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Old 13th July 2013, 06:24 AM   #2321
AndrewT is offline AndrewT  Scotland
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Originally Posted by yammy1688 View Post
Would you mind describing this process in more detail? What needs to be populated in order to test just the CCS? I've just been following the build guide.........
post the schematic you have chosen and we'll tell you the component numbers.
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Old 13th July 2013, 03:25 PM   #2322
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Many thanks for the replies, Im away for a few days so cannot report back. Appreciate the advice and will be testing as suggested when time allows.
Lucas
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Old 17th July 2013, 02:26 PM   #2323
RollE2k is offline RollE2k  Sweden
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One question, i have test-run my SSLV1.1's and my vdrop over the pre-reg parts (choke small choke also) was a bit bigger than i first thought, and the transformer had a little bit less output than i thought (16.5vac instead of 17vac) But anyway:
I will only use this shunt for ~115-120mA total current, and wanted to have 17.5V output. And i have ~19.5v before the current-set resistor and the IRF9610. Using 10R current-set dropping approx 1.18v over that resistor. Leaving only ~0.8-0.9v Drain-Source over the IRF9610.

I will use this to feed the SEN i/v, and when i measure on the V-input of the SEN i do actually have 17.45V and no ripple (with the multimeter at least). I don't have all parts so that i can test it together with the DAC, but i will do so later..
So my question is - will this be any trouble running so low Drain-Source over the IRF9610? looking at datasheet i think it should be possible, but i'm not best at understanding all data.. so any thought about this?
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Old 17th July 2013, 03:06 PM   #2324
Salas is offline Salas  Greece
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Low VDS makes the MOSFET more capacitive as a CCS worsening it in HF, and the whole reg susceptible to pass ripple in case the mains drop more at a point.
The CCS becomes limp for significant current changes on the output side also.
Bottom line is do something to up your input voltage. Lose the input filter if you can't use another transformer for now. Use 1 Ohm/1W in its place just to break EMI byproducts from the charge cycle.
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Old 17th July 2013, 03:56 PM   #2325
RollE2k is offline RollE2k  Sweden
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Thanks for that, i'll see what i can do about the pre-filter.
By the way - approx how low Drain-Source voltage is recommended as minimum?
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Old 17th July 2013, 04:15 PM   #2326
Salas is offline Salas  Greece
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5Vmin, 7V RegVin-RegVout to allow for mains variations. As you saw it isn't about it won't typically work in lower Vdif especially in rather low CCS settings, its about not compromising it.
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Old 17th July 2013, 04:34 PM   #2327
RollE2k is offline RollE2k  Sweden
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Strange thing is that i measure 16.5VAC before rectifier, then after that 1500uF Cap -> common mode choke 2x100mH/3R -> 1500uF cap. And if i measure after my rectifier i only have ~20.2V even though my diodes are low-dropout schottky SMD's. then dropping approx 0.7V over then chokes.. So the LC filter doesn't seem to drop to much, but i rather have to low voltage after the rectifier and cannot figure out why.
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Old 17th July 2013, 05:14 PM   #2328
Salas is offline Salas  Greece
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Constant current taxes transformers more. I would give 1.27 to 1.3 rule of thumb multiplier from AC nominal to DC raw when choosing Tx for this reg. Shoot for 10V dif Vin-Vout to allow for Tx brands tolerances & regulation, mains play.
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Old 30th July 2013, 07:06 PM   #2329
lykkedk is offline lykkedk  Denmark
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Hello there

Just asked some quistions in the other salas shunt thread, which i see, could be hard to understand!

So, i ask straight out instead :

I need 700mA (Raspberry PI) + 25% ~950mA current.
I have a 12vac transformator

Would like to use the excellent shunt for 5vdc feeding the PI.

-Anyone help, with the desired values ?

/Jesper.
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Old 30th July 2013, 09:40 PM   #2330
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Default Hmmm.

Quote:
Originally Posted by lykkedk View Post
Hello there

Just asked some quistions in the other salas shunt thread, which i see, could be hard to understand!

So, i ask straight out instead :

I need 700mA (Raspberry PI) + 25% ~950mA current.
I have a 12vac transformator

Would like to use the excellent shunt for 5vdc feeding the PI.

-Anyone help, with the desired values ?

/Jesper.
I generally do not think the Salas is the best choice for running a computer like this, here's why:
A computer board is going to have widely varying current needs, the listed value of 700 mA at 5V is probably a maximum which the board will draw at boot up, and when working really hard (processor maxed out), the problem is, if you are using it to play music, the processor will probably run at around 5%, and the board will probably draw much less current, perhaps half (350 mA). If you set the CCS on the Salas for 1A to accommodate the max current needed, you will have to have some huge heatsinks to get rid of the other 650 mA at 5V of heat (3.25 watts), plus, you are making the situation even more difficult by having a 12V transformer, as now you also have to handle around 11 volts of drop to reach your 5 volt output. You could make this work, but you will need really, really impressive heatsinking. At least get yourself an 8 volt transformer to make it a little easier...
The Salas is really better suited to loads which do not vary quite so much as a computer board.
I would try around 2 ohms as a start for the CCS resistor, and usually around 220R and two red leds will get five volts. Typically you will have to experiment a bit with these values to get exactly what you need. Note that you will likely have to meet the maximum current demands of the Rasberry Pi for it to boot up, otherwise the output voltage will drop. Make to sure to test everything with a good dummy load: a 7 ohm 10W resistor would give about 715 mA draw at 5 volts output.
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