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Old 15th September 2012, 03:47 PM   #1791
roger57 is offline roger57  Canada
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Quote:
Originally Posted by Salas View Post
OK, moved. You got only 0.5V/10R=50ma for some reason of low Vf Leds and high Vgs IRF610 samples probably and your 75 Ohm load chokes the output circuitry for sufficient current. The load is not wrong, Just see to lower R201 until you will see VdropR201/ValueR201=130mA.
OK, will try it and report back in a few minutes.

Thank you,
Gary
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Old 15th September 2012, 04:15 PM   #1792
roger57 is offline roger57  Canada
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Quote:
Originally Posted by Salas View Post
OK, moved. You got only 0.5V/10R=50ma for some reason of low Vf Leds and/or high Vgs IRF610 samples probably and your 75 Ohm load chokes the output circuitry for sufficient current. The load is not wrong, Just see to lower R201 until you will see VdropR201/ValueR201=130mA.
Improved, better, but still not quite there.
I changed R201 to 5 ohms. The ref LED's are now at "normal" brightness. I can adjust the voltage from -4.85 to -5.4, which is perfect. However, R201's drop is just 370mV, which means I am getting a meager 70mA, a bit short of the target of about 130mA. Here are some other voltages:

Q202 Vs = -7.47V
Q204 Vb = -0.63V
Q206 Vg = -4.7V


Gary
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Old 15th September 2012, 04:27 PM   #1793
Salas is offline Salas  Greece
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Utilize your 4th led position to have room for common R201 choices. What is 3-4leds string drop and Q201's Vgs is of interest, their difference is the voltage across R201.
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Old 15th September 2012, 04:41 PM   #1794
roger57 is offline roger57  Canada
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Originally Posted by Salas View Post
Utilize your 4th led position to have room for common R201 choices. What is 3-4leds string drop and Q201's Vgs is of interest, their difference is the voltage across R201.
Okay. When you say "4th LED position", is that a specific LED?

If the LED drops total across D302, 303 and 304 for 5.1V, it means that I need to add another LED to create a greater difference, and drop across R201?

Please elaborate. Thanks!
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Old 15th September 2012, 04:46 PM   #1795
Salas is offline Salas  Greece
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Most boards have pads for 4 Leds around the CCS. Some early ones 3. What PCB you got?
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Old 15th September 2012, 04:48 PM   #1796
roger57 is offline roger57  Canada
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Originally Posted by Salas View Post
Most boards have pads for 4 Leds around the CCS. Some early ones 3. What PCB you got?
Hi Salas,

I did the schematic capture and made the PCB. I have only 3 LED positions.
Looks like I need 4 ?

Gary
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Old 15th September 2012, 04:52 PM   #1797
Salas is offline Salas  Greece
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You got 1.7V leds at the current they run and 4.73V Q201 Vgs for ~70mA CCS now most likely. Either find stronger LEDs or botch 2x IN4003 or alike diodes in series with your now Leds under board. That will get you near target with 10R R201.
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Old 15th September 2012, 04:56 PM   #1798
roger57 is offline roger57  Canada
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Quote:
Originally Posted by Salas View Post
You got 1.7V leds at the current they run and 4.73V Q201 Vgs for ~70mA CCS now most likely. Either find stronger LEDs or botch 2x IN4003 or alike diodes in series with your now Leds under board. That will get you near target with 10R R201.
Okay.
I guess it's wrong to assume that the Vgs of the power FETs are close. Looks like they can vary by enough margin that one has to accommodate for this in the design.

Thank you for your kind assistance Salas,
Gary
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Old 15th September 2012, 09:11 PM   #1799
roger57 is offline roger57  Canada
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Quote:
Originally Posted by Salas View Post
You got 1.7V leds at the current they run and 4.73V Q201 Vgs for ~70mA CCS now most likely. Either find stronger LEDs or botch 2x IN4003 or alike diodes in series with your now Leds under board. That will get you near target with 10R R201.
Salas,

Using green LEDs has fixed the problem on the -5 supply. I paralleled 2, 10 ohm resistors and there is 140mA total current, 67mA to the load.

Now the -15V regulator is giving me grief. LEDs D205 and D206 won't even light up, and the maximum voltage I can get is -2.9V

The -15V PCB is identical to the -5 now, except that the -15 uses a 5K trimmer and a 1.8K for R203. Across the shunt resistor I am getting 0.73V, so with 5 ohms this is 150mA.

Can you please provide a suggestion as to what the issue might be?

Thanks.
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Old 15th September 2012, 09:17 PM   #1800
Salas is offline Salas  Greece
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See if there is continuity between all the Vref series components. If there is, check if one or both Leds there are not correctly oriented. Q203 should be conducting the Iref there, make sure its healthy if all other checks pass.
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