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28vac transforer to 24vac possible?

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shanezampire:
I have fooled with electronics all my life and have boxes full of junk but stuff like this I really am a newbie. I am learning A LOT as I go and from the people here at EEVblog. I really really appreciate everyone taking their time to reply and try and help me.

And yea, from room temperature... I cut the iron on and it draws over 5 amp and steady drops to about 1.7 amp as it is fully heated. I just tried a 1.5 Ohm connected to a 2.2 Ohm to the iron and it was around 20-ish volts and climbed to 23.9 - 24v on the dot when it fully heated. I had a 15w and 5w ceramic resister on hand and the 5w got super hot! I had to attach it to a heat sink just to do a quick 3 min test. Would for sure need one them big 100w aluminum resisters for the heat.

So that part seems to keep the voltage right but if I power the board without the iron connected the unloaded voltage will still be sending 33v to the board. I just realized that.

Thanks again for everyone sharing their knowledge with me! Google and searching this forum I can find answers to most my questions and problems, but not all.

MagicSmoker:

--- Quote from: shanezampire on July 07, 2019, 05:47:54 am ---...
And yea, from room temperature... I cut the iron on and it draws over 5 amp and steady drops to about 1.7 amp as it is fully heated. I just tried a 1.5 Ohm connected to a 2.2 Ohm to the iron and it was around 20-ish volts and climbed to 23.9 - 24v on the dot when it fully heated. I had a 15w and 5w ceramic resister on hand and the 5w got super hot! I had to attach it to a heat sink just to do a quick 3 min test. Would for sure need one them big 100w aluminum resisters for the heat.
...
--- End quote ---

Well, now you know why I recommended a bucking transformer instead of agreeing with the earlier suggestion to use a resistor: too much power lost in the resistor and this iron does not draw a fixed current (due to closed-loop temperature control), so a fixed resistor would give the wrong voltage most of the time.

That said, this iron regulates tip temperature by varying the firing angle of a triac (ie - phase control) so using a transformer a few volts higher than spec'ed won't hurt. The power supply for the op-amps is semi-regulated down to +/-6.2V using zeners so they won't care about a few extra volts, either. Finally, the bulk filter capacitor has a 60V rating (? - common values are 50V or 63V), so it will also tolerate a higher transformer voltage.

All in all, it looks safe to simply use the 28VAC (loaded voltage) transformer.

themadhippy:
Now you know the resistance  value you need why not switch to a capacitor dropper,it wont get hot like a resistor.

shanezampire:

--- Quote from: themadhippy on July 07, 2019, 12:14:16 pm ---Now you know the resistance  value you need why not switch to a capacitor dropper,it wont get hot like a resistor.

--- End quote ---

I thought about and was going to try that route but I am not smart enough to figure it out. I watched several YT videos on dropping capacitors and used several different online dropping capacitor calculators and I kept coming up with needing a 716uf dropping capacitor which appears they do not come no where near that. All are decimal (point) value. So I am not doing something right.

2 x pie = 6.28 x 60hz frequency x 716uf capacitor = gives you 3.7 ohms resistance

floobydust:

--- Quote from: themadhippy on July 07, 2019, 12:14:16 pm ---... why not switch to a capacitor dropper,it wont get hot like a resistor.

--- End quote ---
A capacitive dropper is not practical at this power level because the capacitor ends up being huge and the power factor and inrush current is terrible.

The heater does not see the full transformer voltage. You lose ~1.3V in the BTA06 TRIAC and a bit more in the cable/connector to the wand, say 1.5V and that corresponds to ~1VRMS.
Hakko 888 station power transformer is rated 26VAC, label says "23.2-24.9V 3.2A" and the PCB labeling "26VAC".
So I think adding a small resistor to drop a few volts at full current/cold heater (lowest resistance) would make the transformer usable with Hakko heating elements. You don't want to hit a cold heater with too much current.

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