Wednesday, October 01, 2008


Squeezebox 2 rebox part 2

This picture helped me figure out how to power the SB2

First, I built a power supply for the +5, 2A input in to the SB2, replacing the cheap walwart that come with it.
To start, the AC is filtered by Felix #3. Then, AC goes to a Signal Transformer, 16 VCT, 25 VA transformer, courtesy of my local surplus store.
I started with a Hammond 12.6V CT, 12VA transformer, but it was a little underpowered, at least I thought so. Powering a SB3 with wireless, Hammond would get to about 130f, with a room temp of 75f, so a 55 degree temp rise. Plus, with the reg I was using, the voltage was a little low. I expected it to provide 6.3*1.4 = 8.8 volts. I got closer to 7.5 volts.

With the larger Signal tx, I get around 11.5 VDC out of the bridge.
After the transformer, I have a full wave bridge, using two diodes, with the centertap going to ground. So, one diode goes to the one secondary tap, the other diode goes to a tap on the other secondary, and the two other taps are tied together, and these two taps form the ground.
This feeds a Panny, 3300uf FC cap, then a small inductor, another 3300uf cap, 0 .47ohm resistor, then a Jensen 10,000 uf cap. The Jensen cap is next to the regulator.
In general, I kept the transformers, bridges, and the first and 2nd caps in the noisy part of the box.
Not the final wiring needs to be cleaned up still.
For the regulator, I am using a PowerReg, a higher power version of the TeddyReg, from PFM.
Link here
I used these values for my PowerReg
R1 = 479R
R2 = 90R5
R3 = 50K
R4 = 100 K (added on top of board)
R5 = 50K (marked as R4)
R6 = 110R (marked as R5)
C1 = 0.1uF X7R Ceramic
C2 = 10uF Tantalum
C3 = 10uF Tantalum
C4 = 33uF Tantalum
C5 = 0.2uF X7R Ceramic
C6 = 3.3uf Wima MK4
T1 = SK117 GR
T2 = BC550C
T3 = D45H11
LR1 = LT1038

Heat sink is out of my junk drawer. Overkill, it hardly gets warm at all.
Ground is connected from the SB to the reg by a VERY short wire, it is under the yellow tantalum cap in this picture. I scrapped off some solder mask, in the ground plane, as close to my PowerReg as possible, to make the ground wire short, so the two grounds would be well coupled.
I removed the power connector, and connected +5 to a pad for an inductor right after the power connector. I also removed the inductor. I also added a 220uf silmic, close to where I wired power in. There were a couple holes, near here, a convenient place to install the cap. You can just see the top of the cap at the edge of this pic.

When I first powered up, I thought it was not regulating well, because it varied a lot over different loads. I had three 15 ohm power resistors to test with, and I would add them in parallel, so I could draw up to one amp with a 5 ohm load at 5VDC out of the reg.
Turns out, my input voltage was not as high as I expected, so the input regulator was not regulating, because I was not feeding it enough voltage. I started with a LM317, but changed to the 1038 because the 317 needs over 2 VDC of voltage difference, the 1038 is around 1.3 volts.
2nd problem I had was the PowerReg comes up too slowly, and the SB did not come up when I powered the reg up. Fortunately, Teddy came up a quick start up circuit, because other people had similar problems with the TeddyReg. adds a transistor, and a couple resistors. Resistors are same value as R3 and R4 (50K and 100K). Transistor he used was a BC639, a NPN with 1A collector current. I found a MPSA42 in my transistor box, a NPN with 500mA collector current.
Added the resistors and transistor to rear side of board, hooked everything back up, and the SB2 came on when I turned plugged in the power :).
Last problem was with tweaking the output voltage. Seems like the voltage drop from the circuit varies with the regulator output voltage, the voltage drop increases as the regulator output voltage increases.
So, I had it working, but output was around 4.8VDC to the SB. I calculated new values, to get a 5.2 V output, but it was lower, because of the increased drop. It took one more iteration to get my current values, voltage is 5.14, where my target was 5.2VDC.

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