Hakko 936 clone, run on DC?

[step_s]

Hi guys.

Just bought the individual parts for the Hakko 936 clone off of eBay, just for the heck of it.
The controller board normaly runs on 24V AC, but I only have DC supplies atm.
Looking at the schematics:
http://dangerousprototypes.com/forum/download/file.php?id=4702&mode=view
The only thing that seems to run directly from the AC is the iron. The rest of the circuit is run after a diode and a 220µF cap, so DC.

So what I thought was, that changing the TRIAC to a N-MOS, sitting on the low side instead of how the TRIAC is now on "highside".
Would this work?

Thanks in advance!

[Zero999]

Why not order a 24VAC transformer? They're not difficult to get hold of.

If you do decide to modify it, you'll need to remove the zero voltage triggering IC. I think it would be easier to use a high side switch (a P-MOSFET or BJT) with an NPN level shifter.

[james_s]

Surely finding a suitable transformer would be easier than the extensive modifications to the circuit? 12V transformers may be easier to find and you can use two in series.

[Strada916]

Few advantages for using AC. Triac doesn't need a heat sink.

Thinner wires for the same current.
No or little volt drop to the element.
The element will last longer due to it going to 0V every cycle. (In theory)


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[amyk]

The triac should be heatsinked, genuine Hakko and some clones do.

The genuine and clone circuits depend on AC for proportional phase-angle control, like a light dimmer.

[Zero999]

Few advantages for using AC. Triac doesn't need a heat sink.

Thinner wires for the same current.
No or little volt drop to the element.

No, it won't make any difference. The AC voltage is given in RMS and the power dissipated in the wire is exactly the same for the same current and power in the load. In fact, with AC the losses will be slightly higher due to the skin effect.

The element will last longer due to it going to 0V every cycle. (In theory)

At higher temperatures, such as in an incandescent lamp, it does make a difference but it's negligible at such low temperatures.

[step_s]

Thanks everyone for your answers

"Why not order a 24VAC transformer? They're not difficult to get hold of."

Couldn't really find any 24V 50W on eBay, for less than 10£ with shipping, making it more than twice as expensive as the 24V DC models.
The point of the project was really to cut the cost a bit, even though it's already absurdly cheap xD Maybe I'm searching for the wrong thing?

"If you do decide to modify it, you'll need to remove the zero voltage triggering IC."

Any suggestions on how to make the circuit switch on and off  like normal, without this component?

[Zero999]

Thanks everyone for your answers

"Why not order a 24VAC transformer? They're not difficult to get hold of."

Couldn't really find any 24V 50W on eBay, for less than 10£ with shipping, making it more than twice as expensive as the 24V DC models.
The point of the project was really to cut the cost a bit, even though it's already absurdly cheap xD Maybe I'm searching for the wrong thing?

Too cheap. A 50VA transformer will cost more than that to make, let alone ship, as they're quite heavy. You might be able to get a second hand one for collection, at that price, if you're lucky.

"If you do decide to modify it, you'll need to remove the zero voltage triggering IC."

Any suggestions on how to make the circuit switch on and off  like normal, without this component?

The µPC1701C is a zero voltage triggering IC. It only allows the TRIAC to be triggered, when the AC voltage is near zero. It has two comparator inputs, one output directly connected to the comparator, and another output to trigger the TRIAC. See the datasheet linked below:
https://www.eevblog.com/forum/beginners/current-controlled-device-vs-voltage-controlled/

The µPC1701C could be replaced with a comparator IC such as the LM311 or LM393 and the TRIAC with a transistor to switch the soldering iron on and off. It's not a straightforward swapping of components. The circuit will need to be modified considerably. Tracks on the board will need to be cut, bodge wires and other components inserted and secured with an adhesive. You will need to understand how the circuit works and partly redesign it.

[Paul Price]

You guys are overthinking the problem. All a 936 station is just a circuit applying voltage to a low-resistance heater (a RESISTOR) and using feedback to control the power from a linear positive temperature coefficient sensor already conveniently installed in the handle.

Here's how I handle it: Make the idea of turning a few bucks of parts into a fun, software and hardware learning project, and ending up with a nice soldering station.

First requirement: Heat the heating element:
 I have several orphaned laptop 18-21V 2.5Amp to 3.5 Amp rated power supplies sitting around and several old desktop PC power supplies and a lot of MCU chips. For power I decided to use two bricks in series, mounted underneath my work bench to be out of the way.

Heating problem solved:

I use two laptop power supplies in series to develop about 37V DC at 2-3.5 amps.  I  make a simple step-down converter to convert the 37V DC to approx. 30V DC. I do this using a PIC MCU PWM output and so I can directly control the power applied to the heating element by varying the PWM duty cycle of the buck convertor from 0 to 100%.  A single NMOS (30N05), an inductor, a catch diode, a 18V zener, a small signal(1N4184 type) diode and a few small-signal transistors, a 78L05 regulator, a few resistors, and a few free capacitors, a couple LEDs, an Arduino or PIC MCU, a little slice of perfboard, and bingo, a precise, high-power buck converter/Lo-Side Switch/soldering station.

Using the bottom 18V brick of the two bricks in series, I can use this tap output to power a 78L05 regulator to provide regulated power and a 5V reference voltage for the MCU chip. Using a single transistor and a zener diode or LED and three resistors I create a precision and stable constant current source to drive the thermistor temperature sensor in the handle.

By biasing the positive coefficient thermistor with a small and constant current,  you feed the soldering tip temperature sensor signal directly into an A2D input on any MCU. By then using the just about any potentiometer connected to the 5V supply and to an A2D input of the MCU (and conveniently mounted on a small case serving as the front panel that can also be used to mount a solder handle rest and a sponge and an on/off sw.), you then have a good-looking functional temperature control that controls the temperature of the tip with a tiny bit of software.

All that is needed now is very simple software code:
If the the iron is cold, increase the PWM drive to the tip slowly (It should take approx 15- 30 seconds to reach soldering temperature from the tip at room temperature while not overloading the power supply bricks.)

Once the tip approaches the setpoint,  the software code reduces the PWM to proportionally approach the required tip temperature without overshoot.  A simple proportional heating method is not so difficult to code nor does the code have to very complicated to accomplish this.

Once the tip temperature is reached, PWM duty cycle reduces to zero and increases again as the tip cools by the soldering operation or just cooling off by ambient air.

Here's where the used PC power supply comes in. A large 50W toroidial inductor is needed for the buck converter and it is already found and already wound and ready to use inside any PC power supply. The PC power supply also is the source of free high-current schottky diodes to connect the drain of the N-MOSFET  to act as the required catch diode for the low-side sw/buck converter and the PC power supply also provides for free the small heatsink ans well as the output filtering capacitors for the buck convertor and control MCU as well.
 
Since a MCU PWM output can be controlled in very tiny duty cycle steps, it is easy to adjust duty cycle to match a wide range of inductance to form a buck converter, since PWM can be controlled in 0 to 1023 steps with a fixed 20KHz to 40KHz PWM frequency.
 
Since I have approx 38-V available from the two laptop bricks in series, I already have a convenient bias supply to drive the N-MOSFET gate. I just need voltage translation to control the MOSFET buck low-side switch.

I use a grounded base BJT transistor (2N3904) whose emitter is connected to a MCU I/O pin and a 470-ohm resistor to ground. The collector connects to the common bases of a NPN-PNP complementary pair (2N3904 2N3906) that is used to drive the gate of the MOSFET.  The common-base BJT transistor's base is biased to 1.8V using a single LED and a resistor to the +5V.

A zener diode connected in series with a 1N4184 diode from the complementary transistor bases to the source of the N-MOSFET limits the gate to source voltage to <20V. The NPN-PNP complementary pair is biased with a single 100K resistor to the top of the brick pair output (approx 38-40V).

If you wanna little fancy, with just a little more code and  three 7-Seg LED displays, add three resistors and just directly connecting to the 8-pins of  a single output port on the MCU tied to the segments and three more I/O pins to drive the CA/CC's, and you have all you need to drive the MTPLX'd segments and common anodes/cathodes. The code needed is easy to add mtplx'd 7-seg digital readout of the temperature. The resistance v. temperature of the sensor in the handle can also be found on this board to eliminate calibration, but requires a little application of ohm's law to your MCU software code.

In fact, only a single LED on the "front panel" can show heat-on heat-off, ready to solder temp,  with only a little code from the MCU.

The fact is, even without any calibration or readout, there exists a precise setting of a control pot that will give you the right soldering temperature as a setpoint that compares to the handle sensor output . You will easily know this "nice" setting by seeing how the solder melts and solders as you slowly increase the temperature by adj. the pot which controls the balance point of this feedback loop controlling  the soldering tip's temperature.

[step_s]

Hi guys, sorry for the late reply, been busy with exams :/

@Hero999
Ye, it would be easier to start from scratch it seems. . .

@Paul
I see the idea of the buck converter, but since it's just a resistive heater element, wouldn't a simple PWM function be as good?
Using a 24V DC PSU, with a reasonable size N-MOS on low side, just making the MCU vary the duty-cycle according to the heat sensing resistor?
Don't know what frequency would work out the best, but this seems a lot more simple to me