LFMWL T200-1003 fan output voltage

I’m having an issue with the VFAN output from J3-2 on this power supply. It is rated at 12V-.5A but I ma measuring 3V and it will not power a fan that it is rated to power. If anyone has nay experience with this please let me know if there are additional methods to test his connection or if this indicates a problem with the supply.


What’s the load on the main output when those measurements were taken?

From what datasheet I can find, voltage tolerance on that output is indicated as give/take 20%. A tolerance that loose suggests to me that it maybe taken from an unregulated auxiliary winding on the main transformer, in which case if there’s no load on the regulated (main) output, the thing might not be chooching hard enough to transfer meaningful energy into the fan output circuit.

The datasheets wouldn’t really appear to declare that and at least in my mind “no minimum load” should mean that everything functions within spec under that condition, but that may not be reality in this case.

If you are seeing only 3v from that nominal 12v output with a modest load on the main output (while not overloading the fan output) I’d be inclined to call that a defect.

I had not tried that, but just now connected a 5.5k Ohm resistor to the main output and still saw the same behavior at the fan output. This is also now on a second power supply we ordered, so its consistent with a sample size of 2.

Was that aux output loaded at all while measuring its output voltage? Did it wiggle at all when the load was connected? Main output looking good?

If the aux winding theory is valid, in might take more than 1/10th of a watt of primary output load to bring things to life. I’d also look very critically at any interconnects used to make sure a person didn’t do something silly like connect to the wrong pin… Done that one a few times…

If you’ve triple-checked that the stuff you’re responsible for is all in order, then I’d be OK with calling defect and proceeding accordingly.

Hi Resonon,

I notice that in Note 3 on page 2 of the datasheet, it states the following:

“3. Fan output voltage tolerance is +/-20%. During V1 full load, VFAN needs min. 20 mA load to be within regulation band.”

Not certain of how to interpret the “full load” wording in this context, but, have you tried placing a load between J3-2 (V Fan) and J3-6 (GND) rather than just measuring the voltage? I would suggest using a fan or something which will draw at least 100mA, just to get it well above that 20mA minimum.

Since this supply should not need the fan to operate under low/no load conditions, they probably did not place much consideration on how it might operate under those conditions. In fact, in order to save power, many power supplies with integrated fans do not turn on the fan at all until the main load reaches a certain level. I saw nothing in the documentation stating that this one operates this way, but it is possible that it does.

1 Like

I placed a proper load (a stepper motor 24V 2.4A) on the main output and did get the correct voltage output from J3-2. My original test using a 5k ohm resistor did not draw enough current apparently.


Amperes = Volts/Ohms

24 Volts / 5000 Ohms = 0.0048 Amps = 4.8mA