boost converter

[EdwardM]

Hi Alun

* Absolutely * ;)

Ed

[scuba14c]

The capacitors are DC. Don't they blow up when charged with AC?

Could you post a schematic of how I would rectify the output using diodes?

When something like this is charging, what should I expect for a charge time?

[Guest Alun]

The output is plused DC not AC, the diode is just connected in series with the capacitor to stop it from discharging back into the transformer.

[EdwardM]

Hi All

hopefully to make it clear...

Ed

[EdwardM]

Hi Alun

The output is plused DC not AC, the diode is just connected in series with the capacitor to stop it from discharging back into the transformer.

Am I missing something, the output is pulsed DC...? are we at cross purposes?

Ed

[Guest Alun]

That's exactly what I said. ;D

[EdwardM]

I didn't wait long enough for my latest post to upload...  :P and it got lost ???

What I said was that the output from the transformer is AC.  And the action of the diode is to provide both DC and secondarily, a nice word for this time of night, to prevent the capacitor discharging back through the transformer secondary.

Ed

[Guest Alun]

Well I woudn't call it DC since when I power a flouresent tube from a flyback transformer one of the ends of the tube goes black before the other because the cathode sputters and not the anode. This is why I always use a two transistor push-pull blocking oscilator in my low voltage flouresent lamp dirver circuits as opposed to a single transistor blocking osilator that outputs pulsed DC.

[scuba14c]

You lost me on the last post. Is the schematic you posted all I need to assemble to charge? If so I can get this done after school tomorrow and report back. Would it work if I just touched the battery to the transformer once a second, or do I need an electronically controlled switch to do this?

[Guest Alun]

No, it's just the output stage, the switch is the transistor in your circuit.

[scuba14c]

I set it up without using a switch, and two 6V lantern batteries and it gave me about 25V off of a salvaged transformer. I'm not sure what the winding ratio is, but plan to make my own. The only problem I noticed was that it would lose voltage very quickly. The capacitor that I was using was a photoflash, 330V. I'm not sure if this was because the capacitor was low quality, but I did have an ultrafast diode connected to it. Can a transformer be wound on any type of ferrite core? Is there any way to electronically activate the batteries so I don't have to manually switch the power?

[Guest Alun]

Just replace the "switch" in Edward's circuit with the transistor in your 555 circuit.

[Guest Alun]

Alright on second thoughts I'll show you:

[scuba14c]

Okay, that's kind of what I thought I needed to do. I will test it tomorrow and get back to you.

[scuba14c]

I tried to wind a transformer, but it didn't work. I used a square ferrite block and wound 1 turn of 24-26 AWG wire on one side and 32 turns on another. What am I doing wrong?

[audioguru]

A single turn of wire doesn't make much magnetism or pickup much in the core unless it is carrying many hundreds or thousands of amps, or unless it is used for low impedance, low level very high frequency circuits like the tiny balun transformer in a 75 ohm TV cable splitter box.
Ferrite is good only for high frequencies.

Try it again with 32 turns on both sides. If the coupling is good and the unloaded transformer doesn't use too much current, try 1000 turns on one side. ;D

[scuba14c]

That was what I thought was wrong, I just didn't want to wind so much. I'll report back when I get one wound.

[scuba14c]

I want to run this by you before I wind this. I have a bobbin shaped piece pf plastic with a hole in the middle. Can I wind around this and separate it in the middle, and then use a steel rod through the middle? Right now these are the only materials I can get.

[Guest Alun]

Or better still go to the local dump, pickup an old TV and use its flyback transformer.

[scuba14c]

I tried to use a microwave transformer, but it didn't give me anything. Wouldn't a flyback charge too slowly because it is outputting a higher voltage?

[scuba14c]

I found some DC Wall transformers. Would one of these work?

[Guest Alun]

Higher voltages will make the capacitors charge up to a higher voltage and higher current sources will make it charge faster.

Microwave transformers connected are very dangerous don't use them unless you know what you're doing as the output voltage and current is more than enough to kill, in fact it's a lot more dangerous than the mains.

Using a microwave transformer in this project is a bit safer, but still dangerous as a 12V pulse can still create a back emf of hundereds of volts on the primary which can be steped up to >5kV.

If you really want to use a microwave transformer in this project connect the mains side to the output transistor but don't blame me if anything bad happens.

A DC mains adaptor transformer is perfect for this, cut it open and remove the transformer, connect the low voltage side to the output transistor and the mains side to the diode and capacitors, if the voltage is too low then try swapping the connections on either the low voltage or high voltage side.

Be warned though big capacitors charged to high voltages can kill you and be carefull the voltage doesn't exceed the maximum rating of the capacitors or the will explode.

[scuba14c]

I know the dangers of the capacitors, and the only reason I tried the MOT was because I was out of options until I remembered I had two of the wall transformers.

[Guest Alun]

I think in general you're better of using a pre-built transformer rather than winding your own. I've done this before, I used a ferrite cube core from an old PC switcing power supply and it worked though it arced over pretty quickly. I tried again and used a layer of insulation tape between each winding it lasted a bit longer but it arced over again. Winding transformers is very time consuming and you need 1000s of turns on the secondary to get very high voltages and it's very frustrating when they go wrong.

[scuba14c]

I think that the wall converter should work, but I have one question. The side that was origionally plugged into the wall and is now my secondary doesn't register with the continuity test on my DMM, but it does conduct when I connect a battery. When the DMM is connected it registers .5 on the screen. Do you know what the number means?