Thursday, March 1st, 2012 09:16 am GMT -6 Thursday, March 1st, 2012 09:16 am GMT -6Thursday, March 1st, 2012 09:16 am GMT -6
Micro Connectors: JST-SH, Molex PicoBlade, JST-ZH

Right now designing an indoor model airplane is a frustrating experience. It does not have to be that way.




Model Airplane Design

I love designing model airplanes. It is a lot of fun, and the sense of accomplishment you get when your creation takes flight is amazing. I also like designing different types of airplanes because of the new challenges that they represent.

I’m in the early planning stages of a new indoor model airplane design. I have been running numbers, sketching out wing outlines, looking at radio gear, and selecting power system components.

The further I have gotten into the project, the more frustrated I have become. This has been a big surprise for me. I honestly expected this to be an easy project. Let me explain the problem I ran into.

Much Better Than It Was

Don’t get me wrong. I know that the indoor RC airplane component choices are better today than they ever were. Just five years ago, most micro motors (less than 10 grams) cost $75 or more. Some still do. They were beautiful handmade little jewels, and just as expensive as real jewels. They were also outside my budget and outside the budget of most modelers.

Affordable Components

Today we have an abundance of relatively inexpensive micro RC components on the market. There are two gram brushless outrunners that do not cost an arm and a leg. Digital proportional servos weighing about two grams cost no more than larger servos. One gram speed controls are no problem. Similarly, there are many receivers available weighing only a couple of grams. A complete flight system can be put together that weighs only about 12 grams (0.4 oz) without breaking a sweat.

Lack of Availability

The first problem I ran into is that these micro components are out of stock a lot of the time. They just don’t have the same level of demand that bigger components have. That is understandable, but it did not make my job any easier. What good is a motor without a matching speed control?

Problematic Connectors

The real Achilles heel did not become obvious to me until I had spent many hours working on the airplane design and selecting components.

The product descriptions invariably say that the servos come with “micro connectors”. The receivers might be described as accepting “JST” plugs. Similarly for the speed controls. Looking at the product pictures, all the connectors looked about the same to me. Not knowing any better, I assumed that they were.

Boy, was I wrong! Care to guess how many different micro “JST” connectors are in common use? Four. FOUR!

Micro Connectors: JST-SH (1 mm), Molex PicoBlade (1.25 mm), JST-ZH (1.5 mm)

Connector Madness

Their pin spacing ranges from 1.0 mm to 2.5 mm. One of the four is not even a JST connector, but is instead made by Molex. Looking at a picture of one of these connectors, it is almost impossible to tell specifically which type it is. They are all tiny, white, rectangular three pin connectors. I cannot tell you which is which, and not for lack of trying.

At first I thought that different brands had standardized on different connectors, giving me hope of selecting a matching set of components if I just stuck with one brand. But I have learned that is not the case. For example, Spektrum uses different JST connector sockets in their 6300 and 6400 receivers. If you buy a servo to use with one of these receivers, better make sure it fits. The two sockets are totally incompatible with each other. Argh!

No Easy Solution

Swapping out the JST connector for a different one is far easier said than done. The wires and connectors are incredibly small and fragile. JST connectors are designed to be put on using specialized crimping tools. The least expensive crimper I could find cost $60. Using needle nose pliers is tricky. Soldering the tiny wires by hand is even trickier.

A Proposal

Frankly, I don’t care much which connector manufacturers standardize on, as long as they do. Given the current trends in miniaturization, my vote goes to the smallest of the four, the JST-SH. The pitch spacing is just 1mm, and the pins are rated to 1 amp. These are currently used on the Spektrum 6400 receivers and on most linear servos. The 1.5 mm pitch connectors have the same current rating, so there is no sensible reason not to use the smaller size.

What do you say? Is this an unreasonable request to make to the manufacturers?

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