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Powering a Rane SM82 Rack Mixer

Andy Betts

I've just bought an SM82 from the US, and got it for good money ($50 inc shipping) Obviously the PSU it came with is for 110v power so i've gathered together all the bits to make my own 230v PSU. they do sell reproduction PSU's on EBay but they go for around £50 and I can make one for around £20. i do have quite a bit of experience with soldering and electronics, but NOT with PSU's, never made one. But it's one of those things i want to have a go at to tick off the bucket list of things to do.

 

A guy previously made one and posted his build on another site (I've also posted on that site to try and gain more information) his build is here https://www.thegearpage.net/board/index.php?threads/diy-rane-rs1-power-supply-for-sm26-and-sm82.1838728/

 

he built his with no thermistors in series, which i don't want to do, I want to play safe.

 

I've purchased a transformer from Rapid Electronics (Vigortronix VTX-120-5412-209 PCB Transformer 230V 12VA 9V+9V) which should do the trick, but my question is around the thermistors. The PCT Thermistors are there to prevent inrush current into the transformer so i'm a little loath to build this without thermistors in place.

 

The schematic for the original PSU is not available, however there is a schematic for a Rap 10. The Rap 10 is the same as the single PSU, just multiplied 10 times, so please ignore the other 9 outputs on the schematic.... Here is the schematic https://www.ranecommercial.com/legacy/pdf/old/rap10sch.pdf

 

As the thermistor is variable resistance how do i know what one to buy?? Is there a way of calculating the zero power resistance? Rapid have the following on their site

 

https://www.rapidonline.com/Catalogue/Search?Query=thermistors&Tier=PTC

 

Any ideas??

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  • Hi Andy.

     

    It's not a symbol I've seen before, but it's not normal practice to have a PTC device on the primary side. It is not normal practice to place it on the secondary side where it is currently placed either. It is normal practice to have it at the input of the end device (optionally) - perhaps theoretically at the output of the end supply, but even there, it really depends on the end device, i.e. you'll need to measure end device current, and then pick the PTC.

    For a safe design it's best to stick to normal practice. As a result, the transformer can (optionally) have a thermal fuse built-in, and then on the primary side the first thing would be an IEC mains connector with built-in fuse, and then connect that to the transformer primary. That's it, no additional component is needed (a filter is optional). The fuse value determination is partially by experimentation, i.e. once the supply is built, you can measure it and then pick the closest fuse that doesn't blow in normal use.

     

    image

     

    Looking at the PSU at the link you sent, this looks well constructed. The underside should be have a cover (could be another piece of proto board) to not allow any fingers to touch the primary side. Only other thing I'd do differently is use proper wire - the primary side wire looks like general purpose wire. At a pinch, a short length of mains cable could be stripped and the insides of that could be used.

    image

  • in reply to shabaz

    So you reckon, don't worry about the thermistors as shown in the schematic... Just add a fuse to the mains plug end? That would then be exactly as shown in those photos, generally our mains plugs in the UK have a 13amp fuse in them so i could easily use one of those.

  • in reply to Andy Betts

    Hi Andy,

     

    That's correct, no need for any thermistors. Regarding the fuse, it's a good advantage to have it in the IEC socket for hobby projects, because it eliminates some of the risk in the construction. Also, often the fuse in the plug won't blow unless there's severe overload, people tend to put any fuse in there or swap them around when troubleshooting, so all round it's better to have the additional fuse.

  • in reply to shabaz

    So..... wire a fuse holder onto the board essentially? i'll also have one in the plug

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