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Nevada Radio PS-40M loud fan mod

I am new into amateur radio, but I knew the importance of getting a nice, quality power supply that should serve to power all my gear for years to come. I wanted a powerful linear unit for clean, ample power. I didn’t want to take the risk on a potentially cheap & noisy switched mode PSU.

Enter the Nevada Radio branded ‘PS-40M. A well priced, well reviewed, great performing linear DC supply that puts out up to 40 amps! (Link above). Now this supply has been fantastic so far, what a great bit of kit. Except for one thing…


Now to be fair, this is clearly an OEM branded unit made large scale by a huge factory in the far east and can be found under many different brand/model names etc. So its important for me to say that Nevada Radio are a FANTASTIC shop, offer excellent service and cannot be faulted for this in any way, because they didn’t build it.

Anyway moving on to the problem in hand…

This unit has a poor cooling design. A thermostat is fixed to the side of the heatsink, power is coming from a tap on the board next to it, via a 24 ohm resistor to limit the available current. The other side of the thermostat goes straight to fan positive, negative then goes back to somewhere else in the unit.

The thermostat in here is a simple ON/OFF type and is set to 50c. And herein lies the problem. There is no airflow over the heat sink until the thing reaches 50c, which of course it will as there is no thermal transfer happening via airflow.

What this means is that you will have a totally silent unit for a period of time, depending on your use, this might be 5 minutes, it might be 30. But when that heat sink reaches 50c, and it WILL reach 50, you are treated to what sounds like a Jumbo Jet sitting idling on the taxi-way at Heathrow airport.

Thermostat on the side of the heatsink

This is a stupid design, because once that heat sink gets to 50c, the fan comes on, drops it somewhat below 50, at which point you are rewarded with silence…. for a brief few minutes until it rises above 50 again, again…. and again. Rinse repeat whatever….

Luckily this is an easy fix. We could just replace the fan with a quieter model such as the one I have used HERE. Its a ‘WINSINN’ brand 24v 120mm unit with a noise rating of only 27dBA compared to the 50dBA of the standard fan. This simple replacement job will bring the noise level right down as intended, but it WON’T improve the cooling design.

The WINNSINN 120x25mm 24v fan is an easy drop in replacement. My taped up connections were for a running test only before soldering.

What we need is for the fan to run at a reduced speed all the time, to keep airflow shifting the heat away from the heat sink, hopefully it won’t ever reach 50c then right? It’s not rocket science, and frankly it’s disappointing that this wasn’t done from the factory. But hey ho, we’re in luck, as this can be achieved with a simple resistor bridging the poles of the thermostat together, only allowing reduced current to flow to the fan whilst the stat is below 50c. If the temperature ever gets above 50c, the fan will run at full speed.

I used a 5w rated 150 Ohm resistor, make sure you choose one with a suitable power rating, not sure I’d want to use a tiny little 1/4W metal film type as used on Arduino projects etc…

Solder the resistor across the two poles of the thermostat and you’re done. Also note here that I have leveraged the existing cables that the stock fan used, and just soldered them to the cables on the new fan, that saved me digging around to find the end of the negative and also provides plenty of length for future fan removal etc…

Resistor soldered across the thermostat poles.

Put it all back together and you are done! No more jumbo jet noise right in the middle of a good QSO to p**s you right off!

Video demonstrating the fan modification to the PS40M.

I’d like to take this opportunity to reference an existing write up that I took influence from. M0LMK has done something similar for the PS-30M model, see it HERE however I noticed the thermostat/fan wiring configuration was different to my PS-40M so thought it was worthy of its own write up!

Cheers all!

Tom – M7CTO

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