Gallery, Projects and General > Project Logs
Mill Power feed
Darren:
--- Quote from: John Hill on April 18, 2009, 10:47:06 PM ---Darren, if the 'top' to the zenor goes to the base of a small transistor you may find the collector of that transistor is connected to the base of the big transistor, this is a Darlington pair configuration and the small transistor is amplifying the base current for the big transistor and allows the use of a smaller wattage zenor.
--- End quote ---
I'd have to draw the circuit out, but the emitter of the 1st transistor should go to the base of the 2nd transistor as shown in this diagram for a Darlington pair.
The two collectors should be common? That's how they increase the current capabilities in the order of 10,000.
Unless I'm missing something?
This is the schematic you are proposing....I think? Would that be correct?
And this is what I have done, replaced the zenner with a variable resistor.
Sorry but I still can't see how the zener helps? You have it in series with the pot with both ends tied to ground. At one end the voltage/current will flow through the zener (high speed) and at the other it will flow through the pot (low speed).
But the high speed will be limited to 12V by the zener, whereas they way I have it I can go to the full 20V of the transformer.
Unless I'm misunderstanding the schematic you are proposing, entirely possible...!
John Hill:
Darren, your schematics are exactly as I was thinking. One advantage of leaving the zener in the circuit is that you wind up with a regulated output.
More significantly however consider what happens in your circuit if you set the pot to near the top of its travel, set up there you have the full supply voltage over a small segment of the pot, high voltage over low resistance means hight current and quite possibly a little puff of smoke escaping from the pot as it suddenly burns out!
Of course you dont have to keep the zener but I suggest you have the pot in the configuration as per my schematic that way you wont be stressing the pot.
I think the pot should be a few Kohms and be aware that pots come in different 'tapers', linear and log(aka audio), I think a linear would be best for this job but might be harder to find.
Sorry for my mistake regarding the configuration of a Darlington, your diagram is correct and shows how the gain of the first transistor multiplies the gain of the second.
[Well, those are my thoughts anyway! :coffee:]
Paul Barker:
--- Quote from: John Hill on April 18, 2009, 05:25:48 PM ---how about this variation?
--- End quote ---
Zenner is upside down
John Hill:
--- Quote from: Paul Barker on April 19, 2009, 04:17:21 PM ---
--- Quote from: John Hill on April 18, 2009, 05:25:48 PM ---how about this variation?
--- End quote ---
Zenner is upside down
--- End quote ---
Antipodean Zener?
[Indeed that Zener is shown upside down assuming it is a positive supply circuit.]
:bow: :bow:
Paul Barker:
--- Quote from: John Hill on April 19, 2009, 04:54:12 PM --- "Antipodian Zenner?:beer: :beer: :beer:
--- End quote ---
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