Power Supply Construction

"Inventor Found Electrocuted in Basement Lab"

"Sunday morning a family member found a inventor who was working on plasma physics slumped over a desk electrocuted by the experiment in progress. The coroner said that the death had occurred several hours earlier and the inventor had been working alone so there was little chance of anyone arriving in time to prevent the tragedy."

I find posting my obituary above my desk helps to make me aware of the hazards working with high voltage. I also put warning signs on my equipment as a continuous reminder not to do something stupid.

If you stick your hand in the back of a TV set you will get a shock that will tighten every muscle in your body by it will probably not kill you. It is 20,000 volts but there is not enough current to finish the job. If you do your own house wiring and accidentally touch the wrong bit of metal it will probably not kill you because it is probably not across your heart and the 120 volt house current (180 volts peak) is not enough to overcome dry skin resistance and deliver a lethal current.

The power supply designs shown here are in that "sweet spot" with enough voltage to overcome dry skin resistance and enough current to kill you. If you have not suffered at least a dozen electrical shocks you should probably stop right here and take up a safer hobby like sky diving.

A technical note on high voltages. 50 ma on a path that crosses your heart (like hand to hand or right hand to left foot) will kill most humans. All you need is 200 volts to overcome your skin resistance and you will get that 50 ma easily. These power supplies will kill you quick if you are not extremely careful.

You can either wear plastic gloves (not fabric - it has holes) or use the "one hand rule" and put one hand in your back pocket whenever the power is on. You put one hand in your back pocket so if something falls over you will not be tempted to grab it with both hands. In any case you will be slow, clumsy or one handed whenever you are working with the power on. Take your time - do not rush - better clumsy than dead.

Float Ground Power Supplies

Normal commercial power supplies are carefully grounded to the third pin on the power cord. This is to ensure a short in the supply will not lead to house power on the case. The problem with high voltage supplies is that means every grounded metal object is a return path for a lethal voltage. I want to know where my high voltage points are so a floating ground means there are only two ends to keep track of. I use a lot of plastic around my high voltage components to make sure the problem solved by the third pin ground is extremely unlikely in my designs.

The simplest way to get isolation and a floating ground is to use two identical transformers wired back to back. This gives you 120 volts AC from the second transformer and with a doubler you get 340 volts DC with a floating ground (120 x 1.414 x 2). In my case I needed a negative supply with a 0-10 volt programming voltage I could drive from a computer.

The current was small enough I could use a shunt regulator. This design means with an output of 0 volts DC the load resistor must drop all 340 volts which is really wasteful. The design however is safer than a series regulator because if you try to ground the 340 volts through your body the maximum current is limited to 15 ma. I use this for new experiments where I am likely to be so focused on the problem I might do something stupid. . . twice actually and I'm still here. When I have stabilized the experiment I use a less safe but more efficient power supply.

Ancient Analogue Meters?

There are two good reasons to use these old analogue meters for high voltage supplies. If you put back-to-back diodes across the meter posts an accidental high voltage short during construction and testing will not destroy the meter (might kill you but the meter will be okay). Put 300 volts on a digital panel meter and its bye-bye $30. Do it a couple of times and you will switch to analogue meters.

The other reason is that the analogue movement makes it easier to see how much trouble you could be in. I use a digital watches and clocks but where the reading is both a value and a warning I like analogue meters. Since speedometers in cars are still analogue most people would agree with me.

Transformer Math

Many transformers have 120 volt AC and 240 volt AC primaries. If you use the 120 volt winding on the input transformer and the 240 volt winding on the output transformer you get a full wave doubling. If you put a voltage double on the output you get 680 volts (120 x 2 x 2 x 1.414).

The advantage of connecting the low voltage secondaries together (in the circuit above 35 volts to 35 volts) is wire size on the primary and secondary are probable engineered to be the "right" size for the ratio of currents they are designed to carry. Connecting them back to back takes advantage of that engineering.

You could connect the 35 volt windings from the input transformer to one half (17 volt winding) on the output transformer. It would get double the output but that would put an excessive current load on the 35 volt windings.

A better way is to try to match the impedance of the low voltage windings better and use two output transformers. Below is a example using dual output transformers and their 240 volt windings. This was not used because there are no cheap 1500 voltage transistors to create a variable supply.

One last way to double the output voltage at the transformer is to use four transformers. The two input transformers are connected by their low voltage secondary winding to the two output transformers. By joining the primaries of the output transformers you get an isolated doubled voltage without sacrificing the engineering margins on the transformers. Needs are lot of iron to make it work so you'll have to get the transformers cheap (free) to make the effort worthwhile.

Diode Doubler Math

You can also double the voltage by building out the diode capacitor change to higher multiples. The problem is every section of the chain gets fewer and fewer AC pulses and so ripple becomes a major problem if you try to draw any amount of current from the circuit. The circuit is called a Cockcroft Walton Multiplier.

1200 Volt Variable Power Supply

The power supply design used for powering most of the experiments is two variable 600 volt supplies that can be chained together like a couple of batteries. This supply was difficult to build because the extra safety circuitry apparently cased oscillation in the high voltage FET transistors. Most of the safety circuitry was stripped away and the power supply was eventually made stable at all output voltages.

The front panel has a separate meter for each side of the power supply so you have to add the meter readings to get the output voltage. Connecting the left hand red jack to the right hand green jack yields 1200 volts across the outer green and red jacks.