How does this fan control circuit from a PTC Ceramic heater work?

[JettTDi]

Hey guys! I'm trying to figure out how this fan circuit works on a PCB from a “400 watt” personal heater. The fan runs way too slow. You have to be within a few inches to feel any heat, and it’s running far below it’s rated 400 watts because there’s not enough airflow across the element. All those them of this model have the same problem.

I love tinkering so I’ve been trying to reverse engineer it and understand what controls the fan speed, and what I could do to improve it. I’m not the brightest bulb on the shelf anyway lol, but I’m having a very hard time understanding how this works. The 120v ac goes thru a MB10S bridge rectifier, which I thought converted AC to DC.. then it goes through a bunch of resistors, diodes, and capacitors, then to an EE13 12v transformer before going through another diode and a few resistors to the fan connector. I thought at first it was maybe a simple voltage divider type circuit and I could just change a resistor or two, but I don’t know.

I ordered a couple buck converters and originally my plan was to solder wires from the outputs of the 12v transformer to the buck converter, then to the fan so I could adjust the fan speed, but I found out the output from the transformer is still AC @ 12V. I probably should have known that, but what I don’t understand is how it’s still AC after going through the bridge rectifier and all the diodes and stuff. Clearly there’s a lot Im not understanding here and I’m hoping one of you can share some insight on this. I’ve been trying to figure this out for days. I did my best to draw on the picture and mark a few things. I would be truly grateful for any help.

 

[ZipZapOuch]

First, I'm sure there was some effort to balance heater power to air movement as the device was designed. I'm guessing they picked something like 390W or more of heating power and under 10W to move air.

The reason for this is, a person need their skin temp to feel 80°F (27°C) to feel warm in the presence of a gentile breeze. The stronger the fan, the more air volume has to be heated so, 400W can't heat large volumes of air above 80°F and the air will feel cool to bare skin. At lower fan speeds, the air is in contact with the heater longer and can heat the smaller volumes of air to a higher temperature (as yours does).

So, the simplest answer would be to buy a more efficient fan, like those found in PC power supplies, and hopefully it moves more air than your current fan - but that increased air volume will not be as warm as the lower efficiency fan.

But, if you do want to modify the board to raise the output voltage. You'll need to figure out the part numbers on the 7-pin chip and the other semiconductors on your board. I'm guessing the 7-pin chip is a voltage/current regulator. You'll need to change a couple resistors that make a a voltage divider and set the voltage at the V(sense) pin (or similar strategy).

Here is a typical device family...

https://www.ti.com/lit/ds/symlink/ucc28631.pdf

 

[JettTDi]

First, I'm sure there was some effort to balance heater power to air movement as the device was designed. I'm guessing they picked something like 390W or more of heating power and under 10W to move air.

The reason for this is, a person need their skin temp to feel 80°F (27°C) to feel warm in the presence of a gentile breeze. The stronger the fan, the more air volume has to be heated so, 400W can't heat large volumes of air above 80°F and the air will feel cool to bare skin. At lower fan speeds, the air is in contact with the heater longer and can heat the smaller volumes of air to a higher temperature (as yours does).

So, the simplest answer would be to buy a more efficient fan, like those found in PC power supplies, and hopefully it moves more air than your current fan - but that increased air volume will not be as warm as the lower efficiency fan.

But, if you do want to modify the board to raise the output voltage. You'll need to figure out the part numbers on the 7-pin chip and the other semiconductors on your board. I'm guessing the 7-pin chip is a voltage/current regulator. You'll need to change a couple resistors that make a a voltage divider and set the voltage at the V(sense) pin (or similar strategy).

Here is a typical device family...

https://www.ti.com/lit/ds/symlink/ucc28631.pdf

Thanks for your help. I finally had help by someone with good enough vision to get the part number. It was OB25134. I found the data sheet online and tracked down the sense pin and circuit. It had a 2000ohm resistor, and the data sheet says to change this resistor to adjust output current. I’ve gathered up several different lower sizes I’m going to solder in and experiment with.

Now, about the first part of your reply. First, it’s rated at 400 but checking with my Killawatt meter it only pulls 300 once it settles out. It starts out high but being a PTC element, it self regulates (resistance increases as temperature of element increases) As the element gets hotter and hotter (Too hot, not enough airflow) the resistance goes way up, therefore current draw decreases. Pushing even just a small amount of extra air in through the back do the heater by another fan raises the current draw back up where it needs to be.

You also mention that the air temp has to be 80 to feel comfortable…The tiny amount of air leaving out of it is upwards of 375*F. Insufficient airflow is definitely an issue on these models. It’s marketed and sold as a Personal Heater, but it doesn’t perform well as one when the heat leaves the heater and practically just rises straight up due to inadequate airflow. It’s plagued with reviews online that complain about the same thing. For comparison I have a different personal heater only rated at 250 watts. I can set it on my desk a couple feet away and its strong enough to blow on me- it’s not too much airflow though, the air is still plenty warm. That’s how a little personal heater should be. It also pulls right at its rated 250w.

 

[ZipZapOuch]

The pin labelled CS is "current Sense) in this case (normally CS means something else). You can try a lower value resistor to boost (i assume - but you may find the opposite effect and need a higher value but I doubt it)

The FB pin (feedback pin) will be used to set the voltage (see the two resistors making the voltage divider on the datasheet?).

 

[JettTDi]

The pin labelled CS is "current Sense) in this case (normally CS means something else). You can try a lower value resistor to boost (i assume - but you may find the opposite effect and need a higher value but I doubt it)

The FB pin (feedback pin) will be used to set the voltage (see the two resistors making the voltage divider on the datasheet?).

Yes I mentioned that I had several lower value resistors I was going to experiment. So far I desoldered the “2001” 2k ohm resistor and soldered in a 1k ohm, the fan speed definitely increased but it sounds like it’s running close to full 12v-too loud. At least I know I’m headed in the right direction now.

I may have to order some resistors between 1.4-1.8k ohms to get it dialed in where I want it. The other ones I have are less than 1k ohms. At first I didn’t think the CS pin was being used, I couldn’t see the circuit but it goes underneath it and around. The resistor going to CS is the one I swapped out.