These days, most people wanting to get into neon will be shopping for used equipment. If you are lucky, you’ll find a system in place that you can demo run prior to purchase. This is not a common occurrence, however, and more often than not you will be looking over a bunch of disconnected and disused equipment and you may very likely not know its history or how it was treated. Since I was recently helping someone come up with a used bombarder, it seemed a good opportunity to show how one can be tested without expensive tools or much effort. The following are a few quick and dirty checks you can perform on a bombarder prior to purchasing it and hauling it home…or at least prior to hooking it all up for real.
Disclaimer– Part of this set of checks involves live power, and since I do not know your level of knowledge and experience and can’t watch you from here, I cannot make specific recommendations about the suitability or safety of anything you are doing….therefore the reader assumes any and all liability for anything that could possibly go wrong. It’s on you. If you are not well versed in these practices and the underlying principles, do not attempt any of it until someone is present who can help you do it safely. Don’t be stupid. I’d like you to be around to read whatever I might write next time.
First off, look at what type of unit it is. If it is a traditional purpose made neon bombarding transformer, the high voltage secondary will most likely be mid-point grounded…just like on a typical neon transformer. This is done because it is simpler for the mechanics of the design….the closest wrap of winding on the core is the part at ground potential….therefore, the windings and each high voltage terminal need only be insulated for half of the full output voltage. The two windings thus function in series for the output, with the center tap grounded to the core and frame. If the unit is a repurposed utility pole transformer, this will not be the case. Most pole transformers have a fully floating high voltage side. Some, however, are end-grounded—one end of the winding is grounded, the other coming out of the case in a single insulator bushing. If the voltage specs are correct, either type can be used as a bombarder provided that all the needed precautions are taken in the design of the system.
In my example today, we will look at a used power pole transformer. These days, the oil in them should not contain any PCBs, and there should be a label to this effect. If in doubt, don’t fool with it…you don’t want a superfund site in your basement.
This specific one happens to be 10kVA with a low voltage side of 120/240 (a 240v winding with a center tap that would normally be fed into a house) and a 12000v high voltage winding. This value is a bit odd, at least around here, as many of our lines are at 13,800 or 14,400. In any case, while a good neon bombarder should have a much higher striking voltage–ideally upwards of 20,000 volts, this one will still allow for processing of shorter tubes without using too deep a vacuum at the beginning of the heating cycle. So long as the user is aware of the limitations, it can work in a neon shop…..and the price was right. This specific one is going to a friend who is setting up his first shop.
Prior to purchasing it, I performed a few basic tests to rule out an obvious dud. A megohmmeter (“megger”) is a great tool for testing insulation resistance and gaining a good idea of the actual health of the unit. I have one, but in this case I will discuss some alternatives because a megger is an expensive tool that most people would not carry around everywhere. (If you make a habit of this sort of thing, you may want one….but most people would hardly get any use from it.)
First off, I used a typical multimeter and checked the resistance on the terminals. In a floating high voltage winding, you should see a resistance value from end to end. Not a dead short, and not an open circuit. AND, there should be an infinite open circuit between either bushing and the case ground. By checking both sides, I rule out a short at either end of the winding. IF this were a center tapped midpoint grounded unit, it would show roughly half of the total if either probe was measured from bushing to the ground and the sum of the two should be in the ballpark of what you measured from end to end.
Next, I took similar measurements on the low voltage windings. As this example is a center tapped one, I measured end to end and then each end to the center. The values of the halves should add up closely to the whole. And once again, there should be an open circuit between any of these terminals and the case ground.
Satisfied that there were no obvious dead shorts or open windings, I then wanted to do something a bit closer to a running voltage test. A relatively safe way to do this is to take a traditional 30mA neon transformer and apply its output to the high voltage side of the bombarder and then measure the voltage on the low voltage side. In the case of a pole transformer, this is closely akin to how it was made to function way up on the power pole, stepping down the applied high voltage. What makes this test relatively safe is that the neon transformer is current limited to 30mA. Keep in mind, however, the step down ratio will lower the voltage but the wattage is still there and the amps will increase on the lower voltage side because power in is roughly equal to power out (minus losses, which are minimal)…so the 30mA at a 100:1 ratio in our test example becomes 3,000mA (or 3 Amps) on the low side….3A at 120v will kill you here just like it can in your house. So don’t be stupid.
In my example today, the neon transformer I used was a 9000v one. Since the transformer under test is a 12000, I am only applying 75% here. So on each of the low voltage terminals I should have roughly 75% of the rating. In actual practice this is not entirely true for a couple of reasons– First off, there are some losses in everything, no conversion is 100% efficient. And secondly, the 9000v neon transformer is a bit of an underrating, it can actually be closer to 10 or 11 thousand without an adequate tube load on it…this is especially true of the older, better made ones like the Sola that is pictured here.
When I applied the power and measured the subsequent output, I had about 100v on each of the 120v windings and 200 on the 240 pair. As I was anticipating anywhere from 90-110 and 180-220, respectively, this is in the ballpark and for the purposes of a quick test it is a satisfactory result. Furthermore there was no buzzing or other concerning noises or any fluctuations in the readings. All nice and stable.
By all accounts, you could stop there, and take it into your shop and proceed to wire it up with your contactor, choke, and safety controls and begin working with it and have a pretty good confidence level that it would be fine.
There is, however, one more quick and dirty test that can be done if so desired. You can power it as a step up transformer and light a tube with it. Because these are not current limited, you must provide a means of regulation. One easy method that works is to connect a 150w incandescent lightbulb in series with the input to the transformer. This will limit the primary current to 150w. The light will glow brightly, but no greater current can flow than that which is consumed by the bulb. The high voltage side is connected to a simple short tubing unit. When plugged in, if all is connected correctly and if no faults are present, the bulb will illuminate, and pass its 150w to the transformer, which will then step up the voltage and light up the neon tubing unit.
Again, these are not as definitive or elegant as a megger test but are merely quick and simple checks to help you separate a good deal from a bad one–or at least save you from compounding a bad purchase by putting a lot of time into it. Using simple tools and some basic electrical knowledge that the average neon shop owner should already have, you can test a bombarder without doing a full installation.
As always, if you are not well versed in these practices and the principles behind them, do not attempt them without the assistance of someone present with the knowledge to help you do it safely.