Not sure exactly what you are trying to achieve with this "mod" - if you are just trying to add "dead-short" drag brakes, they don't work at low speed.
I'm looking for an inexpensive alternative to buying a "crawler" ESC that has drag brake (Tekin FX-R, Rooster, Goat, etc) because there have been situations where the crawler rolled downhill where a drag brake esc would have applied a brake to prevent the wheels from rolling.
I assumed any of the above ESCs in one way or another dead short the motor to obtain this. If I am wrong, I would love to know how they do it. My stock Axial ESC does have a brake feature, but it only works when I switch from throttle to reverse, and I'd have to hold it each time. I'd want essentially a "dead-mans-brake" something that applies this same braking technique unattended.
If you are going to try this, I'd suggest using a DPDT relay to keep the power and brake circuits separate. Connect one common and it's corresponding NC terminal to the motor. Connect the other common to the ESC +ve motor output, the corresponding NO terminal to the motor +ve terminal.
This way, if the relay fails, you aren't left with a dead short on the motor at the same time as the ESC is applying power to the motor - that could kill the ESC.
I definitely want to prevent a fried ESC, because then I will be forced to just go and buy a new one if it does fry. I don't see how the above wiring would work. There are only two wires at play here; ESC (-) and ESC (+) My problem was getting the relay to activate when throttle is removed to dead short the motor, and when throttle is applied it releases the dead short to allow everything to function as normal.
To get over the "pull in" problem, you could try using a small relay connected to the ESC output and use that to pull in the main relay.
Wire the first relay's coil to the ESC output. Run a wire from the battery -ve to the first relay's common terminal , then from the NO to the second relay's coil, then to battery +ve.
Pull in is actually very important, the problem I am having with the 9V relay that I got (275-005) is that it's pull in is too high. It activates beyond the range of throttle input. I tried a 12V automotive SPST NO relay, and it's pull in was lower, somewhere near 7V (don't have an exact figure) and it would activate when throttle is applied, and drop-out when throttle is removed. The problem was it was a NO rather than a NC relay.
Throttle input seemed to be between 5V and 8.4V (with a 2S lipo) at least when the crawler would even begin to move. So I need a relay that would activate within this range.
Example; radio shack PN 275-249, this relay has a Pick-up (pull in) voltage of 9.6V and a dropout of 0.6V. So this would be too high to have pull in to energize the coil. The dropout is nice, it won't release the relay until there's practically zero V or zero throttle input.
It's hard finding out the Pull-In (pick-up) ratings of relay since this is not as important for it's normal operation.
Be aware that ESCs generally work by switching the power to the motor on and off at a high frequency, not by varying the voltage. The harder you pull the trigger, the longer the "on" period is compared to the "off" period (duty cycle). A relay might have problems coping with this.
From my initial testing it seemed that at low throttle there was about 5.2 or so volts being applied to the motor, and at WOT whatever amount of V was in the batteries, 8.4 to 8.2 to 7.9 as the battery drained. So it would seem to me that the ESC varies the Voltage to the motor to modulate speed. Am I wrong?
I also thought that drag brakes just allowed a brushless setup to simulate the natural braking of a brushed motor?
If you want more drag brakes from a brushed motor, couldn't you just fit stronger brush springs (or a motor with stronger magnets) for more mechanical resistance to rotation?
Again, I've got an Axial 55t motor, sealed case, so changing anything about the motor is not optional.
A drag brake - according to the
Novak Rooster Crawler "features"
Novak said:
High-power Drag Brakes provide monster holding power that’s especially useful when embarking on a downhill descent
I want to prevent unwanted motion on hills with a drag brake, but rather than spend about $100 or more to get a new ESC, I thought it was possible to get drag brake working for about $5 with relays.
It's strange that with as many veteran crawlers here, no one seems to understand my intentions, or what a drag brake is for in crawling.
Imagine this: Drive your crawler down a 45 degree slope, let off the throttle. What happens? If it continues rolling (like mine will) it doesn't have drag brake, if it stops, it has drag brake and will help prevent penalty points in a competition.
I had exactly this scenario. I hit a gate because the crawler kept moving on a descent, had I not hit this gate, I would have received a 0 score for the course, but because I don't have drag brake, I got a 10 for the course.
I would have thought someone else figured this out before, because there hasn't always been drake brake capable crawler escs. What did you all do in the "golden" days of crawling when it was "built not bought"?
