I decided that a bit of overkill / future proofing was needed for the PSU required for my SSPA. There are many different PSU manufacturers and specs but I discovered that Kuhne sell the Meanwell units. If they are good enough for DB6NT, it’s good enough for me!
So I have used the RSP-1500-48V supply. There’s nothing much to say about this part
of the design; it just squeezes into a 2U 19” rack case, the lid needs a push
to get the fixing screws in place and the psu is positioned under the case vent
holes. The 50V output is fed via a 50A
fsd panel meter – I couldn’t find a 30A off-the-shelf meter and sorting out a
beefy current shunt was going to take time so I just went for the 50A unit from
Farnell.
The 50V terminals on the back panel are just 25A rated 4mm
terminals and I use two red and two black in parallel. The internal wiring is **mm **A rated cable
used for house wiring, rather overkill but it won’t fail if the output was to
be shorted together.
On the front panel are a couple of LEDs in red holders. These are wired in series and in series with
a pair of 20V Zener diodes then connected to the 50V supply.
The regulator / control circuitry Is built onto the
Veroboard. I need a 12V 500mA supply for
the amplifier fan and a switchable 12V or so for the bias output. These outputs are produced off the 50V
supply. The usual 7812 or LM317 type regulators
are not designed for 50V inputs, so I used a series pass transistor to do a
first reduce to 28V which might be a useful supply for 28V RF relays. I used a *** transistor here and it needs to
be on a heatsink (it is continuously dissipating 22V and 300mA). If I ever use this supply for a 28V relay, it
will probably need a bigger heatsink. Then
there are two regulators for fan and bias, again these are both on small
heatsinks. Finally, the bias is switched
by a simple NPN controlled by the PTT input.
Noise from the 50V line
A few weeks ago, I had everything set up to do a few checks on the amplifier. I discovered that my receiver noise floor rose by a few dB when I turned on the 50V supply. This was with no antenna connected, the SSPA was to be checked into a dummy load. When I’m out portable, everything is plugged together and then switched on so I simply hadn’t noticed this. Although both my 50V psu and 12V psu were connected via earth terminals, I found the noise would appear when I connected the outer of the coax between my transverter and the SSPA. This noise happened when the SSPA is in standby i.e. no current being drawn. So this looked like a conducted emissions problem on the output side. I found a basic generic filter design on an old Meanwell data sheet and a quick filter was constructed.
I had no idea about the spec of the toroid I found in my junk box so I wound 10 turns round it and measured the inductance – it turned out to be an Amidon FT-140-43. Using fairly thick wire (2 x 1.5mm2) I could get 5+5 turns on the core, giving roughly 20uH. For the capacitors, I thought about using low ESR units but didn’t have any to hand (and I don’t know if you can get them with 63V or 100V rating) so I just used what was to hand. I also installed a small earth plate which is connected to mains as it enters the case; it is also connected to the PSU metalwork with a wide strap (UR43 braid).
Once the re-building was completed, I put my station back together, connected up the 50V supply and switched on – no noise! Hopefully that should help pulling weak signals out, I will find out in a few weeks’ time.
