Mezmerize DCB1 Building Thread - Page 14 - diyAudio
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Old 15th January 2011, 09:18 AM   #131
AndrewT is offline AndrewT  Scotland
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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.
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Last edited by AndrewT; 15th January 2011 at 09:21 AM.
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Old 15th January 2011, 12:14 PM   #132
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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.

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Old 16th January 2011, 06:48 AM   #133
nezbleu is online now nezbleu  Canada
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Quote:
Originally Posted by AndrewT View Post
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
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Old 17th January 2011, 12:49 PM   #134
ramallo is offline ramallo  Europe
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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?.
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Old 17th January 2011, 01:59 PM   #135
Salas is offline Salas  Greece
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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.
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Old 17th January 2011, 06:47 PM   #136
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Default 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 View Post
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.
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Old 17th January 2011, 07:57 PM   #137
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Am i correct in assuming the matched SK170's are located next to the BC550 and BC560's>

Thanks
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Old 17th January 2011, 08:03 PM   #138
Salas is offline Salas  Greece
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Quote:
Originally Posted by marcus1 View Post
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.
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Old 17th January 2011, 08:04 PM   #139
Salas is offline Salas  Greece
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Quote:
Originally Posted by westers151 View Post
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.
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Old 17th January 2011, 09:40 PM   #140
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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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