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Old 21st April 2012, 08:34 PM   #7541
RCruz is online now RCruz  Switzerland
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Quote:
Originally Posted by Salas View Post
About the 30 Ohm spec of its standard buffer you are right, for BL should be ~60-70 Ohm. Looks like I had noted max spec for V type Fets that I had measured in a special build. Its an inadequate note. Should read ''under 100 Ohm with average IDSS JFETS''.
Noted.
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Old 21st April 2012, 09:43 PM   #7542
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The mains voltage here in the UK I find is often higher than 240V. The transformer primary is rated at 230V. The net result is a higher than expected DC. I have a 30Vac transformer which gives me 51V DC which is going to be too much for the LM317. I need a 28V trannie to do this right.
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Old 21st April 2012, 10:16 PM   #7543
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Hi
my shunt is not regulating. LedStringV=5.45V VgsQ1=3.6V dummy load=940R VgsQ4= ~0.001VDC??? . Need 28VDC 30mA regulated. Output is like 3VDC. No smoke . Checked several times- wireing, schemo, parts values, all OK. Both mosfets(9240) are cool- not even warm. Any ideas?
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Old 22nd April 2012, 12:31 AM   #7544
Salas is online now Salas  Greece
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Chalk: Congrats for debugging it. Why wrap it in a chip pre reg anyway? Adding circuitry... keep it simple, it has its own input insulation via its CCS.

Milos: Your shunt Mosfet ain't open. Check orientation, health, 4 wires circuit to load, check if you got drop on R1.
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Old 22nd April 2012, 06:16 AM   #7545
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Thanks Salas. I only went with the Lm317 because I thought the input to the shunt needed to be regulated. The 40V comes from a CRC. If this is good enough then I can stick with my present trafo.
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Old 22nd April 2012, 08:42 AM   #7546
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Ricardo

you moved to Switzerland now?
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Old 22nd April 2012, 12:15 PM   #7547
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Quote:
Originally Posted by Salas View Post
Check orientation, health, 4 wires circuit to load, check if you got drop on R1.
Orientation OK.
Health: all parts are brand new and both channels behave the same. I doubt that something is broken. It is mainly P2P, a bit p.i.a. to check, I leave it for the end if everything else (if anything) fails.
4 wires twisted connected to the load.
R1 drop 1.5V.
I feed it with 36VDC (10000uF per a ch. filtered)
Trimmer has no function, turning it in either direction produces only slight variation in voltage output which is ~3V.
Q4 on either ch. wont open.
Layout attached.
Dotted lines=jumpers.
I have no idea what to look for after about 10 hours of checking and staring
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Last edited by Milos P.; 22nd April 2012 at 12:17 PM.
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Old 22nd April 2012, 01:12 PM   #7548
Salas is online now Salas  Greece
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The CCS shows 150mA then. Without pulling parts you can look if all BJTs show Vbe ~0.6V, if Mosfets show Vgs a few volts, if JFET pulls some drop on its resistor in series. Change Q5 if no drop on R7, make sure R6 is connected like a rheostat.
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Old 22nd April 2012, 01:13 PM   #7549
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BC546b=Hfe ~360
SK170BL=Idss ~8mA
2N5457 not matched
No short gate to drain on MOSFETs
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Old 22nd April 2012, 01:15 PM   #7550
Salas is online now Salas  Greece
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Live, vbe, Vgs, drop voltages.
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