SSLV1.1 builds & fairytales

[Salas]

Originally Posted by coolhead

Hi,
I am not really good in electronic. I am not too sure of Rx01 and red LEDs to use.
I want to build 3 kinds of output with this BiB board, and here I list out the red LEDs and Rx01 setting.

1. Input is 6.3Vac, output is 3.3Vdc with 100mA (BII Trident)- Rx01=20ohm 1W, red LED=0
2. Input is 9Vac, output is 5.0V with 400mA (Philips CD player 5V regulator supply)- Rx01=2ohm 5W, red LED=1
3. Input is 12.6Vac, output is 12.0V with 1500mA (tripath 2024 amp)- Rx01=0.5ohm 5W, red LED=4.

Am I correct? Anything I miss out?
Thanks.

Need to know the voltage drop on your red leds at ~3mA current. Use 9V battery, 2k2 resistor, and light up 2 or 3 led for a test. Measure the voltage on them when on. Tell us the average. Also clarify, if those currents you mention are just the load demand in each case. Are you having the newer boards that can take up to 4 leds or the ones that can take up to 3? Then you will get your answers.

[coolhead]

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.

[Salas]

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.

[Salas]

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).

[coolhead]

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.

[coolhead]

Hi Salas,
That is very good explanation, I think should put in the manual. I understand it now.
Thank you.

[cersepn]

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?

[william2001]

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

[coolhead]

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.

[Salas]

Change all small transistors first.