MFJ-4125P Fan Noise

The MFJ-4125 switching power supply  has some notoriety for fan noise. MFJ uses a 60x60x15 mm fan from Yate Loon.  The high noise level is a result of running the fan at full blast.  Any noise abatement comes from dialing back the voltage going to the fan at the cost of reduced airflow, which brings us to the black art of form vrs function.  The problem is to reduce fan noise without adversely affecting the power supply circuitry of the power supply.

How much cooling is enough? Chose a fan, watched the failure rate of the power supply and if acceptable from a business standpoint, good enough.

The fan noise level is described by MFJ as “whisper quiet”.  On the sound scale, “whisper quiet” is 30dB.   My cell phone sound level app does seem to confirm a +- 30 dB level of noise coming from the supply fan, an example of  the double edged sword nature of truth in advertising.

Noise increases with airflow. I plotted the data (see the Yate Loon product data sheet (2) for it’s fan product line further down ).  I plotted out their four data points below which shows the interesting relationship between Airflow and Noise.


To reduce noise, reduce airflow by reducing RPM.  The human ear detects sound intensity in increments of a decibel or two.  For every increase in one  cubic foot a minute  airflow, there is a one to two dB increase in noise.  It should also be kept in mind that the human ear is a very poor temperature sensor without a lot of help.

Reduce RPM by reducing voltage – 4 diodes in series

A number of hams reported using 60Ω to 100Ω power resistors to drop the voltage and reduce fan rpm.  I am trying diodes instead of resistors since I don’t have the needed power resistors.  4 diodes in series produce a 25% voltage drop from 12V to 9V.  Noise decreased noticeably; every diode drops .7 v drop and each results in a small drop in noise, so this might have been 4 to 8 dB.  At 9v, the fan still makes noise. The question is what’s the impact on airflow and necessary cooling.  E.g  a 25% drop in airflow from  22 CFM  would be 16 CFM …  is that enough to make a difference in the power supply temperature?   {9/1/18 after a few months, still seems to be fine with no apparent ill effects.  Using a noise app on my phone, the fan now elevates background noise in the shack from  30 dB to 35 dB with the mic a few feet away from my operating position.}

There are many comments out there of hams who have installed resistors. Nobody has reported any issues with dropping the fan speed using resistors.  We’ll just sort of see how it goes with diodes.

I put a thermistor on the heat sink to enable monitoring the temp on it and set up my arduino thermistor .  (let’s put a thermistor in there – now that we have a small inventory in stock !)  From April to September, the power supply ran with diodes in the fan circuit with no ill effects.

9/1/19.  Did some power supply temperature test with TS590sg  transmitting into a dummy load.  PS heatsink temp rose from 84º F to 103º F  100 watt cw.  That’s not too bad I think.  (the ts-590sg rig remains cold to touch).   Under digital condition the temperature of the heat sink however goes to 123º F.  For this we’d want to be able to switch on full power to the fan.

Temperature controlled fan driver (4)
(Taming the MFJ-4125 PSU Fan)
Below is a schematic from G0KLA who used this circuit for regulating span feed from temperature for his linear amp power supply.   He used the same circuit with an MFJ-4125 and posted it ( see 4 ).   It’s the same analog control concept as ( #6).

4/8/18  I built out the circuit for testing.


C17  – electrolytic,  filter/time constant ?   R31 – the gate resistor 10k seems to be typical to protect the gate from transient voltage at the expense of switching speed.  Switching speed for turning on a fan is hardly critical. (5)

Test Session #1 – Standby noise of about 35 dB and 109º up to 50 dB+  and temps above 118º on 9/13.



9/13/18 Testing temperature driven fan controller-  finally.   Removed the  diode breakout and inserted the test unit.    Insertion notes:  for testing I tapped tge 12V on the rear panel, and attached fan leads directly to fan + and  minus – .   There is a better way.  Make a power cable to get the 12V from the fan power socket on the board, and provide two header pins for the fans power plug. This allows the fan circuit insertion without cutting / soldering  on the MFJ4125.

9/10/18 Results.   The circuit works well.  Maximum temperatures are about 118º F with full power digital operation.   I set the trigger point to occur at something close to 105º F by trial and error .  At this setting with transceivers powered up, the fan idles with heat sink temps around 105º and 110º and fan noise is barely audible with total ambient noise of about 35 dB.    On transmit, the fan speed revs up, and with a lot of transmitting activity like with a digital mode, the fan goes to full speed as the MOSFET switches on to deliver the full input voltage to the fan. The fan at full power adds another 10 to 15 dB of background noise.


The spec we’re most interested in is the junction temp of the power transistors attached to the heat sink .   Then the heat sink temperature might tell us if the cooling is enough.  We’re leaving this to another time.

9/10/18 Production Insertion Notes.   Came up with the better idea for how to make the insertion. Soldered two header pins to plug the fan end connector in.   Used a two pin socket to plug into the fan power supply socket on the rear of the 4125 main board.

4)  – heat sensitive ran speed reduction circuit.

Fan Specifications 

The fan used OEM is a 60x60x15 mm 2 wire YateLoon S60SM-12 run at 12V with the fan load, 13.8v no load.   It’s

The specs and replacement information are included here because the originals bearings are on borrowed time and multiple re-lube operations.

Below: product sheet from Yate Loon (2).  YL has data on four different models of 60x60x25 fans with different motors.  Airfow and noise increase with RPM

fan specs

OEM Source (3) 60x60x20

D60SM-12 Technical data:
Dimensions: 60x60x20mm
Nominal voltage: 12V
Nominal speed: 2700rpm
Airflow: 18CFM
Noise level: 30 dB(A)
Power consumption: 1,2W
With 3Pin Molex plug, 60cm cable and rpm signal

Instead of a genuine YL, here’s what I bought below.  It’s now plugged into the power supply and seems to be running fine.


12/13/2018  So far so good.  With only receivers on, the power supply fan is off.  At some point the fan my idle as the load increases.  On transmit, the heat rises in the PS, the fan rpm rises rapidly and continues until the heat drops.  If we stop transmitting the fan idles and eventually turns off.





~ by marksun on April 7, 2018.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: