Shop OBEX P1 Docs P2 Docs Learn Events
Protoboard Switch Current Rating? — Parallax Forums

Protoboard Switch Current Rating?

DavidGregDavidGreg Posts: 38
edited 2008-07-08 01:07 in Propeller 1
I'd like to use the protoboard's servo headers to power some rather hefty servos. Can someone from parallax comment on the current rating of the switch on the protoboard? Or alternately, give a part no. and I'll go look it up. This might be a good addition to the protoboard sheet?

-David

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2008-07-07 13:42
    The switch is not the limiting element. You really have to look at the whole picture.

    The Protoboard's servo power circuitry includes a series inductor that's a surface mount 1uH device. I believe its rating is under 1A which would be well below any switch rating.

    You're correct that it would be useful to mention this rating on the Protoboard schematic or documentation.
  • Paul BakerPaul Baker Posts: 6,351
    edited 2008-07-07 17:12
    The maximum current rating for the inductor is 1.44A, so whatever you are supplying must come in under that. You can replace the inductor with a patch wire, but the board will be more susceptable to brown outs (ie resetting) from large current swings. Another thing to note is that the 5V regulator is rated for 1.5A, and the 3.3V supply to the Propeller takes it's power from the 5V regulator as well. So if you need to supply more than that you need to provide your own supply for the servos (be sure to connect the ground of both supplies together).

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Paul Baker
    Propeller Applications Engineer

    Parallax, Inc.
  • DavidGregDavidGreg Posts: 38
    edited 2008-07-07 17:58
    I need to run servos (2 of them, at least) that have stall current draws of ~4 amps. It sounds as though I might need to move the power lines for the servos off board in some way.

    What I was hoping to do was use the jumper to connect Vservo directly to VinSW. Vin would be connected to a battery, not to a wall wart. The inductor looks like it is only in line with the Vservo supply, so as long as my battery can supply enough current, I shouldn't have a problem with brownouts on the 5V and 3.3V rails.

    Which brings me back to my original question - can the switch (and the board etc.) handle 10 amps? Or should I plan on running power separately to the servos.

    -David
  • Mike GreenMike Green Posts: 23,101
    edited 2008-07-07 18:07
    It doesn't really matter in one way. It's generally a bad idea to run that kind of current through an area associated with low voltage logic. You can get ground loops and induce noise into the logic power supply, etc. I don't think the power plug is rated for 10A either. Run the power separately to the servos. If you want to switch it all together, use a relay to switch the servo's supply.
  • DavidGregDavidGreg Posts: 38
    edited 2008-07-07 23:47
    I wasn't planning on using the existing power connector but i suppose that's moot. Its good to know that it is not accepted practice to build a board with high current running that way. I'm a gear head by education (and occupation) so board design is not in my skill set.

    A few questions -

    Is it ok to power the board and the servos with the same battery?
    If so, would you recommend some sort of daughter card which had the servo headers and battery connector?
    Conceptually, I'm having a hard time understanding "how far away" the high current lines need to be.
    I don't necessarily mind not having the servo power switched so I think I'll avoid the relay.
  • TimmooreTimmoore Posts: 1,031
    edited 2008-07-08 00:17
    I am not an expert but this is what I do for this kind of stuff. The battery is connected to a fusebox via a switch (or circuit breaker), most high power stuff goes through its own fuse. The fusebox I use has a grounding connector so I connect the battery -ve to it and each ground for high power stuff is connected directly to it, i.e. it is a big star from here to every ground. The prop is on its own fuse (small 1/2A). Anything that isn't run directly off the battery has its own power regular close to it/on the board. In your case if the servos can't be run off the battery (and dont forget batteries when newly charged are over the voltage specified), then give them their own power supply (this helps with noise problems if you are using 1 battery). If you have problems with power supplies for that kind of current, look at car power supplies for PCs. Just make sure all grounds star back to one place - in my case the fusebox. And if you are using 10A definitely have a fuse in the system.
  • Mike GreenMike Green Posts: 23,101
    edited 2008-07-08 01:07
    1) It's ok to power the servos and the board from the same battery. Do add some filtering at the board. A choke (1uH) and large capacitor (1000uF) would be nice, much like the existing filter for the servo connectors.

    2) A daughter card is a good idea. If you're doing your own PCB, you can make the traces wide enough for the currents involved. I don't remember the recommended widths. They depend on the copper thickness. Your PCB supplier should have recommendations.

    3) I don't know "how far away". I'd be happy with 1/4 inch or so, further apart if they run together for a couple of inches.
Sign In or Register to comment.