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Old 21st April 2013, 10:59 AM   #2171
Salas is offline Salas  Greece
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Looks like it.
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Old 21st April 2013, 07:22 PM   #2172
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Hi,

I am using the component IRF kit from Tea Bag.
If I understand correctly I need to configure it like this:
Cneeded=1Amp, shunt headroom =100mAmps so total draw will be 1,1Amps.
Voltage needed=12v
The source is a 1794 dddac with 4 boards of which I have soldered local shunt regulators on the dac boards so probably the current draw will be quite stable?

Acc. To appendix 1c I will need: 2x red led, 1,8kOhm, 5K trimmer.

Now, I am not sure how the current is set, is this with R101 and the amount of green LEDs?
(3xVfLED)-VgsQ101=VdR101, seems I would need 4 LEDs and R101 to be lowered.
(4x2,1)-4,3=4,1v and 4,1/3,9=1,05Amps. This will give a headroom of 50mAmps.
So for more headroom R101 can be dropped a bit more?

Is this the way to do it?

Kind regards,
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Old 22nd April 2013, 02:43 AM   #2173
Salas is offline Salas  Greece
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There is the excel calc also in post#1 for ballpark predictions, but I suggest you start from a likely value, measure and judge the trim needed. Surely with 4 leds in your case. There are very many little tolerances in the semis preventing to be spot on without analyzing each specific led and Mosfet. Cut and try works faster here.
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Old 22nd April 2013, 07:52 PM   #2174
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Hi,

I build the reg today. It is running now on my dac with a borrowed transformer from an amp and a variac to adjust to 15,5vac.

D102/3/4 are 4 green leds measuring 1,76v.
R101=1,2r, 10w
R103=1,5k, r105=2k, max. Voltage adjusted is approx. 13v

CCS current is 1,35A, dissipation of dac is 1,05A.

Main cap is 10000/50v nichicon, other two are 4,7uf mkp.

Adjusting voltage was pretty difficult because it alters when I touch the board or components, I also noticed that if I touch the voltage output leads, the output voltage swings from 12,3v to 13v. Is that normal?

I want to lower the current overhead a bit, how can I obtain this easily? Best by upping r101?
Can I alter r103/105 to any value to make the voltage adjustment range bigger?

Kind regards,
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Old 22nd April 2013, 08:17 PM   #2175
Salas is offline Salas  Greece
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1.35A is at the limit of what a an IRF9610 can safely do for internal temperature, better unload it by 200mA. Up the R101 accordingly. Is the PCB rather hot now? Touching the sense points adds interaction. If the Q103 JFET CCS has plenty of current as a sample then you may try shorting R103 or altering it with a parallel so to see if R105 is enough almost by itself. That will widen its turns effect.
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Old 22nd April 2013, 08:42 PM   #2176
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Hi Salas,

The pcb itself is not hot, I use a pretty big Heat sink.
I will lower the current first and than adjust the voltage components.

So touching the wires interacts, best to keep them short than.

Regards,
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Old 22nd April 2013, 08:54 PM   #2177
Salas is offline Salas  Greece
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If you got an oscope see if there is any EMI or oscillations on the rail too. If not having a scope to know for sure put 100uF across the sense outputs at the reg's connector for good measure. With digital its easy to have interference over sense lines, so keep the power loop short, yes.
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Old 22nd April 2013, 08:59 PM   #2178
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Do you mean 100uf over x102-3 andx102-4?

I will put the scope on it tomorrow.

btw. It does sound pretty good now!

Regards,
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Old 22nd April 2013, 10:07 PM   #2179
Salas is offline Salas  Greece
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Yes, at the connector screws that secure the sense wires directly even. Watch the capacitor polarity.
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Old 23rd April 2013, 02:07 PM   #2180
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So I've been trying to figure out what to put on the board - could somebody confirm?

DC in: 49VDC (35VAC)
V Out: 36VDC
Load Consumption: 80mA (line level preamplifier)
Constant current (Q101): 200mA (lets have a little headroom )

R101: 20R
R103: 7kR
R105: 2kR Trimmer
R102,106: 270R (Mosfet)

D102-4: 4x green, 2V
D105,106: 2x red, 1,9v
D101: Jumper

C102,104: 10uF film

I don't know if the R/film choice is the right one for my load - its Chokys Pumpkin preamplifier, schematics here.

Many Thanks
Sven
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