Gallery, Projects and General > Project Logs
Mill Power feed
John Hill:
Good work Darren :thumbup: :thumbup:
Like I said, more ways than skinning the old moggie! :bow:
However, are you happy with the resistance replacing the zenor? The zenor is a constant voltage reference, how about this variation?
The red lines are the new bits. This will retain the voltage regulation whereas just putting the pot in place of the zenor converted your voltage regulated circuit into a variable resistance circuit where the pass transistor amplified the function of the pot.
[Thats all assuming of course I understand what I am talking about!]
Darren:
I don't think that would work John, you are keeping the 12V zener and paralleling a resistance to it. This would only serve to lower the output voltage as it's the reference to the transistor. You might not be able to get 12V output.
The zener is only a reference device, most of these circuits use a resistor instead. But the zener is more stable than a resistor and you can get them in many voltages. 12V zeners just happen to be handy to the manufactures of the circuits as they are always accurate. Resistors rarely are.
I think if you look at the semiconductors PDF files they will show a resistor and not a zener. The pot is just a variable resistor as you know. Therefore it varies the reference voltage to the transistor.
At the end of the day it's only a simple amplifier circuit.
I use this sort of set-up on my audio valves heaters where quality is paramount. The last thing we need here is noise getting into the circuit to be amplified along with the audio signal.
For this sort of use for powering machinery it's pure overkill. But it also happens to be convenient :thumbup:
Darren:
John, I should have said, on this circuit the zener was not regulating the output transistor but instead the smaller transistor before it.
I've no idea which one it is, didn't look, and the output transistor is a BD142, similar to the 2n3055 but rated at 15A not 35. More than good enough in this application.
I've been playing with the ATX power supply this evening. Interesting, never really considered them before.
You can't add or subtract the voltages as the protection circuit kicks in and switches the whole unit down,
So really stuck with 3,5, and 12V outputs all with good heavy current capabilities. The 3V can only be used with the slow speed circuit on the wiper motor. For some reason it won't work with the high speed. I don't know why there should be more than enough current as it's stated as 14A.
And on my unit the 12V is only 10V. I don't know if this is typical?
It works, and is probably good enough for powering the mill bed.
If anyone wants me to demonstrate please shout up. You lot are so quiet on this thread I'm wondering if the world has come to an end whilst I've been locked in the cellar :lol:
I do hope someone has benefited, cos I didn't really need one, but it will be used now its been made :ddb:
If anyone is still confused speak up.... :thumbup:
bogstandard:
Darren,
Don't be worried, we all have our own specialities, mine ISN'T wiggly amps and electric string, and also, I've got my feeds sorted already.
You just might be way above most of us, and we are just sitting back waiting for the punchline.
Bogs
John Hill:
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.
The zenor conducts while the top end is equal or above its zenor point the end result being that the top of the zenor is kept at a constant voltage. Connecting the pot as I showed is a voltage divider across the zenor, so, wind the pot contact to the top and the full zenor regulated voltage is applied to the following circuit, in this case the base of the smaller transistor in the Darlington pair, the pot in that position is the same as the original circuit disregarding whatever current there is through the pot (which should not be much). Wind the pot down a little and you are tapping off a voltage less than the zenor voltage which means less voltage on the base of the Darlington pair. If you want a voltage higher than the original then change the zenor for a higher voltage one, or you can add a diode to the zenor (in series) or a string of diodes each one increasing the voltage by about .6V.
My own bench power supply (which I made back in the '70's) has 32 diodes carefully soldered in series around a 32 position switch which provides a voltage from about 1.2V to 20V which is applied to the bases of four parelleled 2N3055s, in those days the switch was much easier to find than an integrated regulator! Though technically very crude such a circuit is simple in concept and is relatively 'smoke tight'.
The ATX power supply wont let you series the different voltages because they all share a common earth, the best you could do would be to take one of the positive voltage and 'add' it to one of the negative voltages but in most cases the negative voltages will not handle much current so it is not worth much for our purposes. If you are really keen there are dozens on pages on the web showing how to modify and make use of these power supplies. But to drive an electric motor I doubt it is worth the bother.
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