is that you would be exceeding the 6Vdc maximum for the servo. Why is this okay?

    Thanks.
  • Chris SavageChris Savage Parallax Engineering Posts: 14,406
    edited 2005-10-31 19:51
    Kevin,

    ·· On the standard (Unmodified) servo that we include with the H/W Board, that has been tested to work without permanently damaging the servo, although over time the life of the servo is decreased relative to the amount of voltage over the rated voltage that you apply.· Since the servo included with the H/W Board is there for learning purposes and not really included as part of a robot or other assembly, this isn't really an issue.· Whereas if you were going to include the servo in an assembly where it's decreased lifetime could reasonbly be expected to have a negative impact on using it in that manner, then another solution would be suggested.· The bottom line is that the 9V Battery is okay for use on the standard servo with the H/W Board only.· =)


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    Chris Savage
    Parallax Tech Support
    csavage@parallax.com
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