Dear forum, dear friends, hello!
This is my first post, so I apologize in advance if I've missed the forum. However, I'm stuck and looking for assistance on the matter (in subject).
I'm using 12V 8A power supply and (attached) schematic for automatic fan control (cooling the LED heatsink)
Also, for the LED chip (36V 100W) I'm using DC-DC booster (12V > 36V 3A) feeding the PWM LED dimmer. Everything works OK until I connect the fan circuit. Then, if I reduce the LED dimmer, the fan speeds up, otherwise, if I run the LED at full power, the fan slows down (hence, there's no more automatic fan speed control). However, if I use another power supply for the fan and its circuitry, everything works perfectly (fan RPM is being properly regulated according to LED chip temperature; it increases and decreases as the LED power goes up or down).

Any ideas what I'm doing wrong? If any more info or schematics/layouts are required, please do let me know.

 

What are the two supplies you are trying? You only mention one of them.

Your power supply can't drive the DC-DC converter over it's full range, but as long as you aren't asking it to deliver more than about 80 W total you should be okay.

Are you sure you are connecting both supplies correctly to the circuit?

 

WBahn, hello and thanks for your reply. First power supply used for the LED is lead acid battery charger (max 8A, as it says on the enclosure). Second one (used for the fan) is generic wall wart adaptor (220V > 12V 2A). The fan in question is regular PC CPU cooler fan

However, just few moments ago, I tried using lead acid battery, fully charged and in good condition (12V 75Ah) as primary power source for both the fan and DC-DC converter. Situation was the same as before (no change in behavior).

Regarding your question: yes, I am connecting both supplies correctly. If it might help, I can draw a schematic how I have connected this circuit.

Thanks again for any suggestion you (or anyone else) might have.

 

Yes, please show a schematic. In particular, make it clear what is power what?

How are you powering the LEDs? How much power is actually being drawn from the supply for the LEDs (as opposed to how much power is actually being delivered to the LEDs)?

 

WBahn, thanks again for swift reply. Please take a look at these 2 diagrams (schematics), that's how it's connected.
1ST.jpg shows non operational layout.
2ND.jpg shows operational layout (the one working properly, when I connect the fan circuit to another power supply).

For LED chip, I'm getting 3.9A @ battery charger output. LED is taking around 1.1A @ 33V, which doesn't sound right?

 

For starters, to boost 12V to 36V at 3A you need about 10A at 12V. So your power supply is insufficient.

Secondly, you cannot drive that kind of LED from a constant voltage (36V) you need a constant current driver.

Third, the battery charger is not a regulated power supply. If it is a dumb one, the voltage is unregulated and depends on the load. If it is a smart one, it would not even put out anything unless it is connected to a 12V battery.

 

BobTPH, thanks for your input(s). I've tried using the lead acid battery (12V 75Ah), but the result was the same.
Also, thanks for other 2 hints! Much appreaciated!

Please bear in mind that I'm not a professional or trained in electronics, just a humble hobbyst and enthusiast, so I'm learning a lot of things on the fly.

 

Hi,

You have to figure in the efficiency of the boost converter in order to know the INPUT power to the converter so you know how much power the 12v power supply has to provide.
For 100 watts at 36v and 80 percent efficiency you need about 125 watts input. 125 watts at 12v required over 10.4 amps of input current. Your 12v supply is only good up to 8 amps, therefore when you try to power that 100 watt load it would be too much for the power supply to handle. When you turn on the fan that draws more power, and that is most likely bogging down the 36 volts.

So in short, you need a power supply with a higher power rating (150 watts might be a good start) in order to handle all the loads, or else reduce the load power somehow. Alternately, you may just get away with using a separate power supply for the fan, which from your post, appears to work. You will have to test all this over a longer time period though to make sure nothing gets too hot.