Quote:
Originally Posted by cavorite
I wanted to toss in a brief suggestion to use Kapton (or similar) tape if you are messing about with transformer windings. It is an excellent insulator and can withstand fairly high temperatures that often are seen at either high amperage or voltage. Also very good for masking off PCBs when soldering/desoldering in a confined area.
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The link in the post above is to Kapton (tm) tape, only without the Dupont registered tradename.
That particular product uses a silicon adhesive good to 260c.
See datasheet attached.
Which brings to mind another safety note -
The primary winding (the 'mains', or the 'line') is going to be good old fashioned enameled copper.
Probably intended for a 40c rise in a 20c environment. I.E: 60c maximum.
So consider that (60c) your maximum transformer temperature, not the maximum working temperature of the hand wound secondary's insulation tape (260c).
Cooling:
Your wrapping the secondary with copper strip + Kapton tape on one side, only the "outside" end of the 'coil' is easily accessible.
When you reach your 'turns count' mechanically anchor the strip+tape (so it does not 'buzz' too much), drop the tape cover, bring the strip out an appropriate distance to the other (supported) wire terminal connection.
Now arrange a fan to blow air across the copper strip - I.E: the secondary coil's copper strip becomes its own cooling fin.
Losses:
Running a laser diode is, well, one diode drop (about 2.2volts - in general, think of the range 2v .. 3v) ;
The supply needs to be a constant current supply ;
A bridge rectifier circuit is 2 diode drops ;
An 'on' SCR has a significantly lower diode drop than even a Schottyt rectifier diode ;
So 'secondary control' is the low loss way to go at these high currents rather than the 'primary control' ideas you find in the on-net articles.
And you are talking high currents for laser diodes.
A typical 1W (output power) laser diode has a 2.2volt drop.
It would be nice if it only took 1/2 amp to drive it -
But laser diodes are only 10% .. 18% efficient -
In the case that I am looking at, it takes 5 amps (5 x 2.2 == 11 watts) to drive it to 1 watt output. The other 10 watts is wasted as heat that you have to remove from your laser diode.
A bit more:
So you decided on secondary control of the bridge rectifier (good move) ;
Low current SCRs (anything less than a few hundred amps is 'low current' for an SCR) will probably have an 'on' junction drop of 1.2 .. 1.8 volts.
Presume 1.8v here (because this is only a 'thought experiment').
So your voltage drop chain looks like:
1.8v + 2.2v + 1.8v == 5.8v
Your power loss (at 5 amps) looks like:
8 watts + 11 watts + 8 watts == 37 watts, 1 watt of light, 36 watts of heat.
Those are: Rect SCR + Laser + Rect. SCR voltage and power vectors.
Call it 40 watts consumed.
The transformer for an 800 watt microwave has enough iron in its core to support that flux density.
So if you could (you can't) just scale the above, then as a 'first guess' think of supporting a 20 watt output power laser diode.
Yeah, the other 780 watts is wasted as heat from something (including the transformer - one reason for the (you can't) above).
How many turns?
The bridge rectifier needs a center tapped winding, at least 6 volts per side - since this is an actively controlled bridge, 8 volts per side (no load) would be nice to have.
Transformers for microwave ovens are run at a flux density that amounts to about 0.8 .. 1.0 volts per turn.
Naturally you would make a small gauge wire test coil of a known turn count to check what your transformer is working at.
Err on the side of two many turns, you can always just 'dial back' the rectifier bridge controller -
0.8 volts per turn, 10 turns per side. I.E: a 20 turn, center tapped, secondary , OR
1.0 volts per turn, 8 turns per side. I.E: a 16 turn, center tapped, secondary.
Now you have a lot of the example ballpark figures -
All you need now is to get out the reference chart to figure out what the cross sectional area of the copper strip needs to be for a less than 40c rise, tightly packed, coil operating at the currents you need for your project (a 20watt output power laser diode might require 100 .. 140 amps drive - just about the limits for a microwave oven transformer core.
Way off topic - but more fun than making serial port connectors.
