More Mule then Rover?

[Tommy_T]

Cost saving tip: You can drive the coils of the 80A relays with a simple MOSFET from the Arduino!

jwatte, very good advice.

I started using EATON solid state relays 15yrs ago for work, and over the years have found them to be
very reliable, but not cheap @$36.00 for the 15A units, @$26.00 2A units.

I also like using them because they isolate the 5vdc(control voltage) from the 24vdc(load voltage) in case of failure.

[tician]

Mechanical relay ratings are mostly about thermal limits of the contacts and wiring, and not about welding itself closed. Since a relay failing closed is most likely to happen during the sparking current surge at turn on (arc welding the contacts together), it would be far better to just add a proper pre-charge circuit to eliminate that sparking. A smaller ~10A relay and a low value, multi-watt resistor (e.g. 100 Ohm / 5W) connected from the battery to the load to slowly bring the motor drivers up to ~20V before turning on the main relay/contactor will largely eliminate the risk of the main relay/contactor failing closed.

[Tommy_T]

it would be far better to just add a proper pre-charge circuit to eliminate that sparking.

tician, I agree, something like a RC snubber would add to the warm&fuzzy feeling.

[jwatte]

If the motor controller isn't running, then the current during close is negligible (unless it has really big/low-ESR capacitors.)
You can also pre-charge with a 10 kOhm resistor in parallel, but really, not closing the contact straight into load is the best solution :-)
(If you use raw relays without a controller, you don't have that opportunity.)

[Tommy_T]

baby steps.





I should have the remote controller done by time the lift is ready.

when this mod. is complete there should be 3 sabertooth 2x60 speed controllers,
and I found an extra 4 channel thermocouple board (spi) so it would be nice to
monitor the drive temperatures on the PC software control screen.

[Tommy_T]

mother nature must be checking in on this topic, 20 miles south of me got 10" of snow this week.
she dropped none on me.


I was thinking of using 2 Arduino Nano's for controlling the mule, but I got intermittent results
using the software serial on the Nano at 115200 baud. "SoftwareSerial mySerial(9, 10); // RX, TX"
not sure if it's some type of conflict(maybe spi), but because I have a few Mega clones in the parts bin
I'm going to use 1 Mega and one Nano.

the Nano is only for Drive Enable and uses no serial comm, using the Mega also gives me the option
of adding GPS and other sensors.(hours of entertainment)

I'm interested in testing GPS when the leafs are off the trees.(it's very bad when they are on the trees)

[jwatte]

SoftwareSerial is always intermittent at speeds above 9600 bps :-(
The interrupts used to drive functions such as millis() end up causing the conflict even if you don't do anything else.

For what it's worth, the Teensy 3.2 is about the same size as the Nano, except it has more capabilities than a Mega (except not as many pins -- the Teensy 3.5 gets close, though!)

[Tommy_T]

Teensy 3.2 is about the same size as the Nano

jwatte, I've never worked or played with a Teensy yet. sounds like a good
way to spend a winter.(testing Teensy's,Pi's and Edison's)


I'm also not going to be able the use the cheap plastic relays, while they are Big!(sames as sabertooth 50)
I could find room for them but, within 20min. I started smelling melting plastic, hard to get a warm&fuzzy
with that smell.

It's possible it would go way after awhile, but I'm chucking them in a bad parts storage bin.
plan C should be in the mail today.

[jwatte]

within 20min. I started smelling melting plastic

Oh, yeah, some relays have a limited duty cycle -- for starter motors that aren't intended to run all the time.
In the end, MOSFETs almost always win for longevity! And that's where the Sabretooths of the world come in.
Although, honestly, one could probably build a much cheaper simple H-bridge board with enormous power capacity ... Maybe a business opportunity? :-)

[Tommy_T]

one could probably build a much cheaper simple H-bridge board with enormous power capacity

jwatte, if plan D fails I may just do something like that.

plan D is to use the same thing I would use to enable a drive on a machine.
I can get these with 24Vdc coils.