Losing grip – Watch what the receiver receives

Knowledge ought to save money, but it doesn’t always. Lacking a BEC, a receiver can be powered by a receiver pack, often 5 NiMh cells with a nominal output of 6V. I knew this, and that’s why I just smoked my Traxxas TQi receiver and need to get a new one. Very annoying.

Let’s start from the beginning. I have a TRX-4 kit on the wrenching bench (a.k.a. kitchen table), and it’s a car that between its 2-speed gearbox and lockable differentials provides too many options. This is the original premise. I cannot see when I would want to run it in high gear with all diffs locked, or in low gear with all diffs open. Two gears and front and rear diffs that can be locked or opened independently, means six different options in total. To me and my children, that’s at least three options too many. I will want to run it in low gear with both diffs locked or in high gear with all diffs open. Maybe also high gear with rear locked and front open. Nothing else.

This should not be any problem at all to accomplish, and should free up at least one channel on the receiver as well. That might come in handy. So far so good.

Anyone who has read my column for a little while, will know that I am very fond of Castle Creation’s Mamba series ESC’s, due to their auxiliary wire and its crawler/rock racer mode. This is a very trick little thing. The ESC’s auxiliary wire plugs into a free channel on the receiver, and then it’s possible to change drive modes via the transmitter. Either crawler mode, with 100% drag brake and instant forward/reverse, or rock racer mode with drag brake set to your liking and with forward-brake-reverse. It can be a bit tricky to set up, but they have a good guide to it here. The auxiliary wire becomes really nifty when coupled with a 2-speed gearbox, to have instant reverse and 100% drag brake in low gear, and forward-brake-reverse and lower drag brake setting in high gear.

That’s how I like it, and how I want to set up my TRX-4. I am therefore moving my Mamba Micro X from my SCX10III to the TRX-4, and today I connected the ESC for the first time. While Castle Creations might make excellent hardware, I find them a bit lacking in the software department. The app used to program their ESCs (unless you have a program card), Castle Link, is (still) not available for Mac. Very annoying. I have therefore bought myself a B-link to be able to connect my ESC’s and BEC’s to my smartphone. First try today, with my android phone. Did not work.

This is where my knowledge messed things up. On my first couple of tries, I just connected the battery to the ESC, and the B-link only to the ESC. When that didn’t work, I connected it all to the receiver, thinking that might somehow be a requirement for things to work. I had a feeling the ESC’s BEC was set to 7.5V, and I knew the Traxxas TQi receiver is rated for 6V. I also knew that the receiver can take a 5 cell NiMh receiver pack, with a nominal voltage of 6V. However, while a NiMh cell has a nominal voltage of 1.2V, it actually has an output of 1.4V when fully charged. It follows that a 5 cell NiMh receiver pack has an output of 7V when fully charge, hence the Traxxas TQi receiver can accept 7V, even though rated for 6V.

If it can accept 7V, surely 7.5V can’t hurt it? That’s what I thought when I plugged the ESC to the receiver, hoping that this would enable my phone to connect to the B-link. After but a few minutes, the receiver was pretty hot. Also, I still couldn’t connect my android phone (“bluetooth not available”, the app told me). Ouch! This seemed bad. I decided to put any possible bad news off a little bit, by first concentrating on the B-link. Using my daughter’s iPhone, it all worked fine. Battery to ESC, ESC to B-link, nothing else involved. I connected to the Mamba ESC in minutes, confirmed that the BEC was set to 7.5V (ouch!), and quickly changed that to 5.5V. If the TQi receiver had survived the high voltage ordeal, it should be perfectly safe from now on.

Sadly, the receiver was smoked. A solid red light, and no reaction on signals from the transmitter. Of receivers I know that all the input ports are wired in parallel and that the receiver has an internal BEC that provides its internal circuits with the correct voltage (3V, I think). That means that whatever voltage you feed into one channel, all other channels will put out. I connected a 6V rock light from club5racing to a random channel, and it shone. Diagnosis: internal circuits dead. My fancy TQi receiver now works like a 5-way servo Y-cable, but that’s it.

I knew all the data, still I smoked forty bucks of electronics. Seems to me, that if I didn’t know that a receiver pack outputs 7V fully charged, not 6V, I wouldn’t even have tried. In that case, I would have changed phones first, and would have been fine. Lesson to learn? Well, perhaps not to learn too many lessons?

To read another column, hit the link. But watch out, you might learn something.