TSP #3 - Camera Flash Circuit and Nixie Tube Tutorial (Part 2/3)

In this episode (Part 2/3) Shahriar explores the principle operation of a camera flash circuit. The flash circuit is analyzed at the schematic level and through measurements. He then moves on to power a nixie tube using this circuit and calculates the efficiency of the DC-DC converter for this type of application. There is also a little quiz in this episode! Whoever solves the quiz will chose the topic of the next video.

>> So, here's a circuit for the flash.

So, in order for you to understand how this works,

you need a little bit of electronics background.

But I'm going to do my best to try and explain

at least intuitively how this circuit works.

So, here's the single transistor I was telling you about.

Here, the three resistors,

one is here, one is here,

and the other one is right there.

I told you that there is a big electrolytic capacitor. That's this one.

I said there is a neon electrolytic aluminum foil one.

That's this one right here.

I said there are two transformers,

here's the first transformer,

here is the second transformer,

here's the neon flash shoot.

That's the thing that flashes.

I'm sorry, not the neon just a regular flash.

Here is a little neon bulb.

In the one that I just showed you,

this was an LED,

but it doesn't really matter, the concept is the same.

So, sometimes they use the neon one here.

Here's the trigger switch,

then when you close the flash fires,

here's the on and off switch. Let's go through this circuit.

So, at the beginning the on and off switch is open.

So, let's forget about everything that's on this side of the circuit.

If this switch is open,

and there is, therefore,

no current going through the base,

there is no voltage on the base.

So, this bipolar transistor is turned off,

so there is no current going down.

As soon as you close the circuit,

you can follow this connection.

It goes through somewhere in the middle of the transformer and is

connected to all the way up to 1.5 volts.

So, you will bring some current and

some voltage close to 1.5 volt at the base of this transistor

which will quickly turn this transistor on

causing a large current to go down from the collector to the emitter.

Now, as soon as you induce a current,

to go through this half of the transformer,

this itself will cause a current to be induced on the other half of the transformer.

But as soon as you do that,

the current in here will then lower the voltage of

the base to go down to even below zero.

So, then the transistor turns off again.

But when a transistor turns off this voltage will then eventually go

back all the way up to 1.5 volts because the

current will stop, and then the transistor will turn on again.

So, this transistor turns itself on and

off through this feedback path in the transformer.

So, you could say that this portion of

the circuit is essentially nothing more than an oscillator.

So, by turning the transistor on and off quickly,

you can induce a current in the transformer.

Now, this center tab brings back a small voltage.

It means that the entire voltage across

the transformer does not appear at the base of this transistor.

If it did, the transistor would die.

So, they've taken only a part of that voltage down to the base of the transformer.

But the whole transformer has a huge turn ratio.

By that I mean, if you look at this circuit again,

you can only see the outer coil,

the outer coil of this transformer and I can count it has about six or seven turns only.

That coil on the outside that you can see, that's this one.

On the inside below that yellow tape is a whole bunch of other turns of this transformer,

maybe about a thousand or a hundred times more than what is on the outside.

So, that will cause a huge voltage difference

between the primary coil and the secondary coil of the transformer, and

that's how you are able to generate such a higher voltage using only a 1.5 volt supply.

So, they take the high voltage portion

of the output of the transformer and they connect it to this diode.

They do that for a very specific reason.

It's because the signal that appears here is AC.

It's AC, why? Because it's an oscillator.

Because it turns on and off constantly.

That AC voltage will go below zero by several 100 volts every time on this down cycle.

This diode will only conduct current when

the voltage on this side is lower than the voltage on this side.

So, every time this voltage goes well below zero,

this big capacitor, which is the main capacitor of

the flash gets charged a little bit more.

So, every time during every oscillation cycle you will

put a little bit more charge into this big capacitor.

So, the voltage at the output of the capacitor is a DC voltage

but the voltage on the other side of the diode is an AC voltage.

So, you essentially have built what's called a peak detector,

where you'll keep dumping

more and more and more voltage charge

into this capacitor and the capacitor begins to charge up.

We will take a look at it under a oscilloscope so we can

see the oscillation very clearly.

So, this guy will then eventually charge up to minus 350 volts.

The reason I say minus 350 volts is because

I'm measuring everything with the respect to the ground of the battery.

So, if you assume that this is a zero volts,

every time this thing spikes well below zero,

the diode conducts and

the positive terminal of the capacitor is actually connected to ground.

So, this knot can go all the way down to minus 350 volts.

The other side of this now,

there's a whole bunch of other circuits that is

responsible for actually triggering the flash.

So, there's a big resistor that

separates this half of the circuit from this half the circuit.

So, this is a big capacitor here that's charged up,

and that capacitor is connected on one side of the flash tube,

the other side of the flash tube is grounded.

On this side here we have the neon light or the LED,

and a big resistor in series.

So, if this voltage gets high enough, meaning when the flash is ready to be fired,

the light turns on, it's just an indicator to let you as the user

know that the flash will fire once you push the shutter button.

So, this portion of the circuit is only an indicator.

Then you have another capacitor here much smaller than this one that will

charge to about the same voltage as this capacitor.

So, as time passes and the flash is charged up,

slowly current will flow through here and charge

up this capacitor to the same voltage as this one.

This is a much smaller capacitor,

and here's the triggers switch on the other side and another transformer.

So, the way that flash tube works is that even though we are

putting zero volts here and minus 350 volts here,

that's not enough to initiate the flash,

because the potential is not big enough for a spark to form inside the flash tube.

So, you need to kick it.

You need to initiate

it for the current to start flowing and once the current starts to flow,

then the path, the short-circuit happens in the middle of

the flash, and then you get the flash which is a spark.

So, in order to kick it and initiate the current,

you need to put a very,

very large voltage, in this case,

a very large negative voltage right here to dry the electrons into the tube.

Once you dry the electrons in the tube, then you will fire the flash.

This is done through this transformer and this capacitor.

So, if I close this circuit quickly,

which is done by connecting these two wires together very briefly,

it will discharge this capacitor through

this inductor half of the transformer and to ground.

So, there will be a burst of current,

a lot of it but for a very short time right through

this transformer which will then cause even a bigger,

because this transformer has a big turn ratio,

a bigger voltage on the other side.

So, for a very brief time,

something close to minus 2,000 volts appears

here for a very short time and that initiates the flash,

causes the electrons to be drawn into

the tube and all the way to the other side and to ground,

and then you get the big bright light that you see.

So, this entire circuit is everything that's on this.

I haven't omitted anything that is on this circuit that you just saw.

So, by looking at this,

we should be able to make some predictions well,

we should be able to put our oscilloscope at this node,

and look at the waveform that I told you.

We should be able to measure a very large negative voltage at this node.

Of course, we should be able to see this light turn on.

We will do all of those things right now,

and we'll also take a look at how you would measure something like that.

Then, at the very end,

we'll see how we can use this in

combination with a Nixie tube and do some experiments there.

So, in order to make the experiments safer,

I've taken one of these flash and I've removed

the components that I don't need to and I have some wires to it.

So, it can be easily connected to a power supply,

and we can measure currents and voltages and so on.

So, for example these battery holders, I don't need,

I'm not going to be using the flash itself

anymore because I want to use it for Nixie tube,

so I can remove that and sought of some wires to it so then,

I ended up with something that looks like this.

So, exactly the same circuit, everything is the same.

I've removed the flash, the capacitors there,

and you can see this one actually uses a,

this thing will focus.

This thing uses a little neon tube as

opposed to an LED to tell you when the flash is charged.

So, a little bit different but it's the same circuit.

I have also connected a piece of metal size,

a piece the metal across the on and off terminal,

to permanently keep it in the on position.

So, this would make it a little bit easier for me to do some experiments with it.

I've also removed the low trigger and put a jumper there in its place.

Wire that I inserted through, this wires is just

that across the plus and minus terminal of the flash.

I can connect this to my power supply,

and then we can check to see what is the voltage on the capacitor?

How long does it take to charge that?

Then we will do one discharge cycle by short circuiting

the capacitors which is not recommended, just for entertainment.

Also, then we will show you the wave from the oscilloscope,

and then we'll get to the Nixie tube.

So, let's see what I can do.

So, I'm going to take the positive and the negative terminals.

I'm going to connect them to my power supply.

So, here's a positive,

here is a negative terminal, so I'll put that down.

I will take the positive and the negative terminal of the multi-meter.

I'm going to connect it across the capacitor.

So, I'm going to connect a negative toward the ground of this power supply would be.

That's the positive terminal of the capacitor,

and I will take the other wire and connected to negative terminal of the capacitor.

So, what I'm doing here is that I'm using my power supply to power the flash.

I am measuring the voltage across the capacitor directly.

You will be able to move this side away,

you will be able to see at the same time the voltage that I'm applying to the flash unit.

You will be able to see a current that the flash unit takes.

It will show up right here,

and you will be able to see the voltage across the capacitor.

So, right now the voltage across the capacitor is minus 5.2 volts approximately,

it's the residue from the last time that I charged it.

So, let's do that.

I'm going to turn the power supply on.

So, right now it says zero volts at zero amps. Is that makes sense?

I'm going to slowly raise this voltage at all the way up to 1.5 volts,

and since I have already started the on and off button shut,

it will start charging right away.

So, at the very beginning,

up to about 0.5 volts,

you will get nothing because its bipolar transistor hasn't turned on yet.

So, as soon as you go about 0.6 volts, right there, now,

the flash is starting to charge up the capacitor and you can see the voltage here

is already on minus a 115 volts and it keeps rising slowly.

Minus a 123 votes,

but this is only a 0.6 volt.

An alkaline battery, a double A battery can go all the way up to 1.5.

So let's do that, 1.5 volts.

We'll wait a little bit until this thing charges.

You can see draws a lot of current at the beginning of is drawing almost an Amp.

So, right now the current keeps going down.

So, that makes sense of course because the current

keeps going down because the capacitor keeps charging.

So, resists loading on it.

At the same time, you can see the voltage appear now is at 324 minus 326-7 volts.

If I turn this, I have to be very very careful handling this nap very very careful.

So, if I shift it over,

you can see that little light flashing.

That's the little neon light I was telling you about.

Let me bring it up a bit closer to the camera.

Hopefully, we'll focus on it.

Well, you can see the neon flash going on and off.

There is actually a little quiz in this episode that I like to see

you guys try and discuss it on

the forum or on the comments sections of the YouTube channel.

The question I have for you is,

can you tell me why does this light flash?

I can tell you that in the LED version,

if I were to replace this with an LED,

the LED would not flash.

So, let me know why you think this is flashing.

So, let's put this down.

So, now I have charged the capacitor to minus 350 volts.

There's still some about 296 milliamps of current going through it.

If you leave this for a while longer,

it's not going to go much more than that.

It's going to go about minus 358 volts.

Its probably going to have it around that point.

So, now that capacitor is fully charged, so

what I'm going to do is,

I'm going to turn the power supply off.

So, there's no more current going in it.

You can see this voltage will start to slowly fall for

many reasons that the flashing light is of course is consuming some power,

there's leakage through the capacitor and so on.

So, that voltage will continue to go down.

So, let me get something to discharge it.