28vac transforer to 24vac possible?

[shanezampire]

Is there a simple way, device, or control board to drop a 28vac transformer down to a 24vac?

I have a 28vac transformer I thought about using on a Hakko 936 control board which only requires 24vac.

It seems to be very easy to convert it to DC of any voltage using step down, buck converters, etc. I just cant find anything on AC short of taking some winding off which I really don't want to do.

[bob91343]

Well, it depends.  If you don't care about waveform or efficiency you can put a pair of zener diodes in series.  A series resistor works in case the load current is constant.  Even a stack of rectifiers of opposite poarity in series, paralleled by a similar stack.

I came up with an interesting approach some years ago that worked well.  It was a little transistor regulator that was connected in series with an ac load via a rectifier bridge.  The bridge assured that the transistor always had the right polarity regardless of the polarity of the source.  You can bias the transistor with a zener diode and thus get a constant voltage drop.

One way might be to modify your transformer either by removing secondary turns, or adding primary turns, or something similar.  Or adding secondary turns in opposite phase.

There are dozens of ways to do this, depending on your specific requirements.

[Brumby]

One way might be to modify your transformer either by removing secondary turns, or adding primary turns, or something similar.

If your transformer is of the traditional E-I core type, then this is not an exercise I would suggest unless you were really, really desperate.  Even then, removing secondary turns would likely be the only practical option since the primary is often wound on the bobbin first, with the secondary wound over the top.

If it were a toroidal transformer, then adding primary turns would be a possible option - just wrap another winding around the outside and put it in series (correctly phased) with the existing primary.  However, the number of turns required could be significant plus the fact that you would be playing around at mains potential would be enough to turn me off that idea.

Or adding secondary turns in opposite phase.

If it were a toroidal, this would be my choice.  No need to disturb the existing windings, plus secondary turns would be fewer and you're keeping away from the mains.  Removing secondary turns from a toroidal would probably be possible if you were OK with some surgery (hopefully minor).

[GromBeestje]

Another question. What are the specifications of the control board? How strict is the 24 VAC requirement?

[bd139]

The lazy way to do this is usually stick a fat resistor in series with the output. This makes it load dependent but works for most cases.

[Shock]

What about dual series capacitive droppers on the secondary with a zener?

[MagicSmoker]

Is there a simple way, device, or control board to drop a 28vac transformer down to a 24vac?

Yes, a very simple way to do this is with a bucking transformer. Basically, you wire another transformer up so that it's secondary opposes either the primary or secondary of the first.

But this requires a second transformer so if you have to buy it then why not just get the correct (and far more common) 24VAC transformer instead?

EDIT - found a good link that describes various schemes to drop AC mains voltage by a small amount, including the bucking transformer: http://sound.whsites.net/articles/buck-xfmr.htm

Lots of other good stuff on that website, too.

[ArthurDent]

As usual there isn’t enough direct information given to make a good recommendation on what should be done.  The idea is to use this 28 volt transformer with a Hakko 936 control board that uses a 24 volt transformer. Anyone wanting to give informed hints has to do some research to check the schematic for the board and figure out how it works.

The A.C. into the board goes through a triac to the 48 ohm heating element so maybe around ½ amp load max. When calling for heat with a 28 volt transformer, the element could see the full 28 volts instead of the designed 24 volts A.C., which could shorten the life of the iron heater, so that is a bad idea. The A.C. is also rectified and has 2 zener diodes to give a split supply so the regulators will have to dissipate more energy as heat. This might give too much current for the zeners but a series resistor change could correct this. The big deal would be the overvoltage to the heater which could cause temperature overshoot on warmup and also drastically shorten heater life.

So the bottom line is the best way to replace the transformer for this soldering station would be to use the correct transformer and not try to be cheap which will end up costing more money in the long run, and probably affect the iron in other ways as well.

[ledtester]

Here's a PDF of the schematic (found at http://dalincom.ru/datasheet/HAKKO_936_schematic.pdf )

[mariush]

How did you actually measure the voltage, did you measure 28v AC simply with your multimeter probes on the output?

Most transformers will output slightly higher with no load connected to them but as soon as there some small load connected to them, the voltage will drop....
Also, isn't there some tolerance on the heater element? I would imagine the heater element would just heat up much faster, so the station will just turn on and off the triac more often. 

[bob91343]

All good comments.  The distillation is:

1. Get the correct transformer.
2. Use a dropping resistor.
3. Take a chance and use excessive voltage.
4. Modify the new transformer.

Any of these three will work with varying degrees of success.

[shanezampire]

Well my project got complicated. I had an OK Industries SA-1001 missing the handle and they are hard to find and cost a ton. It was "female" 6 pin. I ordered a 6 pin Hakko 8001 6 pin handle which is a male plug. The pin lay out is totally backwards and nothing like like the Hakko. Ground is like pin one or two on the OKI, heater like 3 and 6 .. just odd and weird than standard. I jumper-ed them correctly and the transformer/iron works fine. The board of the OKI is either fried or don't work right. It will not control the temperature. Its full power/temp.

I said heck with it and ordered a T12 control board, handle, buck converters, and all I need to hook the T12 to the 28v transformer.

Now I have decided to just buy a large project box, a 24v 4.5 amp power supply for the T12 kit, which is a "male" 5 plug. That will cover the T12 handles.

Buy a 936 control board which uses a 5 or 6 pin "female" plug and will accept various handles. I have checked the lay out of various pin outs and resistance on heaters/sensors on Hakko style handles.

So in the end I will solve my problem by having a box with a plug and control board to control which ever handle I find cheap or have (Hakko style that is)

Messing with this OKI station... now I already have a collection of several style Hakko handles, extra tips, with female/male plugs goofing off with this OKI station.

So I already have this good 28v transformer. I hate to just buy another one and spend more $ on a new 24v one > https://imgur.com/a/ixcBOFu

If they are a simple and cheap way to make it work with that 936 board I ordered.

The 936 board I bought is this one - https://www.ebay.com/itm/A1321-For-HAKKO-936-Soldering-Iron-Control-Board-Controller-Station-Thermostat/112719961476

I already have spent a lot of $ trying to make this OKI work but I have learnt (especially reading here at EEV and messing around) a lot so in the end it is all worth it.


mariush: I got the voltage off the transformer 14/28  unloaded it measures 16 / 33 volts VAC   about 20-ish one side VDC through a bridge rectifier and smoothing capacitor unloaded

[shanezampire]

So from what I gather is a resistor will be the simplest.

The transformer is a - A41-80-28A33 is made by Pulse 28VCT 2.8A or 14VAC 5.6A 80VA

The board above says I need - AC adapter 24V 3A transformer OR HAKKO 936 Iron Fist compatible A1321 Central heating Characteristics


Online calculators says if I use a 1.42 Ohm resister for 2.8 amp draw or 1.33 Ohm resister for 3 amp draw that will reduce the voltage to 24v - Minimum Resistor Wattage 12

So If I order a 1.5 Ohm 25 - 100 watt resistor I should be just where I need to be .. Correct?

[shanezampire]

Scratch the above. I just found a 1.5 Ohm resister and tried that. Unloaded voltage from transformer was 33v. I hooked a Hakko FX-8001 handle to it (straight to the heating element) and the voltage dropped to 30v. I then attached the 1.5 Ohm resister and the voltage only dropped to 27v

[bob91343]

So the current draw is less than you thought.  You can play with the resistor value until it's where you want it but it really isn't critical, other than possible overvoltage to the control board.

[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]

...
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.
...

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]

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

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]

... why not switch to a capacitor dropper,it wont get hot like a resistor.

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.

[shanezampire]

The board I ordered had a BT137 TRIAC I think.. if that makes any difference. The control boards with the BTA06 didn't have the screw down photometer to make it easier to attach firmly to the front case cover reason I picked that board.

[Zero999]

Have you considered it might work of 28V and there's no point in reducing the voltage?