Dead HVAC fan repair

Couple of weeks ago, upon returning home from a short trip we found our house much too warm and after quick inspection found that central AC was not working. First suspecting blown fuse i went downstairs hoping for a quick resolution. Alas that was not the case. Furnace (serving as the blower) had power and AC was working as i could hear fan outside as well as the coolant running through the pipes.

After opening up the furnace i found a board with blinking lights repeating pattern 3 green 1 orange and infrequently changing to 3 green 3 orange. These codes translated to blower motor not running at the right speed or not running at all. Starting a search for the solution on the net I’ve stumbled on this long thread which suggested that this might be due to a failed thermistor and that repair could be as simple as replacing it. Taking the power supply off the motor i was happy to find a charred and broken apart thermistor.

The part number was SG348 and after further research i found that this was a Inrush Current Limiter – a thermistor with negative temperature coefficient (NTC) which means that it’s resistance decreases as it gets hotter.  The primary purpose of it is to limit starting current in motors and transformers. This particular ICL thermistor has a resistance of 1 Ohm when it’s cold and can sustain 20 Watt of power. It was made by RTI Electronics under the brand name of SURGE-GARD and was often used in GE motors. The SG348 is not longer in production, but SG100/SG301 can be used instead. In fact any NTC thermistor with room temperature resistance of 1Ohm and 20A maximum current (or 1Ohm*20A=20Watt maximum power). In fact  DigiKey had 3 parts satisfying that requirement with 570-1041-ND being the cheapest at $1.77

Finding the replacement part was easy – finding it locally proved impossible. So after calling all of the local electronics stores I had no choice but to order from DigiKey and wait for the part to arrive. In the mean while i started looking for a work-around. Some people on the forum suggested short-circuiting contacts where thermistor was but i found it too risky. I decided to place a fixed resistor in place, however finding one that would not melt down at 20A was also not easy. Luckily, a local Radio Shack still had some memories of it’s former glory and carried 1 Ohm 10W resistors. Putting them in parallel to distribute the load i had a temporary work around:

 

Workaround using Radio Shack's 1Ohm 10W power resistor

 

This solution was a better alternative to the short-circuit however it reduced the efficiency of the fan essentially by wasting energy in the resistor all the time rather than only during startup as in case of a ICL thermistor. The picture above is after fan has been working for a week or so and you can see those resistors heated up quite considerably and even melted down part of a rectifier nearby. There was also a discoloration on the end of one of the resistors.  so i can certainly can’t call it a perfect solution but it worked for me.

Here’s how the power supply looked like with the replacement parts in place:

Replacement Inrush Current Limiting Thermistor in place

 

At the end, the most impressive part for me was the power internet allowing free peer to peer sharing of knowledge. I was pleasantly surprised how easy it was to find this knowledge on the net and how valuable it was – was I call a HVAC technician it would cost me several hundred dollars in replacement power supply or even entire motor.

 

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Simple Wheel Balancing Jig

I was recently changing tires on my band saw and needed to balance the wheels. I have heard of the method where you spin the wheel and whichever side ends up down the most time is the heaviest, but i wanted a little more precision. After several failed attempts to use a spinning-top kind of a device i came up with (and i don’t claim originality) with following simple device:

Wheel Balancing Jig

MSP430 USB Stick Development Tool (EZ430-F2013) – JTag breakout cable

For my next project i decided to try TI low power microcontrollers and for that purpose acquired a $20 development tool: EZ430-F2013 (doc, digikey):

eZ430-F2013 development tool

Dirt Cheap Light Tent

After making photos of my previous project i was disappointed with the way they came out – wooden grain background and lighting from the table lamp just weren’t good. So i started looking into Light Domes and Tents. A lot of the items i liked were out of stock which made me wander – just how bad would be the diy solution using cardboard box? Well, it turns out not too bad at all. Here’s tent/box:

DIY Cardboard Lighting Tent

DIY Cardboard Lighting Tent - back

DIY Cardboard Lighting Tent - inside

Punch Acceleration Sensor – Part 3

In this part I will describe the software side of the project. You can download the code here. I would highly recommend that you download ATMega328P documentation as well for register descriptions, notes and sample code.

Beginner Arduino Notes.

Any Arduino based program has at least 2 entry points:

void setup()
{
}
void loop()
{
}

Code within setup() is executed only once at the start and code within loop() is executed multiple times. You will also notice syntax like this:
ISR (TIMER1_OVF_vect)
{
}

This is an interrupt handling routine for the Timer1 overflow interrupt. Just writing this code however is not enough as you need to enable that interrupt. Some of the interrupts are already handled by Arduino Wire library (such as Timer0 overflow) so they are not available to user code without additional steps.
There are several ways to do i/o operations. Arduino provides friendly pinMode(), digitalWrite(), digitalRead() functions but you can use native AVR way of reading/writing to various microcontroller registers.
 
									

Punch Acceleration Sensor – Part 2

In this part I will describe the hardware side of the Punch Acceleration Sensor and the next part will be about the software.

You can download Eagle schematics and printouts here

Here’s the basic hardware schematic (click for pdf version):

Punch Acceleration Sensor – Schematics

These schematics use SparkFun Eagle Library for the Arduino board so the pins shown are for the Arduino board and not ATmega328. You can see how those pins correspond to ATmega328 pins by downloading schematics from Arduino Duemilanove specs page.

Punch Acceleration Sensor – Part 1

After practicing a bit with my makiwara and having a mind of an engineer I wanted to know how hard I am actually hitting. Plus I wanted to see if I can hit as hard from short distance (so called 1 inch punch) as with full swing. So that made me go on a frenzy searching spree to see if there’s anything out there and even what number should I be looking for – is it arm speed or acceleration or board speed? There are actually several studies done on the topic and many good articles written explaining the physics behind it. From what I gathered, a hand hitting a board can cause 200g of acceleration in the matter of 10 milliseconds. This gave me the parameters to look for and also the realization that there’s nothing out there that I could afford to buy. In my searches I stumbled upon SparkFun store and from there the whole world of ICs, microprocessors and electronics. So I have decided to make it myself and couple of months later (yes, things go slow when you don’t know what you’re doing :)) I present you the working product (sorry for the grainy video – needed more light):

I will limit this post to the general system overview and will follow up with post dedicated to hardware and another one for software. All code and designs will be available for free download and you’re welcome to do with it whatever you like.

DIY Wall Mounted Makiwara

Sometime ago I decided to improve my punches and strengthen my knuckles at the same time.  After a quick look online I found out that I need a Makiwara – a padded wooden board mostly used in Karate. Original Makiwara is a large wooden board buried in the ground but i found some room versions of them as well for sale. I could not find exactly what i needed – they were either too small or too large and required floor mounting – so I’ve decided  to build one myself:

DIY Wall Mounted Makiwara