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ZD-8915 (SS-331) conversion from 18V to 24V

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Some 19 months ago, I bought a ZD-8915. In a local store even, but it is still China stuff.
And at first, I was very happy with it. Still am, despite what I am about to write, would buy again.

But I started to get small annoyances. Being:

- Very loud FAN. I thought it was broken.
   Like many others, I replaced the fan nearly instantly, but that did not work. Below I explain why.

- Power switch on the back
   Has been moved to the front a while back.

- Extremely slow heatup time. 10 minutes to 400C (which is really only 350C)
   Main mod for this post.

- Temp is off by about 50C (edit: now also fixed)

- Bad sleep mode. Handling does not keep it awake, I have to hit something with the gun

- I may eventually also add a delayed vacuum valve, as I have seen others also do.

So, it turns out the noisy fan and the slow heat-up time are related. The problem is, it has a 24V heating element, a 12V air pump, 12V Fan; But a 18V power supply.

So putting a 24V 90W heating element on 18V yields only 50W. And indeed, checking consumption that is what it takes.
That is the slow heating explained.

Furthermore, the 18V is dropped with 2 ceramic 1R5W resistors to 12V for the airpump. Very wasteful.
BUT, the 12V cooler fan is on the same resistively dropped 12V rail as the Airpump. Ergo, it is nearly always running on 18V !!
Only when you run the pump does it drop to 12V, but I never noticed the fan sounded better when the pump was running. You know, with the added pump noise.
And when the pump does not run, then voltage barely drops, so the full 18V over the 12V fan.

Yeah, that will make it sound as if it is about to explode alright....

And finally, not very critical, but also a bit silly:
The control board is running from a 78L05, is again from the 18V, but again resistively dropped to 15V.
The resistivity dropper is inside the PSU even, so it needs two extra leads out of the PSU. Not sure why. Maybe the 78L05 gets to hot from 18V, but not from 15V?

The Mod:

I modded the PSU from 18V to 24V by tweaking the resistor network around the TL431AA voltage reference.
Adding a 68K resistor parallel to the R8 resistor (24K Ohm) yielded 23.5V output. Which is good enough, especially as the capacitors are only rated for 25V

Next I added a cheap DC/DC board to convert the 24V to 12V, and will now feed all 3: airpump, control board and fan, without any resistive dropping.
I did test if the board could handle a continues 2.5Amp for the Airpump. Not that the Airpump runs continuously, but to be sure...

From the PSU, I removed the thin 15V red/black leads for the control board (the resistive dropper for that was inside the PSU), as well as the 18V thin black/white leads for the airpump. And instead added proper (thicker) Red/Gray wires. The reason for the odd  Red/Gray color combo is that the original colors were also odd.
For example: two of the black wires of the PSU were +18V, and one black wire was GND.... Go figure the logic of that....
So anyway, as I did not want/need to change the Black/White wires to the Iron, I kept Black as +V, and chose different colors for permanent 12V and permanent 0V

--- Code: ---original       new     meaning
Black (thick)  Black   +18V (now +24V) for Iron (unchanged)
White (thick)  White   PWM switched ground for Iron (unchanged)
Black (thin)   Red     +18V (now +12V) for Airpump/Fan (now also Control board)
White (thin)   Gray    permanent ground
Red (thin)     --      +15V for Control board (removed)
Black (thin)   --      ground for Control board (removed)
Orange (thin)  Orange opto-coupler to Switched Iron ground
Green (thin)   Green  opto-coupler to Switched Iron ground

--- End code ---

The power distribution board also was slightly changed. Basically, I removed the two large resistors, and re-used their pads to create a +12V and 0V point for the control board wire. Not too hard, see the images.

Now the device heats up much faster, draws the listed 90W while doing so. And the Fan sounds more reasonable.
I may still changed the fan for an ultra-quiet one.

Mods I may make in the future

- Replace Fan by more quiet one, and add internal shroud for more logical airflow
   (but the airpump is noisy anyway, so maybe not)

- Tune the pots on the control board to see if I can make the temp readout better
   (did not touch them yet) done

- Add small vacuum delay valve in the handle, so the suction comes in with more of an initial burst.
    Something like this: but nicer looking)

- Put a relay for the AirPump, rather then running 3A over the tiny conductors and microswitch of the gun.

- Link the sleep/shake input to the trigger switch

Or not. After all, it is working, and I do not need to wait 10 minutes anymore every time I want to use it, so I may loose interest.
Anyway, let me know if you have suggestions, of if you thought this useful.

Have a nice day.

PS: if someone knows why my inline images are not working, please reply or even dm me.

Andy Watson:
There are some odd design choices in these units. I traced-out the circuit of my 331B version here:
It appears that all version use the ZD-P150 power supply module.

Nice schematic. I did in fact already found that a few days ago, and used it, thanks.

So does your SS331 also have a 24V element, or is there a special/unusual 18V element in it's gun?

Edit: also, your schematic has a 6-pin connector, my device has a 7-pin one. And I cannot find any wire for the shake sensor in your schematic
Now, it was mentioned by the seller (local shop) my device was a new version with sleep. So likely that is why I have an extra pin.
I did not focus on that while it was open, and the device is closed and back in its place now.
But it does indeed have sleep and wake. Be it I have to give it a rather hard tap to wake it.

but thinking about it, having a new version, new handle, who knows, maybe I also have a different heat element. It would explain why I see no-one else complain about the slow heating.


I have several ZD-915 that do not have the sleep function on them so I do have that feature

But I have to ask you about the modification that you did to the switching  power supply

You know that it has a  +5 volt rail but did you know that it also has a -5 volt rail Zener diode with a 1 watt resistor how warm does this resistor get with the extra voltage to this device

How well does this modification work and how often do you use it  for hours at a time or a few times a week the reason I am asking is I might want to do this modification on mine but senses you have already done it I just wanting to know how well is holding up

How badly does the temperature over shoot or under shoot the set point

I have opened up my ZD-8915 to do this great mod.

It has a slightly newer version of the power supply (V4), which uses different values for the feedback circuit resistors.
In this version you need to add an 6K8 resistor parallel to R8 to end up at roughly 23.5V.


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