The Contents of Greg Ingersoll's Brain

This is the second post in my dismantling-a-borderline-antique-Helium-Neon-laser series. In the first post, the project was introduced, and it was mentioned that every laser has three primary components: a power source, gain, and feedback. This post discusses the power source for this laser.

You can put energy into a laser system in many different ways, and the preferred method typically depends on the gain medium in use (which will be discussed in a later post). These methods include flash lamps, electric current through a diode, other lasers, chemical reactions, and electric current through a discharge tube. The last in the list is what is used in a HeNe laser and other similar lasers that use a gas-mixture as the gain medium.

In the picture above, the laser tube is the glowing thing. In the back of the picture is a white power cord going to the wall. So the input is 120V run-of-the-mill AC power, and the output is an electrical discharge through the laser tube. What’s happening in between?

Starting from the input and referencing the picture below, the power cord comes in through the bottom of the case and connects to a terminal block. The ground wire of the power cord connects directly to the metal case ensuring that the metal case will not become energized in the case of some sort of fault. The neutral (white) wire ultimately connects to one side of the transformer primary. The hot (black) wire connects through a fuse and the switch to the other side of the transformer primary. So when you press the switch, current from the wall starts flowing through the primary coil of the transformer. Then what?

Without going into too much detail about how transformers work, basically alternating current flows through a coil of wire (the primary coil) inside the transformer and sets up an oscillating magnetic field. This oscillating magnetic field causes alternating current to flow through another coil of wire (the secondary coil). The coils of wire are wrapped around an iron core, and the ratio of the number of wraps in the primary and secondary determines what appears on the secondary. If the coils numbers are the same (ratio of 1:1), the transformer is an isolation transformer. The input and output have the same AC voltage on them (assuming 100% efficiency), but they are not physically connected. If the primary has more coils than the secondary, the voltage will be stepped-down in the same ratio (e.g. with a coil ratio of 5:1 and 120Vac on the primary, the secondary will measure 120/5=24Vac). If the primary has fewer coils than the secondary, the opposite happens, and you have a step-up transformer.

So what is this transformer doing? Both step-up and step-down. As far as I can tell, this transformer (the brownish thing with the metal bar going across it in the picture) has two primary coils and two secondary coils, so it is basically two transformers in one. The two primaries are connected together and to the AC mains supply. One secondary is in a step-down configuration. It’s only purpose is to provide low-voltage AC (probably about 24V) to the small bulb that is inside the switch. The blue-green wires following the left-most corner of the base in the picture carry current to the bulb. Most laser systems include some type of power-on indicator, and higher powered lasers require one and may require key interlocks and other safety features as well [1].

The other secondary of this transformer is in the step up configuration. Based on some information about similar laser systems, this generates approximately 700Vac which is connected to the one and only circuit board in the system.

This circuit board is shown in the photo below. It is actually quite simple (compared to a board you might find in your mobile phone for example), and most of the engineering complication comes from having to deal with very high voltages. High voltages are required because of the way the laser tube works. Its operation is analogous to a neon sign or a fluorescent light or even lightning. If you put a put a high enough DC voltage across the tube, the gas inside will become ionized forming a plasma, and current will flow through the tube until the voltage is removed. So the purpose of the circuit shown here is to take high-voltage AC and convert it to even higher voltage DC.

It’s even more complicated than that though, because the tube acts as a so-called hysteretic resistor. Like a normal resistor, current flows in proportion to the applied voltage and the resistance (remember Ohm’s law?). However, unlike a normal resistor, current will not flow at all until the voltage across the tube exceeds a certain turn-on voltage and will stop flowing when the voltage drops below a certain turn-off voltage. In the case of this laser tube, the turn on voltage is roughly 4000-5000V and the turn off voltage is below 1000V (but I don’t know the exact number). This means that the power supply must be able to turn roughly 650Vac (rms, about 920Vac peak) into more than 4000Vdc. But it doesn’t stop there. Keeping 4000V across the tube at all times would cause too much current to flow which would cause various side-effects (e.g. overheating, gross inefficiency) not discussed here. So the power supply needs to reduce the voltage across the tube once the tube starts up.

In most contemporary things, this would be accomplished with various integrated circuits (e.g. amplifiers, comparators, microcontrollers running software), but this circuit was designed in the 70s and needs to handle very high voltages that integrated circuits dislike. The solution, though, is quite elegant. At the left of the circuit board are four grey cylinders which are 4.7uF capacitors that operate up to 500V. There are also 1M-ohm resistors (one is visible between the two red wires and below the capacitors). To the left of the grey capacitors is a group of diodes (small black things) and three more capacitors (brown discs, 5nF, good to 3000V). And to the left of those are six, high-wattage, 27k-ohm resistors.

I put the circuit into LTSpice to simulate it, and the schematic is shown below (and you can also download it). C1 through C4 are the grey capacitors. Along with diodes D1 through D6 and resistors R1 through R4, they form a rectifier and voltage doubler. The diodes only allow current to flow in one direction (they are in groups of three because their reverse blocking voltage is limited to 700V each) and the caps act as storage and a filter. In addition, the caps are arranged to double the magnitude of the incoming AC in the conversion to DC. Together this essentially turns the AC voltage from the transformer into about 1800VDC. This voltage is high enough to keep the laser tube running, but not high enough to start it.

To start the laser tube, the voltage multiplier circuit consisting of capacitors C5-C7 (the brown discs in the photograph) and diodes D7-D12 is needed. This uses the AC voltage from the transformer and the DC voltage from the rectifier/doubler to create a voltage across the laser tube of several times the voltage of the doubler circuit alone, in this case around 5000V. The catch is that because capacitors C5-C7 have such low capacity, this circuit can supply very little current. As soon as the laser tube begins conducting, the multiplier circuit becomes ineffective, and the voltage across the laser tube becomes essentially that supplied by the voltage doubler alone.

Plots of the circuit operation are shown below. The green line is the AC output of the transformer. The blue line is the voltage across the laser tube which is modeled as a resistor and a hysteretic switch) and load resistors R5-R10. The voltage across the tube builds until the tube starts conducting, and then it drops to the hold voltage. The current through the tube is shown by the red line. This remains at zero until the turn-on voltage of the tube is satisfied. Then the tube current stabilizes at around 4.5mA (resistors R5-R10 essentially set the tube current; there are six of them to balance power dissipation and keep them relatively small).

So 120Vac from the wall is stepped-up, rectified, and doubled (and multiplied at startup) to ionize a mixture of helium and neon, and this creates laser light. And from the simulation, the power dissipation of the laser tube is approximately 4.6W, but the laser outputs only a few milliwatts of useful laser light. Where is the rest of the power going? Why go through all this trouble for a few milliwatts of useful light? How does this make light at all? Clearly, we’re missing a few steps, and that’s where future posts will come in. Stay tuned.

1. Wikipedia. Laser Safety. http://en.wikipedia.org/wiki/Laser_safety.

 

 

This entry was posted on Sunday, November 22nd, 2009 at 15:34 and is filed under Electrical Engineering, Lasers. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.