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Old 15th October 2011, 03:18 AM   #541
coolhead is offline coolhead  Malaysia
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The board I have is the latest version that can take up to 4 LEDs. Just finish doing it, i put in 2 green LEDs (D302A and D302B) and jumper the other 2 (D303 and D304). Supply Vac=7.2, after rectification is 8.99Vdc, R301 is 20ohm, out put I tie a 2.2k ohm resister to it.

Power it up and adjust the output to 3.3V. Voltage measure across LED at D302A is 1.92v, D302B is 1.93v.

The current I stated is more than what is needed. I shunt about 70- 100mA each. The tripath amp I just follow spec, that is maximum drawn current.
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Old 15th October 2011, 04:13 AM   #542
Salas is offline Salas  Greece
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How much voltage on R301?

P.S. I would expect 3 leds like those you measured, and 14 Ohm R301 for 100mA constant current. Better 12 Ohm or less for good headroom.
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Old 15th October 2011, 05:12 AM   #543
Salas is offline Salas  Greece
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See this easy graphical explanation so you can estimate any constant current source setting. The 1st graph below is the IRF9610 ID current VS Vgs curve. The higher the current the more the Vgs. The two vertical lines are the voltage of 3 and 4 leds groups. Say you need to target 2 points. One is 100mA, the other is 0.96A. You need to pick horizontal lines that meet the curves to the leds lines. You see that after 0.48A the 3 leds like yours come close to the curves. So better use 3 leds for under 0.48A and 4 leds for over. Or stronger 3 leds. The horizontal lines represent the voltage across the setting resistors. One line is at 100mA and is about 1.4 Volt long. Ohm's law says 1.4V/0.1A=14 Ohm. That is R101,201,301 in the PCB. Vrset2 line is about 1.9 Volt long. 1.9V/0.96A=1.98 Ohm.

Every 9610 will not be exactly same, temperature on every sink and setting will not be the same, every different Mosfet type will have other curves to look, even IRF610 has different. But with this graphical method you will always be in the football field. Just allow enough current on top of your load demands in calculation to allow for tolerances, and to have good extra for the regs to work better. The region up to 0.35A gives the steadiest current for temperature variations. Thankfully the positive temperature tracks well up to 0.72A. On hot sinks and higher currents better draw horizontal Vrset lines stopping between those two right hand curves. So your calculated current does not end up less when the sinks come up to temperature.

Always read the Safe Operating Area when planning strong currents and voltages to know the limits of each type (2nd graph=IRF9610 SOA).
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File Type: gif IRF9610_SOA.gif (16.1 KB, 779 views)
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Old 15th October 2011, 06:36 AM   #544
coolhead is offline coolhead  Malaysia
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With 20ohm at R301, voltage across is 0.07v.

Do the changes: R301 change to 10ohm, voltage across is 1.31v. Add 3rd green LED, D302A is 1.91v, D302B is 1.92v, D303 is 1.93v.
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Old 15th October 2011, 10:09 AM   #545
coolhead is offline coolhead  Malaysia
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Hi Salas,
That is very good explanation, I think should put in the manual. I understand it now.
Thank you.
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Old 15th October 2011, 04:29 PM   #546
cersepn is offline cersepn  United States
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I think there's quite a lot of guys using the shunt regs for their legato 3 i/v. Would you care to share the parts list and any tricks/advice specific for the legato 3 application please?
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Old 15th October 2011, 04:57 PM   #547
william2001 is offline william2001  United States
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I use the Legato 2...

R101= 5.6 ohm (5W wirewound)
R201= 3.3 ohm (5W wirewound)

R103, R203= 1.8k (as instructed in guide)
R105, R205= 5k trimmer (as instructed in guide)

shunt current seems about right, output voltage is stable and easily adjusted= happy
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Old 15th October 2011, 06:18 PM   #548
coolhead is offline coolhead  Malaysia
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Ar....... made a mistake while constructing my 3rd board, put BC550C at Q103 and 2SK117GR at Q104, it should be the other way around. Now no out put even Dxx2A, 2B and 3 light up. Dxx5 no light. I tried replace Q103 with new 2SK117GR and reuse back the BC550C at Q104. But same result, no output.

Anyone can tell me which components should I change after a mistake like that?

Thanks.
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Old 15th October 2011, 06:55 PM   #549
Salas is offline Salas  Greece
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Change all small transistors first.
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Old 15th October 2011, 07:07 PM   #550
Salas is offline Salas  Greece
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If it is a negative board, needs 560C.
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