RADIO CONTROL ELECTRICS
Bob Kopski, 25 West End Dr., Lansdale PA 19446
This column follows up on some past topics and related reader inputs, shares some hints for using Deans connectors, and concludes with a Li-Poly ponderance.
ACE Pro 810 / RCD Platinum AM receivers
In the April 2004 column I shared information about a manufacturing/assembly problem affecting older ACE Pro 810 and RCD Platinum AM receivers. I described how a particular tantalum capacitor was installed backward (reversed polarity) in four of my receivers. After a long time (roughly 10 years), one of these capacitors failed, resulting in a crash. I invited anyone who was interested to inquire about the details and how to fix the problem.
I received a very large response — as I write this (at the beginning of October) I have fielded 87 reader letters on the subject. Many readers reported having one or more of these receivers in use or on the shelf. Most indicated they were still working okay, but a few reported receiver failures. All had an interest in correcting the situation before a failure occurred or in fixing a failed receiver.
Everyone who inquired received descriptive information and a photo printout of the problematic part/location and how to deal with it, and some were kind enough to follow up. Although several reported that their receivers were built correctly, most learned they had bad assemblies. Some had both good and bad assemblies, and several had actual failed receivers. A few described recovering dead receivers with the fix I suggested; they "got back" a "lost" receiver.
If you missed the April column, you can still get the info. Send me an SASE and I'll share the details. You may rescue a failed receiver, pre-empt a future crash, or simply find out that yours is one of the properly built receivers.
Dump'r and the AstroFlight Whattmeter — measuring pack capacity
In the October 2003 issue I featured an easily constructed, versatile, and safe discharger for Ni-Cd and NiMH battery packs called the Dump'r. It has proven popular with readers. Three readers independently described the same clever utilization of the Dump'r that I had not thought of: connect an AstroFlight Whattmeter between the pack and the Dump'r to obtain pack capacity in mAh while dumping it. I tried this and it does work.
A few notes and considerations:
- What is a Whattmeter?
It is a compact test device that measures and displays voltage, current, power (watts), and charge (mAh). It’s useful for measuring watts to the motor, current changes with propeller swaps, battery voltage, etc.
- Dump'r discharge rate and Whattmeter accuracy:
The Dump'r discharges packs at an accurate (better than 5%) 0.5 amp rate. That current is low relative to the Whattmeter’s normal amps scale, so the 0.5 A reads in the Whattmeter’s decimal place and may display with a one- or two-digit error (for example 0.4 or 0.6 A). The real current remains the Dump'r’s 0.5 A ±5%. Similarly, the Whattmeter’s reported power and mAh can be slightly off because of the low current.
- Test I ran: CBA vs Whattmeter
To check this I ran a test with my West Mountain Radio Computerized Battery Analyzer (CBA) as a substitute Dump'r, a Whattmeter, and a seven-cell pack charged to an arbitrary level. I connected the Whattmeter to the pack, then the CBA to the Whattmeter, and ran a discharge at 0.5 A with the CBA set to stop at 0.9 V per cell (6.3 V for the pack). The test took nearly 50 minutes and stopped at 6.3 V. The CBA reported 0.41 Ah and the Whattmeter indicated 0.384 Ah. That’s good agreement for this use; run-to-run repeatability is what really matters.
- Important caveat: Whattmeter draws current
The Whattmeter itself takes some small current (several milliamps) from the pack. If left connected after the Dump'r shuts down, the Whattmeter will eventually drain the pack further, and the Whattmeter will even shut down — losing the acquired data. The time for this depends on cell count, capacity, and pack condition, and can be as short as several minutes. Don’t leave it unattended after the Dump'r stops; check in often enough to capture the results.
Remember: the Dump'r is for Ni-Cd and NiMH packs — not Li-Poly. Thanks to the readers who suggested this technique.
Deans connector modifications
Photos this month (described here) illustrate unconventional applications and modifications to Deans connectors that I’ve found useful.
- Four-pin to three-pin for brushless motor/ESC:
I cut the fourth (widest-spaced) pins off a four-pin Deans connector, leaving three equally spaced male pins and female sockets. This effectively makes a three-pin set with equidistant pins (not the same as a factory three-pin Deans). I used that modified connector to mate the three brushless motor/ESC wires. The neat thing is the connector pair can be rotated 180° to reverse motor direction — hence the need for the three equidistant pins. The Deans connector body can be cut with a Zona saw; with care this yields a neat result. I used this in an AXI 2212/26 / Castle Creations Phoenix-25 hookup; peak system current was ~13 A. I guess it would work well upward of 20 A, but I haven’t tried that.
- Deans Ultra: adding grips for easy separation
If you’ve tried to separate a tightly mated Ultra pair, you know it can be hard and often requires tugging the wires. I’ve found a way to improve this by adding “grips”:
- Hot-melt glue short pieces of 1/16-inch-diameter plastic tube or rod to the connector sides just inside the wire end, one side at a time.
- Trim the glued pieces at an angle so there is no abrupt protrusion at the connector edges.
- Add more hot-melt glue over the connector body and over the forward end of the plastic pieces. Let cool.
- Slide 1/2-inch heat-shrink tubing over all and use a covering gun to shrink it tight. The heat briefly reflows the hot-melt glue underneath and secures everything.
The result is two opposing “bumps” on the Deans Ultra body that permit easy finger grip and separation of a mated pair. I’ve used Goldenrod wire pushrod guide tubing and Plastruct rod for the bumps; other materials will likely work as well.
Li-Poly ponderance
Li-Poly batteries are widespread and bring a new level of performance to electric modeling. I have many in use, but there’s one application aspect I’ve been pondering.
Common advice is to avoid “shorting” the outputs of cells or packs; the suggestion is that very bad things could result. I don’t know how severe the consequences of a momentary short actually are, and I’ve been careful to avoid risk. But consider this:
In the August 2004 column I described arming switches with brushless controllers and noted that these ESCs typically have large-value capacitors on the power wire inputs. Those capacitors represent a momentary short when battery voltage is applied. I described how sparking and arcing occurring on switch closure can destroy the switch. The same sparking and arcing occurs when using connectors as a “switch.”
When I connect a Li-Poly pack to a controller via connectors, I see a clear, crisp spark momentarily. No matter how you look at it, that is a momentary short.
My question is: how short can a short be without being a short? In other words, how long (timewise) can a short last before the advisory about not shorting Li-Poly packs is violated? And does a recurring series of very short shorts (every time I power up) accumulate an effect — i.e., do they eventually become collectively long enough to cause the feared damage?
If anyone out there knows, I’d like to hear. Regardless, I’m cautiously enjoying the benefits of Li-Poly technology — always a bit nervous and hoping the fun never turns otherwise. Take care, everyone.
Thus ends the first column of the new year. Have a great Electric 2005, everyone. Please enclose an SASE with any correspondence for which you’d like a reply — everyone so doing does get one.
MA
Transcribed from original scans by AI. Minor OCR errors may remain.
