Hey guys! I'm trying to figure out how this 12V fan circuit works. This PCB is out of a “400 watt” personal heater. The fan runs way too slow. You have to be within a few inches of it to feel anything and because of the weak airflow, it runs about 100 watts shy of its rating and all the heat just rises straight up. I love tinkering so I took it apart to see how the fan speed is controlled and if I could do anything to improve it.

I outlined the fan circuit and marked the 12v + and - legs from the transformer the best I could. Is this a voltage divider type circuit? The positive leg of the 12v transformer goes to an SS110 diode, then up to the positive fan connector. There's also another path from the + transformer lead to a 101 resistor, then a small capacitor up to the + fan connector, then a capacitor and a 622 resistor across + and -.

I do have a buck converter on the way I plan to experiment with to get the desired fan speed, but I really want to know how this particular circuit works and if I can solder in different size resistors to tweak the voltage/fan speed. I’m sorry if I didn’t explain things clearly. I’ll be beyond grateful for any help. Thanks in advance!!!

 

Is there even solder to the leg, beneath the black line you draw?

 

I see a flyback converter, with the secondary inside the line and the primary under the letters TZ.
What is on the other side, to the right of the black line?

 

I see a flyback converter, with the secondary inside the line and the primary under the letters TZ.
What is on the other side, to the right of the black line?

There’s just a 220uf capacitor on the other side of the fan circuit

 

I suspect that it is designed that way.
400W is probably a figure that has been optimistically rounded up, or calculated from the highest possible mains voltage.
Running the fan any faster would just make the air cooler.
Is the heating element a wire? Does it have a thermostat?

 

I suspect that it is designed that way.
400W is probably a figure that has been optimistically rounded up, or calculated from the highest possible mains voltage.
Running the fan any faster would just make the air cooler.
Is the heating element a wire? Does it have a thermostat?

I know it’s designed that way but that’s what I want to change. From what I understand, a 12v brushless “pc” fan is speed controlled by either pwm or by voltage divider with the resistors right? I know there are more ways to control the fan speed but I know they don’t apply here.

It’s a PTC element (ceramic) They're self regulating meaning as the temperature rises so does the resistance. I confirmed that with my Kill-a-watt meter. If I force more air through the back of the heater the wattage increases to where it should be and it just works better. Also there’s no thermostat on this, just an on/off switch.

 

See post #2. It looks like the solder is missing at that spot. Could be an artifact of the photo - it's less obvious in the later photo.

 

See post #2. It looks like the solder is missing at that spot. Could be an artifact of the photo - it's less obvious in the later photo.

It’s just a shadow in the picture, there’s a big blob of solder there on that pin. I would have noticed something like that first thing.

I actually figured out (at least partially) how the circuit works. The 2001 (2k ohms) resistor just to the right of the 7 pin current control chip goes to the current sense pin. I finally got somebody with good eyes to read the tiny part number off the chip and found the data sheet online.

Changing the size of that resistor adjusts the current control. I only had a few 1k ohm resistors and less lying around to experiment with but I ordered several more. I soldered the 1k ohm resistor in its place and the fan speed increased but it’s too much now. I feel like a 1.5k, 1.6k, or 1.8k will be about right, we’ll see when they show up

 

First of all, a brushless DC motor must have an electronic circuit to energize the coils in sequence.
But then a PC fan is mentioned. so what type of fan does the heater actually have??? Was the power supply an original part of the fan assembly??
There is way too much information that we do not have.

 

First of all, I already provided that information. I said it has a PC type fan. So to answer your question, it’s a PC fan. A PC fan is brushless DC, btw.

Of course it’s the original power supply that came with it. I didn’t think I needed to specifically state that it. I don’t see how that would matter anyway, I provided close up pics of both sides of the PCB. I don’t know what more information would have been needed here.

As I mentioned in my last reply yesterday, it’s already been figured out. I’m just waiting on some different value SMD resistors I ordered to show up to change the resistance value going to the CS Pin. That will dial in the fan speed where I want it, as per instructions from the Data Sheet for the OB25134 Current Control PWM Chip.

 

OK, not all PC fans are brushless!! PCs were produced long before brushless DC motors came along, and quite a few still used brush-type DC motors.. And just because a fan assembly has the same frame style as some PC fans, please understand that that frame style predates PCs by over 20 years. Also, many of them were AC powered with small internal induction motors.
Really, the correct description would be a "Box-frame 12VDCFan". Those fans are used in a large array of electronic equipment, not only in PCs. And even in computers there are several different sizes and styles in use.

And I do not check postings very often on busy days.