Mechanics and Construction > Mechanics and Construction

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

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jwatte:

--- Quote from: Duane Degn on May 02, 2013, 05:06:57 PM ---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.

--- End quote ---

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:

--- Quote from: jwatte on May 02, 2013, 08:48:16 PM ---That is wrong, and dangerous advice!
--- End quote ---

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:

--- Quote ---How can I measure peak amp draw
--- End quote ---

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.

 

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