CD Unit - Schematic, pics and hints

[HVD]

I have just completed a capacitor discharge unit that is pretty professional and technically sound. I'm actually quite proud of it :-)

The big difference between this and other salvaged photoflash CD units is that this has a LED that reads the TRUE voltage from the cap bank to tell you when it's reached 330 volts, a power led as well, bleeder resistors to discharge the cap bank after about 15 mins (safer), and i have optimised all the component values so that it charges to 330 volts in about 7 seconds from 2 AA cells. It puts out about 12 Joules and is more than capable of vaporising steel wool at the end of 100m twin core wire and ignitiing large value carbon resistors. (I use 330 ohm personally). Also the discharge switch is (just about) rated correctly for the caps is i fire it though at least a 20 ohm load (ie 100m of twin core wire).

I have included a pic. What i want to ask is if anyone would be interested in a full pdf detailing all the calculations , theory, steps and ancilary info I've gathered in my quest for a nice CD unit like this and of course lots of pics :-) If there are a few people interested then i'll go ahead and do it. There is already a particularly useful article out there by Dennis Griesser detailing how he modified a photoflash unit, but it's not as detailed as i would have liked, and my box has some nice extras that he doesn't go into :-) It's a nice starting point though and Id like to point anyone interested in making a CD unit to that as well.


Cheers
HVD

CD Unit (http://myweb.tiscali.co.uk/bluechip/E&W/CD unit pic.jpg)

[Tuatara]

I'd be keen to see the schematic, at least. A pic of a grey box with buttons and LEDs on it is not of much use on its own. Do show your calcs, those of us with knowledge can then either shoot you down or give you the big thumbs up - either way you'll come out better off :D

[Jumala]

Here is also my last plan. It uses only standart parts no need to make a own transformer.
I choose a small transformer so the thing is insensible to shortcuts but it takes longer to charge the caps. A bigger tranformer will also work.


http://www.technikertreff2.host.sk/phpBB2/files/transformer_12v_-_250v_908.jpg

[Arthis]

I would be interested in a few more details. In fact if I've not made electronics studies, and if I can understand some basics, the scheme you gave isn't that talking, for me.
Could you give us a few explainations about that ?
A pdf would be great but it takes some time. You should decide of a plan and write it step by step, a part every for example.

[HVD]

This looks ... well ... overkill, but maybe that's just because I can't follow all of the schematic :-) My design isn't particularly sophisticated, but it gets the job done. I see you've used full wave rectification , that's something i would have liked but could never get caps charging with it, so i had to just use half wave :-/

However i see you're cap is only rated at 200 uF @ 250 volts. Is this going to be high enough?


Anyway here is the CD unit page (http://myweb.tiscali.co.uk/bluechip/E&W) I've just made, detailing my exploits and asking for some help on the calculations.

Sorry i can't seem to include pics in my actual post. I've used the img tags but to no avail. Any hints?

Cheers
HVD

[Jumala]

The plan seems to have a lot of parts but it works simple. The 4093 (4 x NAND) works as oscillator and voltage regulator. The 4093 drives the two darlington transistor pairs so that the current changes the direction approx. 100 times a second through the primary transformer coils.
This produces at the secondary side a real AC wich can be rectified. If the cap is charged the HV opens over the 10 MOhm the BC546 Transistor and the oscillator stops until the voltage is too low again.

In a self oscillating transformer the current is only pulsed and has always the same direction.
(no real AC) So the fullwave rectifier don´t work. But this is not important.

A self oscillating sytem is very good because it gives high voltage with a handfull parts but the transformer is hard to get. (only from photo flash units and I have none)
I didn´t want to calculate and wound up a ferrite transformer with I don´t know how much turns.
So I must use the second choise, the standart transformer.


The cap isn´t large but it works perfect. If nessesary I can add a second or a third cap.

[HVD]

I see how your circuit works now, and thanks for clearing up the A.C. thing with the self oscillating transformer. I was quite confused over why the full wave rectification wouldn't work :-).

As to photoflash transformers, if you want one or five, then just ask for some old disposable cameras from a camera shop. I'd stay away from the big chain ones and just go to a local one, they're likely to be much more friendly. I've asked a few times now for some at my local shop, and each time they've just given me all they had (usually 5 or 6). I just told them it was for a model rocket launcher (well, it's almost true :-)

Thanks
HVD

[Tuatara]

Are you sure you've drawn that schematic correctly? Some things don't make sense.

1/ Your 'Fire' led and associated circuit is connected in the wrong polarity.

2/ All your cap charging current is going either through the base of the transistor or through R1.

3/ Your LED current calc is out by a factor of 10 - you are putting 65mA through those LEDs, if that is actually a 20 ohm resistor

Don't be surprised at the result of the power calc. e.g Nitrogen lasers produce UV pulses of a few mJ, but they are nanoseconds in length so the power of the pulse is on the order of Megawatts.

That really is an odd setup for a self oscillatiing converter. For more info on how this is supposed to work search for 'self-oscillating flyback converter', or 'saturating core inverter'.

A good starting point (http://members.misty.com/don/samflash.html)

Jumala's circuit would be classed as a push-pull forward converter

[HVD]

Originally posted by Tuatara
Are you sure you've drawn that schematic correctly? Some things don't make sense.

1/ Your 'Fire' led and associated circuit is connected in the wrong polarity.

2/ All your cap charging current is going either through the base of the transistor or through R1.

3/ Your LED current calc is out by a factor of 10 - you are putting 65mA through those LEDs, if that is actually a 20 ohm resistor



I appologise, looking at the schematic it's obviously worng. It was late when i was doin it :-/ I've changed it now to the TRUE circuit, and that clears up your problems bar number 3, which is partly correct. The original calculation was out by a decimal place (65 not 6.5 mA) but on re-examining the circuit i see that in fact I only have 20 mA through the leds (65 would frazzle them). This is because only 0.34 volts is dropped across the resistors. with a 2.5 volt supply, this puts 2.16 volts across the LEDS.

Is this still a strange circuit :-) now what i've modified the diagram? I've seen that page before and thought about using a flyback transformer to generate the high voltage, but given the size of them and the fact i had the photoflash units lying around, i decided to use the photo flash's.

Also i tried to use what i had lying around, hence the 20 ohm resisitors on the leds as opposed to 50 ohms or something (the current is a bit high right now). In fact even the 20 ohm resistors are made up from 2 * 10 ohms ones each.

Anyway, thanks for the info.

Cheers
HVD

[Tuatara]

Ah,yes. Much better. I only provided the link to that page as it has some information on how this type of converter in normally arranged. Using a TV flyback tranny would be silly if you've already got the photoflash parts :D

[matjaz]

Here's my variant on the disposable camera CD box. It's a minor mod only, but works great with fine steel wool (strands 1 mm to a few cm long explode reliably and loudly). I use 2*10m of 0.75mm2 copper cable.

Press the "charge" button until the neon lamp starts blinking. The "test" pushbutton is a continuity tester. When pressed, the neon lamp must glow steadily (instead of blink) to test OK. This all works with the safety switch still "off". Only when really ready to "go", switch it on and press "fire". The design uses the xenon bulb itself as the power pulse switch.

The circuit doesn't need a master on/off, as far as battery drain is concerned, since it only draws nanoamperes when idle.

[HVD]

I don't think it draws any current when idle. If by idle you mean when charge isn't pressed then the transister isn't 'on' and thus no current flows. Unless you mean when the transister is 'off' then it's collector - emmitter resistance is technically not infinitely large , and so some small current will flow?

Anyway it's a moot point as long as your circuit works.

I looked at using the flash tube to switch the HV, but i figured I'd be losing too much energy through it (i fire 330 ohm resistors at the end of 100m of line).

I like yout test switch - i wish I'd designed something into mine because occassionally i get shorts because my line is kinked - although actually thinking about it your test switch would only detect open circuits - doh!

Cheers
HVD

P.s If you ever get problems with your wire wool breaking (i did) then 330 ohm or so resistors really do solve all your problems. They are safe (need high voltage to fire), very sturdy, easy to work with, will NOT break and are just generally great :-) Use carbon film ones though - not metal film.

[matjaz]

Yes, I meant the transistor leak. My ampermeter read 0.0uA, so it was nanoamps at most. But it might be much less. Anyway, that gives more than the shelf life of batteries anyway.

You're right, my test switch only detects open circuits... I couldn't possibly detect a short, since I use steel wool and it has resistance comparable to that of the cable.

The main problems when adding features to the camera flash circuit are
- separation of testing circuitry from the high potentials at the same connectors
- low supply voltage available: 1.3-1.6 volts

So it would be quite a pain to add short/open detection to this one. But if you care to carry a tiny separate 3V or 9V powered gadget, I could think one up.

Yes, I tried the resistors and it works with some models, it's probably the carbon / metal that matters.