Ok so here is the deal for anyone that didnt catch it, why I insist on DC
4 DC heating control, now I know this is where im gonna get flack but I have three
reasons for this #1 Higher frequency than 60Hz, so the heater output can be adujsted
just as fast as the ADC can read the sensor #2 Lately I have been doing a lot of work
to my 96 4runner and plan on replacing the ECU with a homemade modified version of
Speeduino ECU, working all those harnesses and adding a CAN bus will be much easier
if I have a GOOD iron that I can easily run straight from the car battery without inverter! #3 I
just understand a lot more about DC and digital control than I do when it comes to
controlling triacs and and reading AC with a micro.
And to add to that I have already wired and built the transformers as 1 along with soldering T and molex connectors on there in and out, so I can easily plug the station to a car battery, I also built a
4 pole PI filter with 6800uf caps and homemade 5mH inductor/chokes with hundreds of wraps around #26 iron cores in order to get the dc's peak to peak ripple down to less than 100mv at 24v with a 4amp pulsed draw. Im not willing to ditch the over engineering after all that time spent modeling filters, choosing/crimping connectors, and wrapping torroids. So figuring out how to get the heat down is really the only choice..
As I said due to size/orientation in enclosure I stuck two 24V 50VA transformers in parallel instead of just using the 100VA I have on hand. I physically bonded them together by using JB Weld the left side of one transformers El core and sticking it to the right side of the other transformers El core. I did this not only because it made things easier to handle but because JB Weld is killer for heat transfer and I figured it would help too even the heat out between the two transformers when/if one is working harder than the other, I just pulled one of the transformers out of my Atten and the other out of a broke down Yihue someone gave me, they were both fused at two amps, they look identical (except the labels) and are the same physical size, but they do not have matching primary/secondary resistances. I cant find a data sheet ANYWHERE for either of them (big surprise) so I do not know there heat ratings. All I know is that when I hook up a 6ohm load and draw 4 amps, I get exactly 24 volts of rectified DC, using a scope to measure the secondary AC before rectifier shows no unexpected harmonics as it would if the transformers were saturated, lastly the transformers under full 4 amp load seem to top the cores temp at 100c which is 80c over ambient and im not sure if I should worry about that and if so what temperature should I be shooting for too make sure im not wearing them down early, or worse running them on the edge of melt down.
@unitedatoms
You mentioned a resistance to temperature table for the thermsistor, I actually don't have one of those and figured that I would end up having to manually make one using a DMM to measure the PTC's resistance while using a thermo-couple to measure the heater temp. As far as I know a thermsistor be it PTC or NTC does not have a linear ohms to temp relationship that can be extrapolated unlike a thermocouple where the voltage increases linearly with temperature. This usually isn't a huge deal if you just order a PTC with a data sheet, but as far as I can tell hakko does not release the curve of PTC used in there heaters. I could and am hopefully wrong about this, as I haven't done much research beyond reading Hakko literature I can find which has gotten me nowhere. Im just not at the code point yet so it hasn't been a pressing issue, if you have any info on the thermsistors used in hakko a1321 and a1560 heaters, or know where I can find it, I would be grateful it will save a lot of time.
concerning current draw, im not sure if you understand the problem correctly or if I dont understand your advice, Im not sure what you mean when you say I should just be able to use ohms law to figure it out.
The problem is the transformers are rated at 24V 4A, when wired in parallel, if I connect a load under 6 ohms which will draw 4 amps, such as a 3 ohm Hakko Heater which can draw 8 amps according to ohms law, the transformers saturate and the DC voltage then droops down to about 17V, because the Hakko heater is drawing close to 8 amps the transformers where never intended to supply. As I said in my first post a Hakko a1321 heaters resistance is between 3 and 3.5 ohms and I want to support other restive heaters with various specs, so i never know the size of the loa d plugged in to the iron port could be 1ohm, could be 8ohms... I need a way to make sure that whatever the resistance of the heater is is never allowed to draw more than 4amps@24v.
The only ideas I have is to use a 2n3055 and a current sense resistor with switching diodes right after the PI filter as a pass transistor. This seems like a simple bullet proof method to keep the transformers out of saturation. The issue is I would assume the 2n3055 will pump a lot of heat, futher adding to the issue of keeping the enclosure temps down.
Secondly I could use a shunt resistor or a non contact pulsed dc current sensor to directly measure the current the pid loop is supplying to the heater and if it hits 4amps, use code to keep the pid in bounds. As with the solution above this is pretty close to bullet proof no matter what is shoved in to the stations iron connector.
Lastly, and worst of all the ideas(in my opinion). I could just hook my DMM in 10A mode between the pwm/fet output and the heater, and step the duty cycle with the micro until i hit 4A on my DMM. So, say the heater draws 4amps when pwm'ed at a 70% duty cycle, I can now write a profile for that heater that never allows the duty cycle to be greater than 70%. This seems like a real PITA if I want to support multiple heater types though, even worse heaters of the same type are not the same resistance only with in there spec. Also seems like there potential for a bug in the code to screw everything up.
Lastly using ohms law, I could measure the resistance of the attached heater and then adjust the voltage supplied to it to the appropriate level to make sure 100% duty cycle means 4 amps. This way sounds a bit overly complicated though. Im not really sure how I could dynamically raise and lower the voltage of the mosfets pwm output at 100 watts?
I am open to any advice an ideas, they dont include driving the iron handles with mains AC. Im assuming to support an fx100 induction style handle (when i can pop $100 on one, plus tips), I will have to build an some type of high wattage 13.5mhz AC inverter to get good results, unless pulsed DC works fine, but I'm never that lucky!