Checking the Output of a Transformer

Thanks Tornado!

The method for cutting structural foam is almost 40 years old. Prior to this experiment, I've used a 12 volt battery and a nichrome heating element from a toaster as a "variable" resistor. It's crude and the temp control is a hit and miss affair but it works.

Prior to that I used a Variac "auto transformer" wired directly to house current.

Assuming it was a true transformer I was unware of its limitations and the danger. It's only by pure luck and careful use that I did not wind up on a cold slab.

This inquiry is driven by the need for something more predictable and consistent. I'd like something with more precision. Both the Variac and the PWM seem like likely alternatives for regulating current flow.

Even at the reduced voltage, the PWM may not have the wattage capacity needed for prolonged use. I simply don't know.

With the cutting system safely isolated from the line by the step-down transformer, would it then be safe to substitute an auto-transformer (Variac) to regulate temperature?

Thanks

L.J.

If I picture this correctly, it sounds as though you used the variac directly off 120 line voltage. If so, it would be possible to dial down to a safe level, but you could accidentally get up to the 100v range.

I like your basic concept of stepping down to a pretty safe level, and then regulating from there. The closest thing to this that I have actually used is a hot knife for cutting polypropylene or nylon crab pot lines, also sealing the ends to avoid fraying. I don't remember the voltage of the transformer that energized the heat blade. There was no adjustment.

I am weak on transformer principles so take what follows with caution.

The Variac is NOT a transformer per say and because it has no magnetic core and seperate primary and secondary windings, it's still directly connected to the house current even though the voltage may be reduced. As a result the full current potential of the system is there to bite you, even though the voltage is not.

Thanks for the support.

L.J.

I have to disagree to some extent.The Variac IS a transformer and it DOES have magnetic fields. Since it has only one winding, of course the primary and 'secondary' are not separate windings. but different parts of the same winding. It would be quite possible for someone, who did not understand fully, to apply the 'hot' side of the input to the common side of the transformer and the 'ground' or common side of the input to the terminal intended for 'hot' input. They could then set the output for, say 6V, and they would have 120V and 114V at the output terminals (I'm thinking 120V supply). This would heat the cutting wire normally, but could shock the user (could be fatal) when they touch something grounded.

Variacs are not great for large currents, so the correct way to do this is to use the Variac to control the voltage, with the output of the Variac being used as the input to the step-down/isolation transformer, and the output of the step-down connected to the cutting wire. I've done this many times, both for cutting foam (using a mask and very good ventilation), and for 'cutting' glass (initiating a crack in previously scored glass, as making cheap vases out of bottles).

DK I have little knowledge of transformers and acuired my concerns about Variacs from an EE at Underwriter's Labs many decades ago. My ignorance about such things is complicated by the dust that settles on memory after so long.

The whole idea of my using a transformer is to insulate me from the power potential of a 120 VAC circuit. I'm told that the Variac will not do that. I will err on the side of caution.

Thanks for the support.

L.J.

There are actually variacs that are double wound so you can have full isolation between primary and secondary . I use one for a work light on my lathe 240V to 24V it is very handy.

Isolation is only part of the job of the two-winding transformer. The other parts are 'transforming' voltage and current. A step-down transformer is called that because it reduces the output voltage. When you reduce the voltage, you automatically increase the current capability, assuming the transformer is wound appropriately (smaller wire on the input winding, larger wire on the output winding. Since the cutting wire is inherently a low-voltage, high-current device, you need the step-down transformer to supply that combination of current and voltage.

The bell transformer may work for light-duty cutting with a very fine nichrome wire, but such a fine wire is going to break regularly. If you plan on doing much cutting, you need a heavier transformer. The physical size of the transformer is a pretty good indicator of its power handling capabilities. For this job, about 30 cubic inches (3x3x3 etc) should be minimum size, with around 100 being better. If you happen to find a toroidal transformer (I see them commonly in our electronics surplus store), they could be about half the physical size, and would likely not have any high voltage windings.

If you use the filament winding of an old tube-type device (Aurizon's second illustration). be sure to securely protect yourself from the unused windings. My preference would be to cut off all bare wires and apply heatshrink tubing extending well beyond the end of the remaining insulated wires. Tape comes off too easily.

Here's a complete circuit. I've shown the cores of the transformers grounded. That is desirable for safety, but not absolutely required.

I didn't show it, but another fuse between the two transformers would be a good idea.

Good luck. Dick

take an ohm meter. identify the separate windings and those with multiple taps. Some taps are center taps they are 3 wire connected. you can find the max resistance which will be the ends of the wires, with the tap to either end at 50% of the DC resistance.

Then there are line adjusting taps. You will get a resistance of, say 100 ohms, and some at 103, 105, 107, and some at 3, 3 etc.

The 103, 105, 107 are the outer ends. The small ones connected are the tap to tap windings.

make a transformer diagram like this.

This is an isolation transformer

this is filament and plate voltages for a transformer to power a tube set

and here is a catalog

http://www.hammondmfg.com/5cindex.htm

Now you want to energize one of the high resistnce windings with a 12 volt AC source

then measure all the other outputs with an AC volt meter. They will be lower, but they will give you a guide.

The 120 or 240 or tube plate voltage will be one of the high DC resistance ones. The low ones

That helps a lot Aurizon.

I think I'll use a bell transformer for the test supply.

Thank you

L.J.

For the sake of safety, instead of using a 24vac source to feed your test circuit to determine the ratios of the transformer windings- start with a source voltage much smaller such as 3 vac for a preliminary determination. If you were to to feed a 5v secondary winding with a 24vac test source a normally 120vac primary winding would be energized to 480vac. This could be lethal. If you use a low voltage until you have an approximate idea what the ratios are you can make a much more educated and therefore safer estimation of which winding is the primary and what to feed it with to get the output you want.

I have used a set-up similar to what you are planning several times to cut nylon/Dacron/poly rope and also to cut Styrofoam boards. I always use a variac to feed the transformer to adjust the temp of the cutting knife/wire. The variac adjusts the temp and the transformer steps down the voltage and provides the isolation from the line source.

My rope cutting set-up is over 30 years old and still going strong.

Good Luck

Thanks Old Salt

It never occured to me to put the Variac BEFORE the transformer. I'll consider that as well.

It would be wonderful if I can get enough current through a door bell transformer to power the dimmer and regulate temps that way.

There is an event I assist at each year where pitch laps for polishing glass are trimmed. Transporting both the Variac and the transformer would be cumbersome.

We will see.

Thanks again

L.J.

The transformer I refer to in my previous post consists of a "bastardized" 6.3v filament transformer with the secondary winding removed and replaced with 2 wraps of flat copper strip around the primary winding. This forms a high current low voltage secondary winding.

Since the time that we home created this unit WELLER has come out with a similar replacement tip for their soldering guns (not irons) for cutting synthetic ropes. Have you considered using a soldering gun as your power source? It is a high current/low voltage source, although it may not have the power you are looking for. It may be suitable as a prototype for developing the final unit.

Door bell transformers usually don't put our more than 2 amps. One possibility might be using the transformer from a automotive battery charger. Another possibility, which I use when I need a high current/low voltage source is a heavy duty control transformer for the 24vac control circuits for shop machines.

While I applaud the efforts of my more fatalistic experimeters, why not use an inductance meter?

Because very few have inductance meters available!

Lots of people have ohmeters around, I have a GR inductance bridge, who else has one?

By the way, be sure to be careful when testing the windings of a transformer with an ohmmeter. You would not think it, but that little ohmmeter DC source can build up a pretty good magnetic field, especially if the windings are the 480 V or higher types. When you break the circuit a pretty good voltage jolt is developed for a very short time as the field collapses (L Di/Dt). One time I was holding the probes to the wires directly and got a surprise when I broke the circuit with my fingers still touching the transformer wires. Probably not dangerous but still a moving experience!

Normally low voltage windings can be easily determined thru the size of the magnet wires used since low voltage windings do normally are rated to provide a higher amount of current than the primary winding. A variac is an auto type of transformer that can be used to reduce and limit the amount of voltage for testing. Since it is an auto-transformer, you will need a 1:1 isolation transformer as a power source for the variac for your safety. The output of the variac can be adjusted from zero to maximum depending on the input voltage taken from the isolation transformer. By connecting each winding and taking /noting each voltage readings, you can tell whether the windings are in series when a higher voltage is obtained. The opposite will be an indication of series opposing if no increase in voltage readings is noted. A center tapped winding will indicate a doubling of the voltage reading. Remember to complete this first phase of this test without moving or changing your reference point or the common terminal of your tester. Remember to label all windings, this will help to avoid any confusions in your readings.

My 2 cents is that it is better to keep your project simple. Throwing a lot of relatively high tech solutions at it seems to me to be a bunch of work and expense.

My disclaimer is that I am assuming a thin wire that heats up to spec with single digit amps.

Here is what I would do:

Determine the amperage required to maintain the wire at desired temperature with a low voltage, say 12 or 24 volts. ( I would use 12 ) Use of a rheostat is OK for this. While you're at it, get a resistance of the wire when hot and when cold.

DC or AC doesn't matter, I suppose, but if you go DC, a battery charger would make a good power supply... already isolated, voltage regulated, and cheap.

Do the calculations and put a suitable thermister in series with your wire and I think you will have nice regulated temperature.