MFJ-4225MV fan switch

After mentioning the fan used in the MFJ-4225MV power supply (see the blog:

https://wb9kzy.blogspot.com/2023/07/ys-tech-fan-used-in-mfj-4225mv-power.html

Here is a picture of what I ended up doing:

It's an SPST switch in a small plastic box fastened to the cover of the supply.  The fan speed was slowed down by adding a 50 ohm resistor in series with the positive lead of the fan.  The switch just shorts out this resistor when in the UP position giving full fan speed.  In the down position the switch is open so the resistor is in series.

I didn't measure the voltage or the fan speed, it was a hot humid day here so I just buttoned it up and took the picture.

We'll see how it works, I may look into simple temperature sensing to see if there is an easy way to alert me that the supply is getting too warm.  Judging from the dust collected on the cover the bad thing is that the plastic box is covering some of the ventilation holes that were used the most.

One nice thing is that the connector used on the fan was used in old PCs to connect things like reset switches, LEDs and so on.  The mod is completely reversible by just unplugging the connector cable going to the 50 ohm resistor and the header going to the fan.  Re-plug in the fan and remove the switch box and it would be hard to know that a mod had been made.

This should be handier than the Astron linear supply (which had it's own noise emission:  HUMMMMMM).  The new switch makes a big difference, with the switch down (low) it's easy to hear the noise from the radio, with the switch up (high) the fan drowns out the speaker noise.

Best Regards,
Chuck, WB9KZY
http://wb9kzy.com/ham.htm