Monday, June 22, 2009

Cautious Optimism

After talking with the power supply EEs at work, I came home and tried a few experiments to see if I could fix the alternative charge detector circuit.

First, I used an external 12V UPS battery instead of the AUX power output on the Belktronix charger to see if noise on the power supply was causing my problem. Nope: The circuit still behaved as if the opto-coupler latched closed.

Next step was to pull out my oscilloscope and start probing around the system. Fortunately, most of the battery pack and components are floating, so I can attach the ground of the scope to several different high-voltage points to serve as a reference.

Yowzers! There's a hell of a lot of noise in this system. Whenever the Belktronix charger starts up, I get 50V spikes at ~200Khz on the oscilloscope by just holding the tip of the input probe with my finger. Keep this thing away from your pacemaker! I'm guessing that these high voltage noise spikes are causing problems with the opto-coupler.

Here's a modified schematic. The EE folks at work suggested adding C4 (.01uf cap across the opto-coupler input) to help cut down on the noise. I also reduced C3 down to 10uf from 47uf. This reduction increases the maximum switching frequency for the FET (due to the hysteresis in the 555) from 1Hz up to about 5 Hz. While the FET switches at a slightly higher frequency, it's still so slow that the FET stays out of the linear switching region for long periods of time.

I plugged in the system and (gasp), the FET stayed on like I intended it to. After suspiciously pondering the circuit thinking that I had messed something up, I shorted out the OVP signal and the FET quickly turned off and the LED came on. The link-10 meter shows a full 8 amps going into the batteries. In a few hours, the BatMon boards should start lighting up and I'll see if the circuit responds correctly. The charge detector box is a bit warm to the touch, but definitely not hot.

I'm very excited that adding a .01uf capacitor to the opto-coupler input cleared up much of the noise. Someone over in the UK had their Belktronix charge detector blow up too with a lithium ion battery pack, so I hope this circuit can help them too.

After my heated rant last night, I received several supportive e-mails and comments from people. Thanks to all who responded for your ideas and well wishes. I feel good when I hear from folks because I don't know how many people are actually keeping track of this blog, especially since things have quieted down recently.

Let's keep our fingers crossed with cautious optimism. Cheers.

1 comment:

BTX said...

Hi Tim,
After seeing you having so much fun taking pot shots at the charger, I figured its time to inject some reality:

First, the charge detector optoisolator is buffered to deliver 5us transition times for the mosfet gate. For only 8A, this is adequate for the mosfet. Its extremely likely in your many adventure(s) with the charge detector, the totem pole buffer stage probably blew out, so the drive for the mosfet is no longer fast enough to keep the fet out of the linear region during switching. The R/C snubber network across the mosfet absorbs the turnoff spike from the 3 ohm resistor and wiring inductances to keep the mosfet from using the avalanche mode on the internal diode, once again keeping it in a safe operating mode.

Secondly, the kind fella in the UK was well informed the charge detector could be an issue for the Lion charger setup, that the charge detector circuit was deemed experimental and subject to revision. The reason for this is simple, it has to go from full charge current to <1A, requiring an extraordinarily large series resistor (10-20 ohms) and support a much larger voltage drop than yours (up to 20V). He was aware Gen2 was around the corner but couldn't wait for it (I had hoped he would) but...Anyway, he took the time to follow my instructions to beef up the charge detector, and succeeded in his quest. Unfortunately, I can't claim the same victory with your situation. Your choices to attempt repairs to a system you're not familiar with will always up with more problems. Its not that you're unqualified, its your inexperience with the hardware setup of the system that yields the result.
BTW, anytime you float a scope, its subject to seeing the full float voltage + stray capacitances (found in all electronics). Any scope will usually do this.
-And, once you own a pacemaker, you'd be surprised that most electrical appliances have pacemaker warnings. My mom survived heart surgery with one (else she wouldn't be alive today) -
So try a differential probe and you will see a real signal in a floating enviornment. I'm not sure you have access to them at work, but they're available for around $300-$400, less than the price for a Gen2 upgrade. Hope that helps,
Bryan w/ Belktronix.
Ps. Support is generally free...