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Old 15th March 2012, 10:25 PM   #4861
pidesd is offline pidesd  Canada
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Quote:
Originally Posted by 50AE View Post
Excuse me for this annoying and maybe repeated question, but can the transistors from the original schematics (like 2SCK170 or IRFP9240) be substituted with others - how critical is it? I plan using these shuns for the 3.3 and 5V rails for my DAC.
read on from around 3 pages back for 3.3V
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Old 16th March 2012, 06:10 PM   #4862
mns80 is offline mns80  Finland
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Default Help

Now I have come so far in my adventure that I have all 6 shunts in a box and connected for testing purposes. All 4 plus shunts work fine and one of the minus ones. The last one I can't seem to get adjusted higher than -3.9V. The working one is possible to adjust to -2.4V.

I have based the circuit on the following schematic
Click the image to open in full size.
From there I have changed Q5 to a BD245C as I had some lying around, R10 to 6.8K and R6 to 1.5K based on reading and some testing with the circuit

Is there anyone that have an idea what could cause this?
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Old 16th March 2012, 06:44 PM   #4863
pidesd is offline pidesd  Canada
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first of all, what shunt mosfet are you using? (seems i cant zoom on the picture.maybe a jpeg format will help..?or what post is that schematic on?)


second, higher than -3.9 is toward zero or toward more negative voltage?

and maybe a picture of the actual built circuit might help spot potential problems.

Last edited by pidesd; 16th March 2012 at 06:56 PM.
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Old 16th March 2012, 11:34 PM   #4864
mns80 is offline mns80  Finland
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Apparently it went wrong, it is the same I referred to earlier, the one you linked as reply.

I would like the voltage to be -3.5

Last edited by mns80; 16th March 2012 at 11:39 PM. Reason: added voltage
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Old 16th March 2012, 11:49 PM   #4865
Salas is offline Salas  Greece
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Maybe your q9 has much IDSS. Try for lower Vo by paralleling 10K to R10.
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Old 17th March 2012, 12:43 PM   #4866
bismuth is offline bismuth  France
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Hi,
After having greet success with a 5V build of a SSLv1.1 with BJT for feeding the analog side of a DAC chip, I plan to try a +15V/-15V SSLv1.1 with MOSFET for powering a dual Opamp. I do not know where to start for C102,C104 capacitors type and values, I think i also will have to reduce the local decoupling near the opamp (now 22uF tant // 0,1uF) ?

Any hint would be appreciated !

Thank you,
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Old 17th March 2012, 03:07 PM   #4867
Salas is offline Salas  Greece
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Use 220uF for C101,102 to have low enough 1/f noise. You can even bypass the lytics with MKP 0.01u and 1R snub. If the local opamp bypass will interact you will see oscillation on the scope. If not, leave it alone.
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Old 17th March 2012, 09:04 PM   #4868
bismuth is offline bismuth  France
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Efkaristo poli Salas !

I only have 100uF or 1000uF electrolytics at hand, will one of those be OK for C101 ?

And for C103, is there a better choice than a 47uF electrolytic in my case ?
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Old 17th March 2012, 10:52 PM   #4869
mns80 is offline mns80  Finland
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Quote:
Originally Posted by Salas View Post
Maybe your q9 has much IDSS. Try for lower Vo by paralleling 10K to R10.
Tried that now, did not help but worth a shot, thanks Salas for the tips. Then I remembered that I had to lower R6 to get the voltage where I wanted it when testing the other one. Apparently I had to lower this one to 1k to get the desired voltage instead of 1.5k that I have on the other. I am warming heatsinks now to see if it is enough or if I have to rip the card out once more.

May I ask how you guys attach your pcbs as I had big difficulties getting the holes to line up both for the transistors and the pcb? Please excuse the cable salad in and around the box, connectors to be added and outputs/sensewires to be routed (hopefully tomorrow).
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Old 17th March 2012, 11:53 PM   #4870
Salas is offline Salas  Greece
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So you had kept the voltage drop on R6 much and was blocking the low threshold. Very logical explanation.
One basic check is with scope so to know there are no riding oscillations on each reg since 1.2R is sensitive to layout. Did you use a scope?
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