Current Affairs & Power Couples

Current Account

A critical issue for the SS-30M has always been where the extra power for the MIDI interface would come from. If it needs to have it's own supply then there would have to be an elegant way to incorporate that. I mean, I don't want two different power inputs.

Now that the interface is working I have been able to measure the current drawn in the worst case situation of all 49 keys being pressed at once. This isn't a realistic use case but it could be a failure mode of the interface or an accidental occurrence

There's only one positive rail in the SS-30 power-supply, 15V, and that's just right for the CMOS MIDI interface so that's what I tried to use. The +15V rail has a 500mA fuse, so as long as the total current  draw does not exceed 500mA - with some headroom - I can use it to power the interface as well.
Firstly, the supply to the MIDI interface is (ahem) currently coming from my bench power supply. This tells me how much current it's supplying and when all the keys are down that's 144mA, DC. Ideally then, the draw on the 15V rail of the SS30 will be well under 350mA DC.

To measure the current there are two options. Interrupt the DC rail coming into the SS30 and place my multimeter in series there, or simply remove the fuse and measure there. There is difference though. The fuse is on the AC side of the power-supply - before DC regulation. I measured at the fuse and got around 570mA when both the interface and SS-30 are powered and all keys are on. At first I thought thatat was it and the current was too high. Then I recalled that the fuse is rated to the RMS value. The RMS value for a sinusoidal signal - which is what we get from the mains transformer - is I/1.44 (where 1.44 is an approximate value for the square root of 2). I measured 570mA so the RMS current is 570mA/1.44  = 396mA.

To be sure, I replaced the 500mA fuse and tested again. The fuse held and I have my answer: I can power the MIDI interface from the SS-30 power supply.

Power Couple

The second problem with power is that the SS30 power supply won't fit into the space left in the 19" rack enclosure. I need a case just for the PSU and then to get the power rails into the SS-30M I need a connector and cable arrangement with enough conductors.  Carrying a single rail is easy but I  need 6 conductors.
-26V
+15V
-15V
-7V
2x 0V

They also need to be rated up-to 500mA. I don't want to lose voltage through the resistance much either.

I found these connectors.

http://uk.farnell.com/cliff-electronic-components/fc684206/socket-cliffcon-shielded-6way/dp/1908694


And there's a ready made 1.5m cable assembly

http://uk.farnell.com/cliff-electronic-components/cih684216/circular-cable-6pos-plug-plug/dp/2663546?MER=bn_level5_4NP_LastViewed_3

The length is important because the SS-30M could be mounted relatively high up and the supply would likely be on the floor.

This is what Cliffcon say:

CliffCon® Miniature ZC Series

Our ZC series, miniature, locking, multi-pole, shielded connectors are available in a range of 2 to 8 pole plugs and sockets. These are primarily intended for low voltage and signal applications. Contacts are tin plated and other metal parts are nickel plated brass or Mazac. There are also 90° versions of the plug available to order.

Specification: Rated 1A / Pin - 48V max.

I'd like the 90 degree angled plug but Farnell only stock the other kind. Otherwise it's perfect! All I need now is an enclosure for the power-supply.