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AndrewT
R.I.P.

Join Date: Jul 2004
Location: Scottish Borders
when using a bjt as a switch, aim for hFE ~10 which puts the device into saturation and ensures the Vce is very low (~0.1V) This stops the bjt heating significantly.
The base current must be limited by that base stopper, so the [base supply voltage - base voltage] / [base stopper value] ~= Ic/10

eg.
if the base sits at Vf + Vbe above ground ~2.4V
and Ic~13mA
and Vbase supply = 5V (from the digital chip)

Then 0.013/10 = 5-2.4 / Rbs
Base stopper required would be ~2k (from rearranging to give Rbs=[5-2.4]/[0.013/10]

now to the LED bypass resistor.
most LEDs when viewed indoors look bright enough with 2mA to 5mA.
Let's take the higher value to start, because the modification to reduce brightness is very easy.

Vf of the LED @ 5mA ~ 1.7V
current to bypass is 13-5 =8mA = 0.008A
Rbypass = Vf / Ibypass = 1.7 / 0.008 = 220r.
If this is too bright then add a second resistor in parallel, try 470r or 300r or 270r.
You can leave the second parallel resistor in place or you can replace the pair with an equivalent resistor for your selected brightness. You can even make the second resistor selectable with a jumper for users who want different brightness.
Quote:
 Couldn't all 6 LED's share a common power source but be individually switched on by their associated transistor?
yes, just tap off a single supply voltage and insert a dropping resistor to feed all 6 LEDs. I knew someone would see a better way.
__________________
regards Andrew T.

Last edited by AndrewT; 15th January 2011 at 09:21 AM.

 15th January 2011, 12:14 PM #132 westers151   diyAudio Member   Join Date: Dec 2008 Thanks Salas, I've done what you suggested and am now getting sensible results (but I also realised I wasn't measuring correctly last night - it's been a long week!!). Now I'm getting consistent Vf of 1.74 on the LED's so that would give me 8.7V for Vref and 5.22V for CSS. I think those voltage numbers are close enough to the 9V and 5.4V recommended, but if anyone thinks otherwise then please advise. Thanks
nezbleu
diyAudio Member

Join Date: Dec 2009
Quote:
 Originally Posted by AndrewT when using a bjt as a switch, aim for hFE ~10 which puts the device into saturation and ensures the Vce is very low (~0.1V) This stops the bjt heating significantly.
[snip]

Wow, thanks once again for such a detailed reply! I think I am now leaning toward the parallel-LED approach, just to be sure there are no issues with the relay coils. So I will ditch the emitter resistors, and with the hints you have provided start filling in some values. This might actually work!

Bill

 17th January 2011, 12:49 PM #134 ramallo   diyAudio Member     Join Date: Nov 2004 Location: Madrid After I heard the weekend the DCB1, I'll want to go to a full Hod Rod (First stage). I have a few questions (200mA Hot Rod): 5w or 6W 10ohm wirewound resistors are ok or I need get a metal resistors? (Low inductance), the MUR120 are ok for the 200mA?, will be my chassis enough for dissipate the heat?.
 17th January 2011, 01:59 PM #135 Salas   diyAudio Chief Moderator     Join Date: Oct 2002 Location: Athens-Greece Since you only got 1.55V average across your setting resistors, use 8R2 5W to near 200mA, any type your heart desires. Non critical. MUR120 have taken 200mA in reported builds enough times in the black PCBs original building thread.
marcus1
diyAudio Member

Join Date: Jul 2004
hot rodded mesmerize

Trying to decide on what board to buy from the latest GB, so:
From what I understand/read, the mesmerize standard version runs at 60 mA - has anyone built a hot-rodded version (of the mesmerize) that runs at 600 mA or close to that?

Thanks

Quote:
 Originally Posted by Salas Since you only got 1.55V average across your setting resistors, use 8R2 5W to near 200mA, any type your heart desires. Non critical. MUR120 have taken 200mA reported builds enough times in the black PCBs original building thread.

 17th January 2011, 07:57 PM #137 westers151   diyAudio Member   Join Date: Dec 2008 Am i correct in assuming the matched SK170's are located next to the BC550 and BC560's> Thanks
Salas
diyAudio Chief Moderator

Join Date: Oct 2002
Location: Athens-Greece
Quote:
 Originally Posted by marcus1 Trying to decide on what board to buy from the latest GB, so: From what I understand/read, the mesmerize standard version runs at 60 mA - has anyone built a hot-rodded version (of the mesmerize) that runs at 600 mA or close to that? Thanks
Someone did it and it was on a flimsier black board if I remember well. Maybe it was a Hypno, but still black first run, equivalent. Don't go that high, its not like the Hypno Blue with its more filter caps, space for fat setting resistors, bigger diodes etc. 200-300mA will play very well.

Salas
diyAudio Chief Moderator

Join Date: Oct 2002
Location: Athens-Greece
Quote:
 Originally Posted by westers151 Am i correct in assuming the matched SK170's are located next to the BC550 and BC560's> Thanks
The must be matched are where it says 4x2SK170. Those are the audio fets. The others are PSU.

 17th January 2011, 09:40 PM #140 westers151   diyAudio Member   Join Date: Dec 2008 Thanks Salas - As for matching, I want to check my understanding of this too. My understanding is that each pair of SK170's should have the same Idss, but the two pairs do not have to have the same Idss as each other, although it is good to get the Idss of each pair as close to each other as possible. This part of the build has always confused me, but I'm trying to learn. Thanks

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