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Topic: Making a needle glow (1000°C) by Battery (Read 4 times) previous topic - next topic

P_Wood

Quote
Test with 150V at 220uF on a 400V capacitor, energy 2.475J piece of stranded copper wire.

Nice. Was this taken from a video frame?

jazzar

Thanks, nope tripod, 1 second exposore and a steady hand.

jazzar

I'm currently working on a power supply...

For stepping up 9v to 35V I decided to use a boost converter, these are normally controlled via a 555 IC and I'm sure I could use the arduinos pwm to modulate it... BUT I want to use the pwm from an astable multivibrator!

I already got somehow mixed results... the frequency of my multivibrator was ~325Hz
calculated by: f = 1/ C * (R1 + R2)
(only one c because both values are equal)

For the boost converter I used a self-wound coil with probably 300uH (can't measure it)

And with 12V input I got around ~100V output without load... with load (capacitor) its ~11V wich is input voltage minus diode drop.

Also the Transistor on the boost-converter gets extremely hot..

Can anyone point me in the right direction on what frequency this should be run best, or what kind of coil to use?
I'd like to get along without ordering new parts... (shipping costs...)


Schematics:
Multivibrator:
http://en.wikipedia.org/wiki/Multivibrator

Boost converter:
http://en.wikipedia.org/wiki/File:Boost_circuit.png

P_Wood

Quote
~100V output without load... with load (capacitor) its ~11V

I think that the measured voltage drop across the load (capacitor) will change as it charges. If you put 100V across a cap it should start out with a small voltage drop and eventually reach 100V once it has charged..

As for the boost converter circuit.. I've looked at how they work and the math involved. I would need to do some experimentation with it to understand more about how frequency and voltage are related. I'd say just use a potentiometer as a variable resistor in your 555 circuit so you can vary the frequency and see what happens. Worth a try.

What kind of transistor are you using and what are its ratings? For example: Is it say 'N-channel MOSFET' or bipolar-junction..? What is the max voltage and current ratings? I found that the MOSFETS I used needed to be switched at the gate with at least 10V. It says 5V in the data sheet but 5V wasn't 'completely' closing the 'switch'. SO when I used my 555 PWM circuit @ 5V to switch the MOSFET it got very hot very quickly. The beauty of a 555 is that they can run on up to 18V.. So I just ran it with 10V and the MOSFET didn't even get warm to the touch while switching a 90V 15A DC motor. Pretty amazing.

I used one to control a huge dc motor for my Mechanical Engineering project. It was an N-channel MOSFET(200V & 100A max). That worked well as long as it was protected from transients, of course.

retrolefty

Quote
What is the max voltage and current ratings? I found that the MOSFETS I used needed to be switched at the gate with at least 10V. It says 5V in the data sheet but 5V wasn't 'completely' closing the 'switch'. SO when I used my 555 PWM circuit @ 5V to switch the MOSFET it got very hot very quickly.


"It says 5V in the data sheet"
I suspect the 5v spec you saw was called gate threshold voltage. This is the gate voltage that cause the source/drain path to just start to conduct and certainly can not pass much current at this gate voltage. The gate voltage to pass a specific current at a specific drain voltage is not really a fixed spec, but rather always shown in a graph in the datasheet. Anyway there are N-channel MOSFETS (they usually have a L in their part number) that are designed to wire directly to 5v logic and they are called (surprise) logic level mosfets. They have a gate threshold spec in the 1.5-3 range and work great with Arduino output pins.

Lefty

P_Wood

I found this equation that relates Vout, Vin, Vdiode, Duty, iload, frequency, and whatever "min" is.

I also read in an app note that the frequency in these circuits ranges from 100KHz to 5MHz. So that's something to consider as well.

P_Wood

Quote
I suspect the 5v spec you saw was called gate threshold voltage.

Wow, thanks Retrolefty. I never knew. This would be a good time to mock my school's motto.. "Technology driven, results proven." I feel like this would be a handy thing to have learned in an electronics class. Oh well. Thanks again.

jazzar

Guys you are great!

The multivibrator:
I used electrolytic capacitors, but with frequencys above 100kHz I think I should use ceramic ones.

The transistors used were the standard 2n 3904 but anything shoulld work with a switching time below 10 us (=100000 Hz if I'm not mistaken).

Voltage will be 9V in the final circuit so with 5v thershold on the boost converter mosfet this should work.

The boost converter:

As I understand the "on" time should be less than the time that takes the coil to saturate.
And the back emf usually is al lot faster than the forward(?) emf so the multivibrator should oszillate like this:
----||----||----||----||----||----|| rather than:
--||--||--||--||--||--||--||--||--||

To archieve this one resistor has to be greater than the other, no problem here.

The capacitor used in the bc. was a ceramic capacitor, is this ok? As I understand the purpose of this thing is to buffer the pulses nothing more.

---

I also used a standart npn transistor instead of a mosfet in the bc. This might explain why it got so hot...

---

Unfortunately until the 15th I have to study for three upcoming tests so I can't work on this until then. But after that I will be able to get my hands on an oscilloscope and will post some pictures of how I adjusted the circuit and if everything works out also a quick and simple tutorial on how to create a boost converter without a 555.

cheers!

p.s.
I also bumped into this site which was.. not as helpful as I hoped since there is little explaining which paramaters change what.
http://uzzors2k.4hv.org/index.php?page=450vboostconverter

jazzar

I've been able to do some work on the cap bank, the fotos you see below are run off a 0.188F cap bank at 19V, which makes a total energy of 33.934J
However in the fotos I took thin strands of copper wire to short the bank.
To trigger the short, a relay was used.
The finished thing will be enclosed in a case and will have safeguards.

cheers!

http://img64.imageshack.us/slideshow/webplayer.php?id=dsc046321.jpg

Grumpy_Mike

#69
Jul 05, 2010, 01:49 pm Last Edit: Jul 05, 2010, 01:50 pm by Grumpy_Mike Reason: 1
Quote
I also read in an app note that the frequency in these circuits ranges from 100KHz to 5MHz.

Go back and read that again. I think you will find it said from 100Hz to 5KHz.

Quote
I used electrolytic capacitors, but with frequencys above 100kHz I think I should use ceramic ones.

See above.
You can't get ceramic ones with the big capacitance values you need. You need to have capacitors with a low ESR (effective series resistance) ones designed for switch mode power supplies. These are more expensive than conventional caps.

jzzar, nice photos, I thing a bit of insulation tape would not go amiss on the large wires.  ;)

P_Wood

Quote
Go back and read that again. I think you will find it said from 100Hz to 5KHz.

I am no electrical engineer, but I can read. Since reading your reply, I have poked around and looked at several boost converter IC's--many having fixed frequencies in the MHz range.

Also, look at this and tell me what you make of it.. http://www.egr.msu.edu/classes/ece480/goodman/spring/group05/docs/AppNote-reemmerb.pdf look under "Design" at the fourth line down.

jazzar. I really like your photos. Very nicely done. How long do you think the contacts in your relay will last--shorting large amounts of current through it? I wonder if it would be possible to use multiple power-mosfets in parallel instead of a relay. Kind of hard to tell without knowing the actual amount of current being switched. It would be really neat to do with solid state components, though.

jazzar

Hey there,
been a long time since I started this, now I finally "finished" it. What you can see in the pictures is a needle during triggering and afterwards, the needle didn't only get hot but blew away. although it is not powered by a single 9V block it sure is possible to charge it by battery. I declare: Concept proven :-)

The energy that was consumed to blow up the needle was approximately 60J as you can see the capacitor bank is still a bit charged, before triggering it, it was at ~30V.



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