Recently we’ve purchased a couple of these cheap chinese rotary quadrature encoders with markings LPA3806-600BM-G5-24C on them. I’ve went and looked inside of one of them as documentation available online is rather scarce.

LPA3806-600BM-G5-24C construction

There’s not much to it - just what you would expect - a quadrature disc, and a LED/phototransistor module.

LPA3806-600BM-G5-24C pcb

I’ve traced the schematics:

LPA3806-600BM-G5-24C pcb

The circuit draws around 30mA. Outputs are open collector with 20 Ohm resistors for protection. Nice to see that they have added a “fools diode” D1 that will short out your power suply if you switch GND and VCC connections.

One thing to note - the encoder has a 5V linear regulator inside, so you want to power it with at least 7V power supply. Make sure the power dissipation in the internal regulator is within reasonable limits. The heat management is not spectacular in the encoder - case is connected to the chasis ground, not circuit ground and the internal LM7805 regulator in D-PAK packege relies on a big ground plane to act as a heatsink.

Let’s say we are powering it off 12V supply. The regulator has to dissipate (12V-5V) * 0.030A = 0.210W. Considering the worst case scenarion of RthJA beeing 100°C/W it would experience 21°C rise of junction temperature above the ambient. Considering the ambient to be 85°C we get the junction temperature to be at 106°C which is a bit too close to the absolute maximum of 125°C for my taste. As it’s a chinese noname product, I’d aim for the junction temperature not higher than 100°C. YMMV

Published May 26 2014 by miceuz