Mezmerize DCB1 Building Thread - Page 15 - diyAudio
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Old 18th January 2011, 01:15 AM   #141
Salas is online now Salas  Greece
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Quote:
Originally Posted by westers151 View Post
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
See an example. Building a symmetrical psu B1 buffer The inner ones are to be the slightly less in IDSS per pair, and the pairs should be around same ballpark not to have much difference in dissipation and to behave similar dynamically.
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Old 18th January 2011, 01:44 AM   #142
nezbleu is offline nezbleu  Canada
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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)
OK, I'm coming back to the well one more time to be sure I understand this. In trying to select parts it is difficult to find transistors which list hFE on the order of 10; for example, the 2N3904 data sheet (http://www.fairchildsemi.com/ds/MM/MMBT3904.pdf) lists hFE's of 30-300. However, those are all at Vce=1.0V and varying collector currents. Then it shows collector-emitter saturation voltage of 0.2V, with Ib=1.0mA and Ic=10mA, which to me implies an hFE of 10. Further, it lists the base-emitter saturation voltage of 0.65-0.85V. Can I take it that these "saturation voltages", as opposed to "breakdown voltages", are what I am interested in, and that hFE is not some fixed gain ratio but a more fluid quantity that varies a lot with other operating conditions? Would the 3904 be a good choice? Would a Darlington with higher current gain be better? The 3904 seems better to me than the 2N2222 because of the lower base current, and the lower base-emitter saturation voltage.

I really do appreciate your pointers, and hope I'm not hijacking this thread with what is really a side-bar.

Thanks.
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Old 18th January 2011, 08:42 AM   #143
AndrewT is offline AndrewT  Scotland
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Hi,
yes and no.
Use any cheap To92 that has sufficient Ic max. Speed, Vce0 and gain are unimportant in this slow switching application.

Using an excess of base current one pushes the switching transistor into saturation, where Vbe is higher than the usual 650mVbe +-50mV and the Vce is well below 200mV. At an effective 1:10 ratio of base:collector current the Vce can usually be in the region of 1mV to 50mV.

But the transistor has no internal mechanism to protect the base from damaging base current values. You must have a base current limiter. This is usually just a resistor that limits the worst case base current to ~10% of the collector current.
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Old 23rd January 2011, 05:36 AM   #144
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Originally Posted by Horio View Post
I'm in the process if building my Blue Mez with 2 x 20ohm 2W resistors in place of the 68 ohm ones (per Tea-Bag's recommendation). This gets you in the range of the "standard" hot rod from what I understand.
Iíve been looking for your source to find out more info, just to confirm the 20R/2W x 2 units replaces the 4 x 68R/0.5W is that correct?
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Old 23rd January 2011, 09:00 AM   #145
AndrewT is offline AndrewT  Scotland
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There are two CCS in the Mez, the +-ve supply and the -ve supply.
Each supply has two locations for resistors. That is 4 resistors to each Mez PCB.

I use 600mW resistors, my stock of power resistors is tiny.
I find that 30r with ~1.8V across it dissipates ~110mW. this will run fairly cool.
I also use the bottom side of the PCB to add extra parallel resistors, if I need more current.

Now you could use up to 8off 30r resistors on 1 PCB.
Each 30r gives about 60mA of CCS current. This needs to be checked at time of build.

If you fit 3off 30r you will get ~3times the current.
If you fit 7off 30r you will get ~7times the current.

Using this methodology I don't need to buy any new resistors, they all come from my standard E24 stock.
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Old 23rd January 2011, 09:15 AM   #146
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Thanks Andrew, that’s a great help indeed.
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Old 27th January 2011, 06:43 PM   #147
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I just finished building a Mezmerize dcb1 and powered it up for the first time. The power LED lights up as well as both clusters of 5 LEDs, but the two clusters of 3 LEDs don't appear to light up at all. I hear one of the omron relays click after a few seconds. The LEDs came from Ed Lafontaine's group buy. I built it per the BOM except with a 12v transformer.

I haven't done any measurements or tests yet, I'll check the DC offset and whatnot once I remember where I put my DMM.
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Old 27th January 2011, 07:03 PM   #148
Salas is online now Salas  Greece
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Is the orientation of the triplets OK?
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Old 27th January 2011, 08:31 PM   #149
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Yup, one was backwards in each triplet, now they light up. I've gotta stop drinking and soldering at the same time...

Last edited by blue lander; 27th January 2011 at 08:35 PM.
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Old 27th January 2011, 08:38 PM   #150
Salas is online now Salas  Greece
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Poke the DVM around for V+/V- PSU, DC offset, drop across current set resistors, for a general health check. Let us know. Congrats for putting it together.
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