Howdy Nathan,
I hope to begin soon on the parts procurement for the 20lb Locky. Right now, I have a little project that's begging to be completed. It's a 30 foot, all weld, galve steel tower, with an 18 foot diameter wind turbine on top. Since it's summer here, you could say I am 'making hay while the sun shines' :) .
As winter makes it's unwelcome approach, the Locky will start to become a reality. That way, I can use it as a heater, while making a major quantity of noise hehehe. (evil laugh)
Yes, you have correctly remembered the pinouts of the IRF640. To help things along, I have included a pinout chart here.
Our mosfet connects to the circuit, thusably:
The 'G' terminal goes to the point where Q1 and Q2 drive the fet,
The 'D' terminal goes to the sparky coil,
The 'S' terminal goes to the zero volt supply rail.
If I understand correctly, you are still searching for NPN and PNP transistors?
Are there electronics stores near you?
Some of these sell collections of general purpose, silicon, transistors. They are as cheap as dirt, so you'll be able to get a swag of them for a few dollars. These are almost certain to work nicely in your circuit. When you get them, a search on the net will identify each one for its characteristics, polarity, and pinouts.
Generally, you should find that any general purpose trannies will do the job quite nicely :) .
A little clip on heat sink will surely add some time to the heating period of the mosfet.
When you mount the mosfet to the circuit board, you might like to bend it over and strap it down to the board, so as to make it less likely to break off, due to vibrations.
In order to enhance thermal efficiency still further, you could add a second IRF640.
You'd essentially be wiring these in parallel, S to S, D to D and G to G.
To make this work correctly, you'd need to add a 10 ohm resistor to the gate of each fet, and then connect those resistors to the junction at Q1 and Q2.
This will halve the current in each mosfet, and effectively reduce the dissipation to one quarter of its former value, as the forward voltage will be halved also.
So, instead of 6.48 watts, you'll have 1.62 watts in each mosfet. The heating effect will be quite small, and may allow you to avoid a heatsink.
But, before going down that track, try a single fet, without a heatsink, in open air.
Run the circuit for 5 seconds, stop and feel the fet for heat.
Run again for a longer period, stop and check again.
You may find that the fet's heating period is not going to be a problem after all :) .
Yes, those linear engines do look good, and their low profile makes them look like a good solution for jet karts too.
For a beginner, like me, anything would do, just to get a feel for the ways of these grand little power houses.
Valuable, experience is what we'll get when we build one of these noisy beggars.
Sure, our neighbours will all move away, but hey, we'll have a jet engine. Yippee :) .
Oh, I just can't wait to 'strap one on and light it up' hehe.
'He strapped it on, he lit it, he vanished from the surface of the earth..........'
"Hey, can you get me down from here - I seem to be stuck on the moon!"
Best wishes, and powerful ignition molecules.
Steve Tee.
Nathan wrote:Hey Steve. Got any stainless to start that 20lb Locky yet?
I'm still trying to finish parts procurement for my ignition circuit. I can definitely find an IRF 640. The problem I'm having is memory. Number 1 problem is that I can't remember the pin connections on the 640. I think the connections are G, D, S with the tab D. How does that hook up to the Ig Mol Gen diagram? Number 2 problem is that I can't for the life of me remember where I put all my "antique" books on transistors. Rats.
Since the 640 is a TO220 case do you think that for PJ ignition usage that the little clip-on heat sinks will be enough or do I need to mount it external to my (plastic) box on a more substantial sink?
I am almost decided on building a 55lb'er according Lockwoods specs. I want to experiment with a few ideas on liquid fuel injection and throttling so I really don't need an unproven design, yet. I really like the linear engines best for looks if nothing else, especially the big, fat, jumbo size. They look more like a jet engine "should" look instead of a large commercial sink drain or a truck exhaust system. Thanks.
Nathan