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Homemade Camera Flash Type Transformer

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Voltwad

Tue Jun 02 2009, 09:56PM

Registered Member #1829 Joined: Sun Nov 30 2008, 01:06AM
Location: Raleigh N.C.
Posts: 74

Hello everyone, I've recently finished winding myself a transformer similar to the ones found in disposable cameras only bigger. I used a core and bobbin from a computer monitor. (I think it was from the SMPS section) The secondary is the first coil with about eight layers and a total of about 500 turns, each layer separated with overhead sheet. on top of that the drive and feedback coils are side by side (as opposed to the feedback coil wrapped over the drive coil).

1243977325 1829 FT0 Setup

Here she is ready to go.

And here's a schematic of the setup I'm using. I've tried a HOT for the transistor and a 2n3055 so far. All the following traces are with the 2n3055.

What I was hoping for was a transformer that would be somewhere in between a LOPT and a camera flash in terms of both size and power. However, I don't think mine is processing as much power as it could be. When I turn my supply on it oscillates nicely. As I crank up the supply voltage the output increases to a point, but past about 10.5-11 volts the transistor base peaks slowly decay until it just stays on all the time and the output voltage drops way off.

Output (top) vs. base voltage (bottom) with 5V in.

You can see the transistor base voltage has two peaks here. Also the white lines are the ground levels.

Output vs. base voltage with 10V in.

As the input voltage is increased the first peak on the transistor base gets weaker and the second peak develops more noise.

Output vs. base voltage with 11V in.

After a point, the first base peak will continue to decay even though the input voltage remains constant. When the peak fully decays the second peak and the output voltage follow quickly.

As far as what's going on, there's only two things I can think of. Maybe the core becomes saturated past a certain voltage (I'm not sure of the material properties, it's an SMPS ferrite gapped double E core) but I thought saturation would be something that would happen much more quickly once the critical point is reached. Otherwise it could be the properties of the transistor changing as it heats up. When I get my hands on some better transistors I'll see if they make a difference. My only other unanswered question is, what do you call these type of transformers? (The camera flash type) I want to say they're just flybacks but if I say that I think people will think it's a LOPT. Finally, here's some shots of the CE voltage vs. the base voltage at 5, 10 and 11 volts. (CE is the top trace).

CE at 5 volts
CE at 10 Volts
CE at 11 Volts

Sulaiman

Wed Jun 03 2009, 11:33AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Try a diode (1N4148, UF400x etc.) across the transistor base-emitter (cathode to base)
and a small capacitor (10nF ?) from the junction of the 330R and feedback winding to 0V.

P.S. this type of inverter is called a 'blocking oscillator'
it works because the feedback winding voltage disappears
(allowing the transistor to turn off) when the core saturates.
The core must saturate for this oscillator to work, so;
- don't worry that the fault is due to core saturation - it must saturate
- the core WILL get hot, it's normal.
(This is why high-power inverters are NOT 'blocking oscillators')
- the core saturates faster at higher voltages,
(The oscillation frequency should rise roughly proportionally to supply voltage.)

klugesmith

Wed Jun 03 2009, 08:37PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Nice schematic. Did you draw it using a circuit simulator tool?

As Sulaiman said, you have made a blocking oscillator. Not unlike the popular Joule Thief circuit, for example:

It's also a kind of flyback transformer or converter, as mentioned in our HvWiki at

Flybacks don't necessarily depend on saturation, but they do depend on energy storage in the core. Energy is alternately put in via the primary winding, then taken out via the secondary winding. Unlike "ideal" transformers, where power input and output are concurrent.

Voltwad

Thu Jun 18 2009, 09:32PM

Registered Member #1829 Joined: Sun Nov 30 2008, 01:06AM
Location: Raleigh N.C.
Posts: 74

Alright, it's been a busy few weeks. First off, a big thanks to Sulaiman. Your suggestions have worked wonderfully. I've gotten ~3500 volts out and the core is now getting warm so I think I'm approaching the power limit for the core in this topology.

1245359502 1829 FT70608 3055 Out V Base At 20 Vin

1245359502 1829 FT70608 3055 Out V Base At 20 Vin

Base vs. out with 2n3055. (-3500 volts peak)

Next, thanks Klugesmith. I drew the circuit using Orcad capture from the Pspice program. Here's a copy of the updated circuit using Sulaimans suggestions. While using the HOT, I was able to push the input up to about 14 volts before the oscillation died. Here are some base vs. out traces using the HOT at 5 volts in and at 10 volts in. And here's a shot of the setup. Now, here are some shots with a 2n3055 which performed better. I was able to push this setup up to about 22 volts before the oscillation died resulting in the 3.5 kV output.
5 Volts
10 Volts
15 Volts

And here's the 2n3055 setup.

There's some frequency modulation that becomes more apparent as the voltage and frequency increase. Also, the transformer was also hissing like mad after 10 volts. I might try vacuuming it under oil but I may just wind another one with kraft paper and a ZVS primary for more power.

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