Needed: Electron-Minded Guru

I wonder if the resistance in the dimmer switch on the secondary of the transformer is much too high. It is designed to give a smooth range with a

110 volt input. With 6.3 volts out of half of the secondary, it might act more like an off/on switch.

Using a 3 volt secondary transformer just halves the voltage to the nichrome wire. That may be more in the range of what it really needs. You might mess around with a resistor with a slider, moving the slider until you get the heat that you want and then lock the slider in position. It would need to be a fairly high wattage resistor but without knowing something about the current flow in the secondary I can't begin to give you a starting point.

There is one other possibility, both the dimmers you tried might be bad. Not likely but maybe. Do you have any measuring equipment?

Harry

Bill,

Is the diac part of the dimmer switch? I assumed, probably incorrectly, that the dimmer was just a variable resistance.

Bill, I was with you until you got to "secondary." :)

Let me translate. Those dimmers are designed to work on 120 v, not 3 v, not 6 v and not 12 v.

They are not variable resistors because if they were they would develop heat (lots of it) when you are running dimmed. The energy which would not be used by the bulb (or tip in this case) has to go somewhere and heat in the 'variable resistor' is the result.

The way these dimmers work is to chop the ac. They are either on or off and not something in the middle except for as short a time as possible. A fully off switch generates no heat. Same for one fully on (I won't bore with details here but it does seem intuitive, I think). Think about a wall switch which should never be hot.

They switch on and off quickly in such a way as to control the AVERAGE voltage the bulb (or tip) sees. If you have one in your house which causes problems with AM radio -- that is why. The fast switching generates hash which feeds back through the wiring in your house. Better dimmers have logic to control radio interference but I'd rather not go into how it is done -- it is beside the point.

These things just won't work with a low voltage input. I don't see how the circuit could possibly have worked as shown. It seems that you should do away with the dimmer in the tip circuit and use only the other one -- the one on the ac line. But, the transformer would have to be sized so that when you turn the dimmer all the way up, you won't burn out the tip.

Hope that this helps...

Bill

Since I have only a very dim idea of what you guys are talkng about, of course, I must jump in. For those of you who share my ignorance this might brighten the light a little bit. Arch

Fortiter,

:(

Hmmmm. I tried placing the dimmer before the transformer and it just wasn't variable enough at the tip. It seemed to be off until the dimmer was almost at full blast - then it was red hot over all the nichrome.

Are there any run-of-the-mill household items that can be scavanged for necessary parts or modified slightly to work - like a model train transformer or something like that?

I work in an industry where we use the commercial version of this for micro-soldering. They are available with tweezers or pliers from Micro-Mark, although we use a very small 'wand' for soldering under a microscope. The devices are pretty expensive to purchase, hence the DIY. I'm curious... what do you use for the 'wand'? i.e. how do you attach the nichrome wire?

Let's try this again:

1. I use a surplus hot wire type wire stripper transformer for powering my nichrome wire. I've measured voltage, it's about 2 volts, closer to 1V at the tip. current is several amps, don't remember exact value.

1. if you dont' have access to surplus stuff, you can use a power supply from a dead PC - the later ones have a 3.3 V output (as well as 5 and 12 volts). You can adjust the heat at the tip by shortening or lenghthening the wire (more/less resistance), or by switching a diode into the circuit (0.7V forward drop for silicon)

Owen, about the note on the second page: Let's say your nichrome wire tip has a given resistance R. Power (which leads to heat in the tip) is P = V^2/R

Assuming your controler output supplies the desired voltage whatever the load is, the power with a 12.6V output is ~18 times higher than with a 3V transformer. A Nichrome tip resistance is in the range of 0.2-0.3 ohms. which gives a power of 530 Watts. obviously too high, so you need a way to adjust it (thus the dimmer idea, but it doesn't work on secondary side as explained above) with 3V transformer, the power is reduced to 30W, which is much more reasonnable, and could eliminate the need for the second dimmer (after the transformer), and even the first one. (though I feel it would still be a little bit too hot)

As William says, a PWM is a good solution (as Bill explains, it switches the output on and off to the tip) I posted a while back how to make a woodburning controler on Woodcentral, from an old computer PSU. I've put all the instructions on my website at

the left menu, look for woodburner in the tool section. you could adapt the 555-based output stage to the output of yourtransformer.you need a bit of knowledge in electronics, but not that much. takecare when working with stuff connected to the mains.hth, Pascal

Why not just sign the bottom of your work with an actual woodburning pen? I've been doing that for years, I have three different ones and a simple dimmer switch CAN be used to control the heat on a woodburning pen since it is a resistive load unlike a transformer. And woodburning tools can be had at any price from 10 buck and up, and even the cheaper ones come with two or three different styles of tips and many others can be had at very reasonable prices. I have a dimmer in a square electrical box beside receptacles with a double pole switch in the circuit so I can choose either left or right burner with different tips on each and also a 600 watt dimmer in line so that I can cut the heat back on the tip I'm using at the time. Some woods such as butternut and basswood burn very easily and require less heat, and this is a very easy to build and useful tool. Just a thought. Like the others have mentioned, dimmers only work on 110vac. You could very easily put a hefty diode in the incoming AC line on the 110 side OR a diode in the

6.3 output side of the transformer which will cut the voltage from 6.3 to roughly 3 volts, the voltage recommend > I'm attempting to build my own nichrome wire burner - mainly for signing

One idea...

Normal lamp dimmers only work for resistive loads (light bulbs) and you are trying to use one for an inductive load (transformer). However they make a special dimmer which is designed for low volage halogen lighting which is fed via a transformer. I had tried using a resistive dimmer with a halogen lamp and it definately did not work. I bought one of the "low voltage" dimmers and it worker perfectly (with the lamps).... The thing to remember, although they call it low voltage, it is designed to work on the mains side of the transformer (110 v side). It will take the mains down to almost nothing and provide you with an output waveform which can be "transformed".... A normal dimmer provides a kind of chopped DC voltage which will not work in any inductive circuit...

Hope this helps.. Cheers from Scotland Glen

====================================== A chopped DC fed into a transformer will give an AC output at the same frequency as the chopped waveform. This is the way many power supplies work. An AC input is converted via full wave rectifier to DC. The DC is chopped at frequency above the standard 60 HZ (50 HZ in UK, etc.). This is transformed to various voltage levels for internal equipment use. This way is more efficient since the higher frequencies are coupled using smaller transformers and filtered with smaller capacitors.

Ken Moon Webberville, TX

I was sent a private email response to my quandary suggesting the use of

500 ohm, 1 watt potentiometers in place of the dimmers. Not being an electron-minded guru myself would anyone like to comment on this rec?

Here's the second PDF from the FishCarver.com website about the handset construction. I've built all but the hollow tube for the "pen" section just so that I could test the main control box.

Using Ohm's law (I=E/R)

110 volts / 500 Ohms = .22 Amps = 220 milliAmps

The current through that 500 ohm resistor at 110 volts is about 220 milliAmps. Then calulating the power dissipation using P=EI;

110 volts X .22 Amps = 24.2 Watts

The 1 watt potentiometer would burn up.

If I'm understanding correctly, you're only applying the formula to the input side of the transformer. If I keep the 120v toggle on the line side and put the potentiometer on the transformer output...

6.3v/500ohms = .0126 amps = 12.6 milliAmps 6.3v X .0126 Amps = .08 Watts

(or, using the full transformer output>

12.6v/500=.0252 Amps 12.6v X .0252 Amps = .32 Watts)

Wouldn't the pot work on the transformer output side at either full or half total output? (Looking at Radio Shack, it appears they only have

5K, 10K, 50K, 100K & 1M Ohm pots, all with a .5 watt rating. They also come in "linear" and "audio" taper... Would one of these work in the above location and if so, which one?)