Building a symmetrical psu B1 buffer - Page 248

Salas · 22nd February 2012, 01:57 PM

CCS current=((3xVf)-Vgs)/Rset. The strings of 5 are going to set the +/- Vo. CCS currents are to be in the same ballpark and Vo polarities near enough. If the DC offset is going to be low in the audio output that is the only important criterion.

KatieandDad · 22nd February 2012, 02:00 PM

So the 5V (3 x LED) are only setting the current through the regs.

KatieandDad · 22nd February 2012, 02:05 PM

I've got approx 5.38V and the series resistors are currently 22R

I'm aiming for about 200mA - I did ask earlier about the voltage across the series resistors but no-one replied.

Using my BJT knowledge. With Vb at 5.38V, Vbe=0.6V so the voltage across the Emitter resistors needs to be about 4.78V.

I=E/R so I would be 217mA with a BJT. If that is still true with a MOSFET, I just don't know.

Salas · 22nd February 2012, 02:06 PM

Quote:

Originally Posted by KatieandDad

So the 5V (3 x LED) are only setting the current through the regs.

Those, the particular Mosfet Vgs, and the particular setting resistor. PMOS and NMOS are not having the same Id-Vgs curve in most cases anyway, no same Mosfet is guaranteed to have the same curve, and so it goes. But not crucial at all.

Salas · 22nd February 2012, 02:12 PM

IRFP9240 shows about 3.5V Vgs @ 0.2A and Tj 25C. The junction will go hotter, so something like 1.6-2V across Rset is probably expected given the variations. Most use 8R2-10R 5W for circa 200mA CCS.

KatieandDad · 02:41 PM

So my 22R 3Watters may burn up then ???

I'm still not sure about the Nichicom caps. They do test OK.

Salas · 22nd February 2012, 02:44 PM

They will just give less CCS current than you expected. Around 100mA maybe.

KatieandDad · 22nd February 2012, 02:49 PM

I can play around from that value.

Some guys have tried as much as 600mA.

KatieandDad · 05:37 PM

Well I've finished the PSU part of the DCB1.

I've got +/- 9.58V give or take 10mV or one digit on the DMMs.

Running current with the 22Rs is 63mA - a bit lower than intended but everything is running cool.

The voltage seems to be dropping slowly as warm them up, not much but it started at 9.6V and after a 10 min test they were 9.5V.

The IRFPs have no heatsinking, they are cool to the touch.

Next step is to fit the elusive matched 2SK170BLs and then it's finished.

udailey · 23rd February 2012, 05:56 PM

Remember, they dont have to be matched. AndrewT has covered that multiple times.

23rd February 2012, 05:56 PM	#2480
udailey Passive Aggressive diyAudio Member Join Date: Mar 2008	Remember, they dont have to be matched. AndrewT has covered that multiple times. __________________ You can purchase LDRs anytime to build a standard LDR attenuator or to build my new LDR Attenuator "A Lighter Note". Email me. diyldr@gmail.com

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22nd February 2012, 01:57 PM	#2471
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	CCS current=((3xVf)-Vgs)/Rset. The strings of 5 are going to set the +/- Vo. CCS currents are to be in the same ballpark and Vo polarities near enough. If the DC offset is going to be low in the audio output that is the only important criterion.

22nd February 2012, 02:00 PM	#2472
KatieandDad diyAudio Member Join Date: Nov 2011 Location: UK	So the 5V (3 x LED) are only setting the current through the regs.

22nd February 2012, 02:05 PM	#2473
KatieandDad diyAudio Member Join Date: Nov 2011 Location: UK	I've got approx 5.38V and the series resistors are currently 22R I'm aiming for about 200mA - I did ask earlier about the voltage across the series resistors but no-one replied. Using my BJT knowledge. With Vb at 5.38V, Vbe=0.6V so the voltage across the Emitter resistors needs to be about 4.78V. I=E/R so I would be 217mA with a BJT. If that is still true with a MOSFET, I just don't know.

22nd February 2012, 02:12 PM	#2475
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	IRFP9240 shows about 3.5V Vgs @ 0.2A and Tj 25C. The junction will go hotter, so something like 1.6-2V across Rset is probably expected given the variations. Most use 8R2-10R 5W for circa 200mA CCS.

22nd February 2012, 02:44 PM	#2477
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	They will just give less CCS current than you expected. Around 100mA maybe.

22nd February 2012, 02:49 PM	#2478
KatieandDad diyAudio Member Join Date: Nov 2011 Location: UK	I can play around from that value. Some guys have tried as much as 600mA.