Monday, December 16, 2013

One Bit ADC and a (Not) Wheelbarrow Shaped Object

With the semester winding down, and final project(s) nearing completion, the coil was revamped and the project part of my final project was made.

Mmm remounted IGBTs
The goal was to have a working analog interrupter which would take an analog signal, such as the one coming out of an mp3 player, and then be able to play it through a Tesla coil.

The block diagram is the following:

Signal -> Amplification -> Triggering -> Poopy sparks

This led me to use an LM358 and 555 timer due to their availability. A 74HC14 was added to buffer the output of the 555 to preserve waveform integrity.

Both channels of the audio signal are sent through an inverting adder and then to a monostable 555. The trigger voltage is set using R5 and the gain of the opamp can be adjusted using R3. The output of the 555 is then buffered by the inverter and sent to the opto-out. The interrupter is optically coupled to the coil to avoid the potential ground loop. Unused inputs are grounded and decoupling caps are added appropriately. 

At this point, I was running out of time, so I quickly breadboarded the circuit which also made the system incredibly noise prone. Oops :P

Input and Output Waveforms Lookin' Legit.

It even did the interrupting thing!

Further testing involved hooking up a dinky little speaker to the output to test if audio came out intelligibly. 
The results:

It's important to note that because this is, for all intents and purposes, a one-bit ADC, the audio will be pretty terrible.

Nevertheless, it seemed to work on the revamped coil.

Modifications: new secondary, primary,heatsinking, and rectifier diodes.

One concern was that the bridge was blowing prematurely due to the rectifier diodes failing - perfectly valid given that they were rated for only 4A (MUR460). They were then replaced with complete overkill: some minibrick diodes Bayley bought a while ago.

Rate for 96A at 600V. Yummy. 
The assumption was that the diodes were failing short due to transients, sending X amps of 60Hz AC to V+ and V- of the inverter. Eww. Another useful feature of these diodes is that when your bridge blows, the diodes won't.

Power testing also revealed that the new bridge is now capable of doing up to 75% duty cycle.

Heatsinking was improved by bottom mounting the IGBTs and increasing thermal mass substantially.

A squashier primary was wound using the old chassis as a coil winding jig. A power drill was used to speed up the unbearable process of winding 1330 turns of 36 gauge wire.

Only took five tries...

The final secondary dimensions came out to 7" long, 3.5" OD compared to the old 10", 2.4"OD. The resonant frequency also sank from 300kHz to ~150kHz making it suitable for brick coil use. (Whether it'll stay a coil is another question).
Unfortunately, the demo involved swapping out the secondary for a smaller one to reduce coupling - in this configuration, the coil had a tendency to be quite hot and burn-y.

For more details on the driver, you can view the project proposal here (Dropbox link).

Other news:


After relentlessly avoiding HSMXpress, I finally got around to generating the G-code to mill out the fork of my electric scooter on the MITERS CNC mill. 

And while HSMXpress seems to be incapable of informing the user why it failed to generate a toolpath, it can do this:

Totally worth it. So, sit back, relax, and listen to the elevator music courtesy of youtube's audio edit function.

And IRL:

Front assembly sans mounting screws, rear plate and brake. 
I opted to use a caliper brake as it requires a single mounting point on the fork and a shorter pull in terms of brake lever travel.

No longer a sad wheelbarrow!