PS-LM40 Power Supply cooling fan mod

Hi All,


Recently I'd asked for some info with power supplies on the forum and thank everyone that responded. I didn't quite get to the bottom of what I was after but during the process of investigating gleaned some information that helped me in the end. Eventually I chose to go down the linear power supply route (despite formerly being happy with a switch mode type) especially after getting the following information direct from a manufacturers site :-

Switch mode power supplies (SMPS) employ high frequency switching and thus, are a source of radio interference, a recipient of radio interference and a conduit of radio interference. (Older linear type transformer based power supplies do not employ high frequency switching voltages and will be quieter as compared to switching type of supplies.)

The document then went on to explain some of the issues that arise, suggestions to improve the situation and then the usual disclaimer of how their products comply with the limits laid down in the European Standards for emission and immunity.

This all goes to explain why my noise floor rose by some one to two s-points on the lower amateur frequencies when I switched from a battery power source to my original switch mode! (No, not 'birdies' but general wideband hash!)

In the end I was gifted (by my lovely wife) a linear power supply in the shape of the Sharman PS-LM40. (It comes in many manufacturer 'flavours' but also reminds me of the Alinco DM340.) It's a big heavy beast being a linear type and I'd quite forgotten how my last big linear power supply was such a 'foot warmer' with this unit being no exception. The first time the 120mm 24v fan came on I was taken by surprise and hadn't expected it to be quite so noisy! (It's quiet in comparison to some fan units and if placed sensibly wouldn't be an annoyance.)

I could have gone down the route that many computer style 'modders' do with fancy/specialist fan manufacturer replacement units but instead dug through my drawers to see what I had available and came up trumps with a stock of 60mm Evercool fans.

Time to open the power supply (and completely void my warranty! )



There isn't much room inside but a space above the regulator board and behind the (lovely) analogue meters looked promising. The heatsink had some 'spare' tapped holes being a mirror of the other side where the temperature sensor is mounted so looked a good candidate for mounting the fan hardware.

After digging through my workshops/sheds I came up trumps with a sheet of aluminium plate so set about with the fiddly (but weirdly enjoyable) task of cutting, drilling and shaping the mountings I required.

I don't think it turned out too bad....



These are 12v fans so I simply wired them up in series to the cigarette lighter/accessory connector allowing them to run continuously but at a slower speed. They were set to blow in to the PSU being able to drawer air from the left vent slots and wouldn't fight against the original 120mm fan at the rear which is set to blow out of the power supply when it triggers from the thermal switch.

It is all fantastically quiet now and despite running several radios from this PSU as well as firing up some 25W FT8 action the main (noisy) fan didn't come on once. (I did however plonk a big load of 100W power resistors onto the supply drawing some 25A for about half an hour to ensure the main fan came on with the extra heat produced.)

Here's another shot of the additional fans in place...



Overall I'm very happy with this modification but some might ask why you would buy a new power supply only to have to fit extra fans to it. I didn't have to, but I had the parts, was willing to undertake the work and very pleased with the reduction in noise not only RFI wise but also audibly so.

It would have been lovely to have obtained an Astron or Microset PSU with their huge fan-less heatsinks but I didn't want to take advantage of my fantastic wife's generosity whom I thank immensely for her support of my radio interests.


Boring stuff for some I suppose, but for me the radio interest is much more all encompassing including such frivolities.

Enjoy your radio pursuits no matter what.  

All the best,
Victor

i know the equipment i have are cheap, how ever tried an ex-maplin switch power supply up to 15V / 4A & was giving some noise like whisper at PNI 65000 with AM when was at RX mode ... a cheap regulator 13V / 3A of 10 euro the ones having big heavy transformer is way more clear no problems atoll with noise (i do have some where in this forum pics of them) , Thanks for sharing experiences

Hi Victor. Glad you got it designed/made/mounted. Looks good though I suppose you have voided your warranty now.

By the way since you seem to have everything, in your drawers/sheds have you got the parts to fix the HMS Queen Elizabeth carrier's prop shaft ?

Cheers Babis.

Always nice to build something but by the time I priced up the parts it was cheaper to simply purchase a high current linear PSU and was such a nice gift from my wife. Glad you got sorted with your DIY routes.


------------------------------------------------


Hi David,

Thanks, much appreciated. Warranty definitely out of the window but at 9kg the 'return postage' would have been expensive and besides, if it ever goes wrong I'll just simply fix it myself.

After decades in the electronics sector plus a passion as a hobby I have drawers full of all sorts of everything!



Much appreciated both, nice to share some of my antics and get a response.  

73,
Victor