Transformers
Current (Amperage or A)
Current is the quantity, volume or intensity of electrical flow.
Voltage (Volts or V)
Voltage is the electrical �pressure� or force which causes current to flow through an electrical conductor.
Resistance (Ohms or R)
Just like it sounds, resistance �impedes� or holds back the electricity.
Power (Watts or W)
This is the voltage multiplied by the amperage. Basically it�s how much energy or work that can be preformed.
These four pieces are always in direct relation to each other no matter what. This is an undeniable fact. This is called Ohm�s Law and is expresses in simple mathematical terms below:
------ = 1
A R
P = A V
One of the easiest ways of obtaining this kind of power is with a �step down� transformer. The transformer will do two things. It will provide the high current needed and also provide a �limit� to the amount of current available depending on how many watts it can produce. This will ultimately lead to the maximum rating for the unit. For example, the system I built will provide approximately 50 amps (current) at about 4 volts. Using the above law, P=AV this equals 200 watts (50amps x 4volts = 200watts). This is all the transformer can supply when an extremely low resistance load (the joint to be soldered) is applied to it. Why is there a limit to the supply? The windings in the transformer cannot produce any more current. How low of resistance? Well, going back to Ohm�s law V/AR or 4volts/50amps x R. R=.08ohms. The low resistance allows a high current pass through causing the joint to heat up.
Basically the transformer should be capable of delivering about 3vac @ 75amps. This will equate to be about 225watts. More than enough power to do most jobs the hobbyist or model builder will encounter. Besides, this type of soldering lends itself more so to precision than brute force.
One option is to find a transformer specifically designed to provide a high current and low voltage. If you can find an old TV (the ones with the tubes) they usually had a transformer capable of the values we are looking for but I haven�t come across one in years.
Another way to go is to get a transformer custom wound either by your self or at a motor rewind shop. I have read about people going this route but I have done this as of yet.
I took a different approach to my unit since I wanted to use what was readily available. The transformer in my system is an 110vac to 12vac step down transformer rated for 15 amps. So once again, going back to our old friend Mr. Ohm and his law, this works out to be�180watts (12v x 15a). So the transformer is capable of producing this kind of wattage but the voltage is too high and the current is to low. Now think about resistance soldering in the whole. All that is happening is the �shorting out� of the secondary windings of a transformer, the �short� being the low resistance load applied which is the joint to be soldered. Now since power, amperage, voltage and resistance are all in direct relation to each other lets see what happens�
The maximum output power of the transformer = 180watts
The maximum output voltage of the transformer = 12volts
The maximum output amperage of the transformers = 15amps
Now using Ohm�s law: 180watts = 12volts x 15amps 12volts / 15amps x R R = .8ohms
So we can apply an .8ohm load and still be within the specifications of the transformer. So this means the unit can supply this for 100% duty cycle. Remember that.
Now let�s try and solder with it. If a low resistance is applied (the solder joint), what happens?
The maximum output power of the transformer will still be about 180watts because of the physical limitations of its internal windings. So if the power output won�t change, what does??
Now this is where this little beauty comes in handy:
Ohm's Law Calculator
If we input our 2 known values, 180 watts and .08 ohms, the Ohms Law Calculator computes the amperage at about 47amps and the voltage at about 3.8volts. Exactly what we need to solder! When such a �heavy� load or �short� is applied (like a solder joint) to the circuit, the voltage drops and the current rises. The transformer supplies all the current it can and to compensate the voltage drops.
Now here is the compromise�
We are asking all we can from this transformer. This is all it can do, well out of what the manufacture designed it for. The transformer will supply the power in this way but not continually. This is just an estimate but I think realistically the transformer should be rated for a 25% duty cycle. That�s to say of every minute it energized, it should cool or rest for 4. With out letting the unit cool down, the internal windings will heat up. As they heat up, that actual current it can supply will drop. The hotter it gets, the less it can perform and if it gets too hot the windings will either short out against each other to short to ground destroying the unit. This type of duty cycle works very well for resistance soldering as most cycles last only for a few seconds at the most! I suspect a transformer, say outta� an old TV or a custom wound one wouldn�t need such a duty cycle but I believe this is overkill for our application.
One side note� The transformer can and will produce the amperage needed (about 50 amps), but a normal household outlet is only rated for 15 amps. The power available will be again, amperage x voltage, 15amps x 110vac or 1650 watts conning out of the wall. This is more than enough to power the transformer. One thing I discovered is when on full power, say 200watts, the actual amperage the unit is drawing is about 4 amps. 4 amps @ 110vac is 440 watts. The unit is drawing almost double to what it is supplying. This loss (240watts) must be due to the fact that the transformer becomes less efficient at these high rates of current. Since heat will be generated, I�m cooling it with an inboard fan. This may not be necessary but I felt it would add to the duty cycle.
In conclusion, no matter which way you go, try and determine the available wattage of the transformer. If no data is printed on the unit the best way would be to, determine the primary windings, apply the mains voltage then measure the secondary windings (the low voltage side). If you are in range and the voltage is low enough, then the best way to figure the current output is by the physical size. Remember, the wattage is in direct proportion to the current it can supply being limited the secondary windings. The bigger the windings, the more current it can produce, the more current, the more available wattage. The only realistic way to do this with out ruining the transformer, is to look at the size of the unit. It should be rather hefty and well built