That's a damn good idea Tom. I really like that idea. I have never dealt with a CSS, but I am very interested in learning more now.
It just so happens that I have a spare PCB mountable heat sink that is meant for transistor-like devices.
My PCB can't be larger than 60 sq. in but I doubt it'll be that large.
I assume that using a current regulator from my B+ to ground with a control resistor dumps a specified amount of charge to ground? That's perfect for my application if I am understanding correctly.
It just so happens that I have a spare PCB mountable heat sink that is meant for transistor-like devices.
My PCB can't be larger than 60 sq. in but I doubt it'll be that large.
I assume that using a current regulator from my B+ to ground with a control resistor dumps a specified amount of charge to ground? That's perfect for my application if I am understanding correctly.
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The CCS'es are pretty easy to use. They're really J-FETs or depletion mode MOSFETs, but they're marketed as constant current sources. Digikey has them... The IXCP10M45 is a popular one. It can handle up to 450 V and I think 0.1 A, though, probably not both at the same time.
I used them in my 300B amp. See the 300B_Amp.pdf schematic in This Post.
Q1, Q2 are the CCS parts. R1 prevents parasitic oscillations. R2 sets the current. Similar for R3, R4. See the IXYS data sheet for a graph of which resistor to use for a given current. The CCS only needs a few volts across it before the current through it stabilizes. This makes it possible to test out the CCS on a low-voltage lab supply before you stuff it in the PCB and make it dance at 325 V.
And yep. You connect the CCS from B+ to GND to bleed the charge off the caps. Put the LED in series with the CCS if you want LED action. If the LED is to be mounted on the front panel as an ON indicator, I suggest placing the LED "on the bottom" of the CCS. I.e. between the CCS and GND. Safety first and stuff...
There's plenty of room on your PCB. The components are currently miles apart. If you place the parts so the signals flow better (minimize the rat's nest) rather than lay them out like they're placed in the schematic, I'm sure you'll find that you can save a lot of area. Just remember to keep hot components away from the electrolytics if you can.
~Tom
I used them in my 300B amp. See the 300B_Amp.pdf schematic in This Post.
Q1, Q2 are the CCS parts. R1 prevents parasitic oscillations. R2 sets the current. Similar for R3, R4. See the IXYS data sheet for a graph of which resistor to use for a given current. The CCS only needs a few volts across it before the current through it stabilizes. This makes it possible to test out the CCS on a low-voltage lab supply before you stuff it in the PCB and make it dance at 325 V.
And yep. You connect the CCS from B+ to GND to bleed the charge off the caps. Put the LED in series with the CCS if you want LED action. If the LED is to be mounted on the front panel as an ON indicator, I suggest placing the LED "on the bottom" of the CCS. I.e. between the CCS and GND. Safety first and stuff...
There's plenty of room on your PCB. The components are currently miles apart. If you place the parts so the signals flow better (minimize the rat's nest) rather than lay them out like they're placed in the schematic, I'm sure you'll find that you can save a lot of area. Just remember to keep hot components away from the electrolytics if you can.
~Tom
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You have access to this forum. Curious as to why you didn't inquire earlier in the design process.
Sheldon
I have been using this forum for quite a while. I had a few threads on different things. Most of my research has been from my text books.
I would do what Microsoft does - it's not a bug, it's a feature!
Skip the bleeder resistor and tell your professor that the system is designed for field use where electricity is not available.
Just plug it in for a few minutes before you leave for the field, unplug, and you will have have power for those 12AX7s to last days.
Skip the bleeder resistor and tell your professor that the system is designed for field use where electricity is not available.
Just plug it in for a few minutes before you leave for the field, unplug, and you will have have power for those 12AX7s to last days.
Haha, Loren. That is beautiful. I'm not in marketing; I'm in engineering. My job doesn't entail lies
You have to just think of yourself as management material and expand your horizons.
Texts are fine, but here you have access to professional engineers. I was just curious as to why you didn't seek their guidance a little earlier. I know that it's a school project, and you probably want to test your own capabilities. Yes you have to do your own research, but if you can, also seek input from knowledgeable practitioners. I wouldn't consider that cheating as long as you have really learned the principles behind the design.
A step that seems missing to me, is the first step in the process; namely specifications requirements. If the design starts with rational requirements, it can do its job properly and efficiently.
Sheldon
I agree. Asking for a "design review" after you have put a proposal together is a good idea. Not only that, you will be doing just that in the real world anyway, so get some experience now.
That, right there, is a big part of the problem of "education". You can learn just so much from books. Believe me, I learned a helluvalot more sitting at my work bench with a hot soldering iron and a stack of parts from Rat Shack than I ever learned in some lecture hall as some TA droned on and on, or from something I read once.
Engineering has always been part science and part art. It's the art aspect of it that can only be cultivated by actually doing.
I'd say that "book learning" and "lab learning" go hand in hand. I have working knowledge of *a lot* of circuits from my time in the lab, but understand the fundamentals of how those circuits work thanks to the "book learning" required for my college and university degrees. Combining the two makes me a creative engineer and I wouldn't want to be without either.
~Tom
~Tom
Here we are now with the constant current source.
I also squished together some of the components on the low power section, picked out a PCB mount transformer, and moved the heat further from the large caps.
An externally hosted image should be here but it was not working when we last tested it.
I also squished together some of the components on the low power section, picked out a PCB mount transformer, and moved the heat further from the large caps.
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I am in my last semester of my Associate's degree. I have had a lot of labs from digital to communications, but I learned more tinkering, googling circuits, and tearing things apart than I ever did from reading!
When I continue on with my Bachelor's degree, I will understand transient analysis and more analytical approaches to circuits.
By the way. It looks like you're trying to make 6.3 V DC from rectified 6.3 V AC. You won't have enough margin for that.
6.3 V will give you roughly 1.3*6.3 = 8.2 V on the reservoir cap. Allowing for, say 1 V of ripple (which would require a much bigger cap that what it appears you have) you'll only have about 900 mV of drop available for the regulator. Unless you're using one of the newfangled LDO's you won't have enough headroom for the regulator. Things become worse if you take variations in AC input voltage into account. The AC mains generally vary at least +/-5 % with sporadic drops of 15 % or so. The filaments have enough thermal mass to not be affected too much by the sporadic drops, but if the AC is 5 % low one day, it would suck to have hum in your amp...
In addition, I would suggest moving BR1 and associated components a bit further north of TR1. Unless your footprints are very accurately defined, you could easily end up a bit too close for comfort on those. -- Especially if BR1 ends up needing a heat sink (have you done the math?)
The trimpot for setting the 6.3 V out appears to be connected from the ADJ pin to GND on the regulator. Hence, when the pot fails, the output voltage will shoot up. This is not a desirable mode of operation. I usually make R1 a fixed resistor and put the pot (with a series resistor) in parallel with R2. See the PSU for the 300B amp I linked to.
~Tom
6.3 V will give you roughly 1.3*6.3 = 8.2 V on the reservoir cap. Allowing for, say 1 V of ripple (which would require a much bigger cap that what it appears you have) you'll only have about 900 mV of drop available for the regulator. Unless you're using one of the newfangled LDO's you won't have enough headroom for the regulator. Things become worse if you take variations in AC input voltage into account. The AC mains generally vary at least +/-5 % with sporadic drops of 15 % or so. The filaments have enough thermal mass to not be affected too much by the sporadic drops, but if the AC is 5 % low one day, it would suck to have hum in your amp...
In addition, I would suggest moving BR1 and associated components a bit further north of TR1. Unless your footprints are very accurately defined, you could easily end up a bit too close for comfort on those. -- Especially if BR1 ends up needing a heat sink (have you done the math?)
The trimpot for setting the 6.3 V out appears to be connected from the ADJ pin to GND on the regulator. Hence, when the pot fails, the output voltage will shoot up. This is not a desirable mode of operation. I usually make R1 a fixed resistor and put the pot (with a series resistor) in parallel with R2. See the PSU for the 300B amp I linked to.
~Tom
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