Not That Kind of Transformer!

September 3, 2007 8:13 AM

ElectronicsFilter: How do I drive a step-up transformer to get hundreds of volts out of an AA battery? They're only 1.5v.

I have a tiny step-up transformer I salvaged from a disposable-camera flash circuit. I know it worked before I took it apart, but I was reluctant to poke around in there with my meter or 'scope probes because I didn't want to fry anything (or zap myself), so I don't know what the driving frequency/duty cycle is (was). I'm using a 555 oscillator to drive a transistor which pulls current pulses down through the primary, and I'm rectifying the secondary and (theoretically) putting charge into a capacitor, just like the original flash circuit. However, my pulses aren't right, so I'm not getting anything out of the secondary.

What's the easiest way to figure out how to drive the transformer so I can get the highest voltage out of it? I have a couple of meters and an oscilloscope, but no signal generator or advanced tools. I'm hoping to get 80-90v at a few mA to drive nixie tubes.

posted by spacewrench to sports, hobbies, & recreation (7 comments total)

Have you connected the secondary directly to the oscilloscope (without the rectifier or cap), and then just see what you get from it as you drive the primary?
No matter what you're doing with the primary, you'll see something that way, and once you're looking at the output, you can adjust the oscillator to get the best spike. If you see nothing, something is broken :)

I don't use them often, but if you can set the o-scope voltage scale to something safely insanely high (2kV?), and then keep stepping it down until you see something, there shouldn't be a risk to the instrument.
posted by -harlequin- at 10:08 AM on September 3, 2007

(You could also put a low-value high-voltage cap in series with the oscope and secondary)
posted by -harlequin- at 10:11 AM on September 3, 2007

you used to hear those things whistle as they charged, so the frequency is probably in the tens of kHz.

isn't the problem impedance matching? what's the inductance of the transformer seen through the input? that would give you the frequency (since you want the impedance to match whatever you are using to drive it). disclaimer: that is a wild guess.

1mA @ 100V is 0.1W. if you're 10% efficient (is that reasonable?) that means you need 1W going in. batteries won't last long. still, woz's watch exists...
posted by andrew cooke at 10:20 AM on September 3, 2007

First answer - working flash circuits are dirt cheap. I've found them at surplus stores for like $1 a circuit board. I also got a basketful (and more stuff besides) by buying opened disposable cameras from a photo developer and pulling the flash circuits out. So, in your position I wouldn't try to repair a flash pcb, I'd just get another.

Moving on, there's also the issue that a flash circuit is just designed to pump a capacitor up to 100v, it's not meant to provide a steady current at the peak voltage... so there's a possibility that the flash transformer won't be up to the job.

But it's certainly worth a try. I'd consider getting an oscillator that goes up to at least 1MHz, hook it to the transformer's primary, hook a high-voltage AC meter to the secondary, and change the frequency till the secondary output peaks. This will give you a ballpark idea of what frequency the transformer is most efficient at.

Then I'd make your 555 circuit oscillate at that frequency, load the transformer output (1mA @ 100v -> 100k load), and start tweaking the 555 frequency and pulse-width to maximize the output voltage.

If the flash transformer proves unsuitable, you might look for other transformers, or wind some of your own using toroids from a trashed PC power supply.

I expect that you should be able to get 60 to 75% efficiency, maybe even more with care.
posted by Artful Codger at 11:15 AM on September 3, 2007

Bonus response: The high-voltage electrolytic capacitors in disposable camera flashes are great for use as coupling or in/out capacitors in tube audio circuits. :)
posted by Artful Codger at 11:21 AM on September 3, 2007

If the flash transformer proves unsuitable, you might look for other transformers,

I'd suggest CCFL drivers. These boards include an oscillator, are cheap, plentiful, small, available in computer stores as well as electronics stores, designed for continuous operation, and once rectified you'll usually get more than 100V out of them even using a 1.5V source (they're normally designed for 12V, and sometimes 5-6V, but they don't need modification to run at 1.V volts, though they can't drive a CCFL at that voltage)
posted by -harlequin- at 11:28 AM on September 3, 2007

You are experimenting, and making assumptions about the operational environment of the transformer.

You need some data. If you can't get it from a data sheet on the transformer, the first thing I would do is to get some idea of what the transformer specs are. You can get DC resistances from the primary and secondary from a DMM. Also, perhaps the inductance of the primary. If you don't have a meter that can measure L, then you can estimate it with a DVM on AC, using the AC voltage drop across the coil at a know frequency and series resistance. Good thing to use for this is a 6.3V AC filament transformer. That will give you a safe, low voltage 60 Hz source. Otherwise, you can use an audio oscillator set to 1 KHz sine at 1 V or so, and measure the drops across a series resistor (1K) and the primary. That will give you the primary's inductance with some simple calculation effort.

Determine the turns ratio of the transformer. You can estimate it, but again, the easiest thing is to impress a signal on the primary and see how much the secondary steps it up. That'll give you a ballpark. Keep the AC input low enough and load the output of the transformer with a resistance about 10x the DC resistance of the secondary. Again, it's rough, but you are experimenting, not designing.

To maximize the output voltage at minimum battery drain, you shouldn't put current pulses into the primary that are longer than needed to fully charge the inductor. Also, run the 555 at a sufficiently high voltage, because they don't run at 1.5V. You have the Vce of the output transistor to deal with and remember to check the output of the 555 to be sure it's open collector and not totem pole. I fergit! I'd stick a .1 ohm resistor in series with the primary (on the low side) to allow monitoring of the primary current. Then, you can see with a scope if you are wasting energy by keeping the switch on too long. Once current is at a maximum, it is time to turn it off, of course, so that will tell you what frequency to operate it at.

You may be better off switching with a super low RDS on FET as a driver, versus the bipolar output transistor on the 555.

There's more you can do, but that is a start. Monitoring a working system with a scope will also get you to the answer faster without so much work, but won't be as informative.
posted by FauxScot at 5:22 PM on September 3, 2007