First attempt at a RC bot, PLEASE help! :-)

[jwatte]

Also, any clue where one might get a big red emergency shutoff button that will not melt with 30A-60A running through it?

http://www.trossenrobotics.com/store/p/6362-Huge-Red-Panic-Fire-Button.aspx

This button does not do 60A; you'll need a relay for that. I *think* it's spdt, so you can wire it to break current; use one button to energize the relay coil; let the relay latch itself; use this button to break that latch.

[Duane Degn]

The big red button jwatte linked to was the first button that came to my mind when you mentioned you wanted a big red button. I have a couple of those big red buttons and they are cool by they might be a bit too big for your application. The red dome is also prone to scratches so you'd want to be careful how you used it.

I do think it's one of those parts just crying out to be used.

Have you seen MPJA's buttons?

http://www.mpja.com/Push-Button-Switches/products/63/

I think they have some fun buttons and pretty reasonably priced.

I'm looking forward to your future progress on this robot.

[ROBOT420]

Thanks for the links and info fellas! Thant BIG red button is a bit big for the space still available in my design, but, IT IS pretty damn cool so I will use on on something at some point.
Here is a diagram of the run/charge circuit on my bot. Right now I manually disconnecting the ground wire from the battery to the motor controller to turn it off when not in use. To charge, I unplug both quick connects going from the battery's to the motor controller and plug them in to the charger's quick connects.

http://s1362.photobucket.com/user/420photo420/media/20130502_103217_zps0add7ea2.jpg.html?sort=3&o=0

Where can I find a 60A DPDT (a DPDT is what I need for this correct? )switch that I could wire all 6 leads (+&- from 24V battery pack, +&- from the 24V charger, +&- from the motor controller) to, and simply switch between "Run" and "Charge" mode? If we can figure this out it would double as an "off" switch by just switching it to charge mode weather the charger is plugged in or not. Thanks again for all the help! Steve

[jwatte]

You actually only need a SPDT, as you can keep the grounds connected at all times (charger, battery, and bot, can all share a ground.)
Breaking the ground may be advantageous in some cases (if you get ground loops, say,) and may actually be a disadvantage at other times (if you use a big fuse, you may lose that protection, for example.)
60A is a lot. You need a "contactor" instead of a "relay" at that point. However, are you really drawing 60A continuous? If the 60A number is just transient spikes, and average draw is, say, 20A, then a 30A relay will serve you just as well.

You can also build a 60A breaker using a P-channel MOSFET. It might actually be cheaper than going with contactors. Just beware to specify the P-channels based on your highest load-dump voltage -- for a 24V system, 60V minimum, and ideally higher than that. (A TVS across it might help protect it, too.)

SPST isn't so bad, though. Keeping the charging port connected to the battery at all times is convenient when you have the bot on blocks and are testing.

Here are some parts from Digi-key you might want to look at:
http://www.digikey.com/product-detail/en/1393276-7/PB1319-ND/3318138
http://www.digikey.com/product-detail/en/ADQM16024/255-2597-ND/2125665
http://www.digikey.com/product-detail/en/LEV100A5ANG/A101164-ND/2362833

Note the lack of insulation on the first (cheap) part! Also note that it's just barely within your specification. I'd go with the second.

Or, if you want MOSFETs: http://www.digikey.com/product-detail/en/IXTP96P085T/IXTP96P085T-ND/1995412
Just make sure to have a sufficient heat sink! (I might also put two in parallel, just to make sure there's sufficient overkill.)

[Duane Degn]

Those relays sure seem expensive.

I think this one would work just fine. Edit: As long as it meets your current requirements.

https://www.sparkfun.com/products/10924

Since the current rating is when used with 220VAC I believe it could take even more current at lower voltages (I'm not at all sure about this). I've used them to control kitchen ovens.

I believe the above relay has a 5V coil (which you'll likely have handy). You don't wont to power the relay directly from a microcontroller pin. You'll want to use a transistor to power the relay. Never mind. You want to control the relay with a button.

Here's a similar relay with a 12V coil.

http://www.mpja.com/12VDC-SPST-NO-30A-Omron-Relay/productinfo/19059%20RL/

I'm confused why it's rated at 30A @ 250VAC but only at 15A @ 125VAC. Does that mean it would be only good for 3A @ 25VAC? I'd think it would be the other way around with the relay being able to handle higher currents at lower voltages?

MJPA has lots of other relays as well.

http://www.mpja.com/Mechanical-Relays/products/134/

Solid state relays tend to last longer than mechanical relays in my experience. I doubt you'd need one but MPJA also has some SSR.

http://www.mpja.com/Solid-State-Relays/products/133/

Be aware with SSR, you need to make sure you get the right kind for your needs. AC and DC SSR are not interchangeable.

Edit: I had lots of bad advice in this post. Make sure and read jwatte's post below.

[jwatte]

Since the current rating is when used with 220VAC I believe it could take even more current at lower voltages (I'm not at all sure about this). I've used them to control kitchen ovens.

That is wrong, and dangerous advice! The reason the high-amperage relays are expensive, is that the contactors have to be a lot higher quality. The reason for this is Ohm's law. The voltage switched does NOT MATTER in the current rating. The reason for this is, again, derived from Ohm's law.

U = I * R
P = U * I
-> P = I * R * I
-> P = I-squared-R

So, the heat generated by the contacts is proportional to the SQUARE of the current. For a given level of heat, the resistance (electrical or thermal) has to be four times lower for twice the current. Comparing 20A relays to 60A relays is like comparing an RC car to a Tesla Model S :-)

This is why I asked whether the 60A rating was continuous, or just occasional spikes. If the occasional spikes are less than a second, and the average is well below 20A, then a 20A rated relay may do just fine, and an auto relay or other cheap sealed relay is usable.

If the current will actually be 60A for some significant amount of time, do not use a 20A relay. The contactors will weld, and you won't be able to break contact, and the next emergency you have, the robot will run away from you, cause a traffic accident, and then have its battery burst into flames, burning down the nearby daycare center. If you still do this, make sure to have a camera at the ready; perhaps the YouTube advertising gains will help defray the costs of settling the lawsuit :-)

The voltage rating has to do with the arcing that happens as the relay makes and breaks contact. A higher voltage rated relay is "better" at making/breaking contacts, and might be expected to mechanically last longer if used below its rated voltage. I don't know the exact physics of this, though, so if you're really interested, go to an Omron seminar or something ;-)

[Duane Degn]

That is wrong, and dangerous advice!

Yes, you are absolutely right. Thank you very much for catching my error. (At least I expressed some reservations about the advice.)

I was thinking about the problem wrong. I was thinking the voltage of the system somehow influenced the power in the P = I*V equation so a lower voltage system would increase the current carrying ability of the relay. But the voltage I should have used in any power calculation was the voltage drop across the relay (current times resistance of the relay) so you're right about the current being the important factor.

Thanks again for minimizing the damage my dumb advice could have caused.

[ROBOT420]

Thanks for the links Duane

J, as for the REAL Amps I am not too sure. The stock power-chair had a 30A re-settable in line fuse, so I was worried that the 25A Sabertooth motor controller might not cut it so I went to the next bigger I found and that was the 60A Sabertooth. I found out that "full throttle" on the power-chair was WAY slower (restricted for safety reasons no doubt) then what the motors are actually capable of, so I did away with the 30A fuse (bad move, I know). How can I measure peak amp draw when the only way I can get a meter on it is with it up on blocks and I am sure that the weight of the bot will increase the actual amp draw. Like I said not to sure....I suppose this is where I need to start so that I can put the fuse (or a bigger one) back in line somewhere and also so I can choose the right switch. 20ft leads on my amp meter maybe? Thanks again!

[jwatte]

How can I measure peak amp draw

Get a bench dyno? :-)

Clamp the motor output shaft with steel clamps. Get a really beefy power supply. Put the multimeter across the terminals, measuring resistance (when not powered.) Then measure voltage (when powered, and stalled.) Your current draw is resistance times voltage voltage divided by resistance.
Note that stalling the motor for any large amount of time is likely to overheat it / burn it out.

[ROBOT420]

Hello all, this is the switch I went with.
http://s1362.photobucket.com/user/420photo420/media/20130508_102303_zps71f5209d.jpg.html
http://s1362.photobucket.com/user/420photo420/media/20130508_102208_zps363a2032.jpg.html?sort=3&o=1
It was a bit pricey from ZORO.com but it is VERY high quality and HEAVY duty construction, it works great!

Here is how I wired it up so now I have "run" mode as well as "charge/off" mode.
http://s1362.photobucket.com/user/420photo420/media/20130508_102241_zps49f8d4ab.jpg.html
http://s1362.photobucket.com/user/420photo420/media/20130508_102326_zps76b50f2f.jpg.html?sort=3&o=5
I used my adjustable power supply (such as it is) and my Volt meter to put 5V on it and test it to get a feel for how it worked.
http://s1362.photobucket.com/user/420photo420/media/20130508_113243_zpsf356fd52.jpg.html?sort=3&o=0

Next I cut the whole bird nest of wires that you cann see in the "phase one" video I posted off and re-wired it all so that it is more smooth looking and easier to work on.
http://s1362.photobucket.com/user/420photo420/media/20130508_102737_zps5239506d.jpg.html?sort=3&o=11
http://s1362.photobucket.com/user/420photo420/media/20130508_102719_zps3437b044.jpg.html?sort=3&o=10
http://s1362.photobucket.com/user/420photo420/media/20130508_102418_zpsf460593c.jpg.html?sort=3&o=9
Looks much better this way.

I also got the mount for my mini water pump working from my transmitter, this will give me a "up down" control, and I will use the bot to move left and right.
http://s1362.photobucket.com/user/420photo420/media/20130508_102336_zps124b1624.jpg.html?sort=3&o=6
http://s1362.photobucket.com/user/420photo420/media/20130508_102355_zps298d0a5f.jpg.html?sort=3&o=7
http://s1362.photobucket.com/user/420photo420/media/20130508_102403_zps9fc52714.jpg.html?sort=3&o=8
I will post a video of it once it is mounted and working.

 

[ROBOT420]

Here are the 2 switches (glowing green) and 2 volt meters that I wired in.
http://s1362.photobucket.com/user/420photo420/media/20130510_105930_zps63565ccf.jpg.html?sort=3&o=0
http://s1362.photobucket.com/user/420photo420/media/20130510_105937_zps26f3f01b.jpg.html?sort=3&o=1
http://s1362.photobucket.com/user/420photo420/media/20130510_105945_zps3682544f.jpg.html?sort=3&o=2
One turns on the volt meter to the main batteries and the other turns on the headlights and taillights as well as the accessories battery volt meter.

I am now mounting the water tank (a rabbit water bottle) under neath my bot (now I need a wheelie bar to protect it from scraping when I wheelie) I am now sorting out the plumbing, and getting the water pump relay to work via my RC remote.

Any suggestions for other cool stuff that might fit on there? Thanks, Steve.

[ROBOT420]

My bot made a friend!

http://s1362.photobucket.com/user/420photo420/media/20130513_171434_zps533a5650.mp4.html

I think the next one (not even quite done with this one yet) I build will be a RC lawn mower. Yes it has been done before but I would love to cut grass from the shade!

[jlizotte]

lol. made a friend. that's the perfect caption for that video. lol.

J

[Sisco]

Does your bot have any sensors or is it just R/C?

Rick

[ROBOT420]

Does your bot have any sensors or is it just R/C?

Rick
[/quote

Just RC for now. I would like to build a quad rotor that just hovers about 4 feet above it though.... LOTS of room left on there for expansion.