NEON SIGN TRANSFORMERS - TROUBLE OR GODSEND

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Richard Hull
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NEON SIGN TRANSFORMERS - TROUBLE OR GODSEND

Post by Richard Hull » Mon Nov 18, 2002 8:21 pm

The neon sign transformer can be a blessing or a curse.

I have done this before but here is the final pass.

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Neon sign transformers are designed to be inefficient and waste power at full load!!!!

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No neon sign transformer ever produced will supply rated current at rated voltage!

All neon sign transformers are current limited devices.

All neon sign transformers have a severe voltage plunge well before delivering rated current.

Most neon sign transformers when supplying rated current supply only 500-1000 volts! This regardless of whether the transformer is a 5,000 volt transformer or a 20,000 volt transformer.

Power is power is power and nothing you can do will ever change that. In each step of any power tranformation or modification there are loses.

You can never ever get out of passive electrical conversion systems more than a fraction of the energy that is put in prior to multiple conversions.

The neon sign transformer is of a special magnetic design called a magnetically shunted transformer.

As all magnetic systems are current driven, the neon sign transformer in starting to output its maximum current brings its magnetically shunted core into play more and more as more and more current is demanded. In doing this the voltage plunges rather percipitously. So a 15kv 30ma sign transformer at 0 draw outputs 15kv. At 5ma it may only be down slightly to 13kv. But at 10 ma it may only have 10kv and at 20ma would be 7kv and finally near 30ma it might only produce 1kv output.

Now, 15kv X 30ma = 450 watts and that is exactly what the tranformer is rated at on its name plate. The transformer can actually almost draw that much power at near dead shorting of the secondary. However, the energy actually output under max load is nearer to 30-40 watts while the core is being wastefully heated due to its looking more like a simple inductor shorted across the line. The waste is marginal and real, but part of the price paid for a simple magnetic solution to neon signage current limiting.

Why all this weirdness and deception?

Real neon signage might involve an actual gas spark gap of 30 to 50 feet!!!! It requires a huge voltage to arc that much gas. However, once arced, the gas becomes highly conductive and is ohmically a very low resistance. It now only requires a few milliamps of sustaining current to keep it hot and ionized. Likewise, only about 1000 volts is needed to maintain the arc across the gap now. The tranformer obliges by shunting some of the flux past the secondary dropping the voltage, but maintaining the current. In the early twentieth century when signage became popular, there were no fancy electronics to put in charge of all of this so the transformer designer and magnetics engineers came to the rescue with a lossey but very simple and workable solution.

Behold the NST.

Now, to reduce insulation requirements they made two HV windings and grounded the inner most turns of each to the steel core, which in turn was connected to electrical ground. This meant that in a 15kv transformer no wire or terminal was at more than 7.5kv above ground potential.

Now, in comes the 80's and 90's and the new millineum with amateur electronic hobbyists wanting to make hv DC supplies out of NSTs.

Most think the nameplate is fully applicable to the device and IT IS NOT.

The problem is that once rectified,the output of the transformer and therby the DC output is highly current demand dependent. A 15kv DC supply would be a 15kv DC supply at 2ma or a 12kv DC supply at 5ma or an 8 kv DC supply at 15ma, etc., etc, and you get the idea......

As if this isn't sucky enough already, there is that center tapped ground. You can't bridge rectify the transformer to full wave or ever ground any part of the supply positive or negative outputs or the case ground of the transformer, thus making it a death sentence to touch the transformer body!!

The best you can do to get full wave rectification is to put one diode on each knob and ground the case. This will get the full wave, but at half the transformer voltage. ( a bit more than that, actually, due to peak voltage considerations).

So you are stuck?

The story is the same only more so for voltage multipliers. To utilize full voltage, you must make a lethal, ungroundable case scenario with weakened internal transformer break down characteristics.

Finally, let us say you made a voltge quadrupler using the lethal case design. If you really need the 15kv quadrupled to nearly 60 kv then only a tiny fraction of 1ma might be avaialble. For 60 hz multipliers have efficiencies in the 30-50% range. Last, if you need low ripple then some big and very lethal HV storage caps are needed.

In the final analysis NSTs are great for demo fusors, but are just non-functional for a real fusor where voltages above 20kv and a minimum of 5-10 ma are demanded.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Richard Hester
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Re: NEON SIGN TRANSFORMERS - TROUBLE OR GODSEND

Post by Richard Hester » Mon Nov 18, 2002 11:10 pm

You can do a voltage multiplier with a NST without floating the case. A biphase multiplier circuit is shown in the voltage multiplier section at Jim Lux's high voltage site. However, since you are working with a center tapped winding, you get half the output voltage -therefore, you need 2X the number of stages. Coupled with the other inherent disadvantages of NSTs, such an exercise may not be worth the trouble. An attached gif file shows the proper (safe) multiplier arrangement for a center-tapped transformer with grounded center tap such as a NST. The circuit shows a doubler.
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Re: NEON SIGN TRANSFORMERS - TROUBLE OR GODSEND

Post by Richard Hull » Mon Nov 25, 2002 6:36 pm

Another nice point is that this is a full wave circuit using two linked half wave doublers. Pretty slick.

Still the current limitation is gonna' suck the life out of this arrangment. Worth a try for those trapped into using, or over invested in, NSTs. I would think that a 15kv 60ma unit would be the best shot, but the rectifiers should, ideally, have PIV ratings of ~20kv each!!

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Richard Hester
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Re: NEON SIGN TRANSFORMERS - TROUBLE OR GODSEND

Post by Richard Hester » Mon Nov 25, 2002 8:02 pm

Two series connected microvave oven diodes (for each rectifier) would be the way to go. Each single microwave oven rectifier is 12-15kV, with avalanche rating. Appliance supply houses sell these as replacement parts, and I've seen them on Ebay for a pittance. The caps will take some scrounging, though.

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Re: NEON SIGN TRANSFORMERS - TROUBLE OR GODSEND

Post by DaveC » Tue Nov 26, 2002 9:44 am

Neon sign transformers are built to use the reactance of the secondary as a current limiter.. More accurately, the secondary's leakage reactance stores energy in its magnetic field to keep the arc stable. By using a reactance to limit current rather than a resistance, the transformer draws lots of volt-amps but not too many watts and thus stays relatively cool in operation.

There is also a safety reason for the center tapped secondary.. When the arc has been struck in the sign tubing, the plasma voltage drops to only a couple hundred volts during each half cycle. By grounding the center tap, the sign itself is kept to relatively modest voltages, not too much higher than 120/240 to ground, . except at the start of each half cycle when then ends will be near to max. voltage, briefly.

The reactive current limiting feature can be useful in a fusor...but mostly in the demo mode, as the voltages wil be too low for good ion energies.

Dave Cooper

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