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Need help regarding a relay switch . . . — Parallax Forums

Need help regarding a relay switch . . .

ElvisElvis Posts: 8
edited 2005-04-11 14:00 in BASIC Stamp
I do not have a strong background in electronics so I need some help. I want to use the Basic Stamp (with the BOE) to control a large servo. However, the servo requires more power than the Stamp can provide. I need the Stamp to control a switch that would be connected between the servo and auxillary battery. For example, if I wanted the servo to be on, the Stamp would turn the switch on and allow power to servo. From what I have been advised is that I need a relay switch that operates using an LED rather than a magnet and coil. If anyone has any idea about what I am talking about and give me any advice I would really appreciate it. Thank you.

Comments

  • JonbJonb Posts: 146
    edited 2005-04-09 23:11
    What you refer to is a SSR(Solid state relay). It is not at all needed in this scenario. From the servo you have 3 leads, power, signal and ground. Plug the Servo's power to the power supply needed, the grounds should be common and the signal line to the stamp. It is simply a matter of supplying the servo with a seperate power supply but the signal to control position doesnt change. No fancy relays or doodads necessary.

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    Post Edited (Jonb) : 4/9/2005 11:16:48 PM GMT
  • allanlane5allanlane5 Posts: 3,815
    edited 2005-04-09 23:16
    Agreed. Basically you want the Servo supply voltage to be connected to a separate power supply. Then the ground of that supply should be tied to the BS2 ground (Vss). Then the Signal of that servo should be tied to a BS2 I/O pin.

    This assumes the "Servo" you are talking about is a typical air-plane model control servo, which uses a pulse of 1 to 2 mSec width, repeated every 20 mSec, to control its movement.

    If you're talking about some other kind of "servo", then this may not apply. It has three wires, right?
  • ElvisElvis Posts: 8
    edited 2005-04-10 00:04
    Yes it has three leads. Thanks for the advice, that seems like a very good idea. One thing that concerns me is that the servo is very large (330 oz/in of torque made by KoPro). We are waiting for the manufacturer to send us info regarding the current that the servo requires. Will there be any danger of this method allowing the high current and votlage supplied by the external battery to damage the Stamp?
    Sorry if these questions sound stupid. Thanks again for the advice, wish I knew about this forum a week ago.
  • JonbJonb Posts: 146
    edited 2005-04-10 01:23
    Without knowing the specs on the servo it is all assumptions.

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    Post Edited (Jonb) : 4/10/2005 1:28:59 AM GMT
  • allanlane5allanlane5 Posts: 3,815
    edited 2005-04-11 14:00
    Well, a Servo is an 'active' device. This means it has its own comparator and drive electronics "inside the can", powered by the Servo power supply. The 'Signal' pin should be pretty well isolated by the vendor so that it does not damage 'upstream' control devices like the BS2.

    So the BS2 should be able to treat the Servo as just another digital output, which gets the above pulse signal to control it. All the power devices should be inside the Servo, being controlled by the Servo comparator. You only need the common ground so the Servo can properly read the BS2 control signal.

    If you are really concerned, you should be able to put a 470 ohm resistor in series in the control signal between the BS2 and the Servo to limit any 'disaster' current -- but it shouldn't be needed.
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