Hi,
When looking for a good but cheap supply for digital circuits, there is a lot to find on this site. Too much is a better word, lots of schematics and even more opinions. However this seemed like a good startingplace for a cheap but good supply for the digital parts of my dac. When i come to feed the DACs itself, i'll try the more advanced stuff. This is for the digital circuits and maybe the xtal.
So i remembered a small trick to use a fet to drive a led over a big voltagerange (also mentioned somewhere in this post, allthough i connected gate to drain ??) Well i made it as attached. The result is a 5.31V supply.
However, i have no good measuring equipment (old tube-scope.. with 50Hz without signal) and no DAC yet to test this by ear. So it would be nice if somebody would try this and report back :->
I find the LT1086 too expensive for my needs (need 3 supplies for the dig. part alone), and the other stuff too complicated to make three. Other options, zener+transitor, shunt?
Would there be an improvement by using a 3.8V ref instead of the 2 LED's (other than getting closer to 5V)?
I could try red LEDs to get the voltage down a bit.
No caps attached yet, that is for later, on in and output terminal and accros the LEDs.
Greetings,
GuidoB
When looking for a good but cheap supply for digital circuits, there is a lot to find on this site. Too much is a better word, lots of schematics and even more opinions. However this seemed like a good startingplace for a cheap but good supply for the digital parts of my dac. When i come to feed the DACs itself, i'll try the more advanced stuff. This is for the digital circuits and maybe the xtal.
So i remembered a small trick to use a fet to drive a led over a big voltagerange (also mentioned somewhere in this post, allthough i connected gate to drain ??) Well i made it as attached. The result is a 5.31V supply.
However, i have no good measuring equipment (old tube-scope.. with 50Hz without signal) and no DAC yet to test this by ear. So it would be nice if somebody would try this and report back :->
I find the LT1086 too expensive for my needs (need 3 supplies for the dig. part alone), and the other stuff too complicated to make three. Other options, zener+transitor, shunt?
Would there be an improvement by using a 3.8V ref instead of the 2 LED's (other than getting closer to 5V)?
I could try red LEDs to get the voltage down a bit.
No caps attached yet, that is for later, on in and output terminal and accros the LEDs.
Greetings,
GuidoB
Power supply
The current for the the BS170 at 1.25 volts gate to source varies with the turn on voltage of the mosfet. You will get different output voltages for different fets. Replace it with a 1K ohm resistor.
The current for the the BS170 at 1.25 volts gate to source varies with the turn on voltage of the mosfet. You will get different output voltages for different fets. Replace it with a 1K ohm resistor.
Ok,
But does it matter? I am not interested in a supply which is exact 5.000V, but more in one which keeps the voltage as stable as possible.
Doesn't matter then if it varies a bit then from supply to supply.
So my question is, would the fet give a supply with a better regulation (noise, load) than the resistor.
Or does the fet vary more on load or temp or something else then than a resistor ?? Would think it is not as depending on Vout as with the resistor.
Thanx,
GuidoB
But does it matter? I am not interested in a supply which is exact 5.000V, but more in one which keeps the voltage as stable as possible.
Doesn't matter then if it varies a bit then from supply to supply.
So my question is, would the fet give a supply with a better regulation (noise, load) than the resistor.
Or does the fet vary more on load or temp or something else then than a resistor ?? Would think it is not as depending on Vout as with the resistor.
Thanx,
GuidoB
Data sheets for BS170:
http://www.fairchildsemi.com/ds/BS/BS170.pdf
http://www.futureai.com/electronics/datasheets/fet/BS170-D.pdf
http://www.semiconductors.philips.com/acrobat/datasheets/BS170_CNV_2.pdf
http://katalogi.iele.polsl.gliwice....-all--&ile=50&start=0&szukaj.x=16&szukaj.y=14
The typical Vgs threshold voltage is 2 volts for 1 ma. You should have about 1 to 3 ma through the LEDs. You might have to sort quite a few to get one with a turn on voltage around 1.25 volts. The drain to source impedance should be much higher than a resistor picked for 1 ma. Read the data sheet for the LM317 closely to see how the adjustment terminal works. You could add a diode biased by a current source to get your nominal 2 volts and not have to sort through as many BS170s. It is a good circuit if get the right fet Vgs. Temperature stability should be fine. Try it, its a pretty simple circuit. I would add a 1k gate resistor to the MOSFET as they are prone to oscillate without one.
http://www.fairchildsemi.com/ds/BS/BS170.pdf
http://www.futureai.com/electronics/datasheets/fet/BS170-D.pdf
http://www.semiconductors.philips.com/acrobat/datasheets/BS170_CNV_2.pdf
http://katalogi.iele.polsl.gliwice....-all--&ile=50&start=0&szukaj.x=16&szukaj.y=14
The typical Vgs threshold voltage is 2 volts for 1 ma. You should have about 1 to 3 ma through the LEDs. You might have to sort quite a few to get one with a turn on voltage around 1.25 volts. The drain to source impedance should be much higher than a resistor picked for 1 ma. Read the data sheet for the LM317 closely to see how the adjustment terminal works. You could add a diode biased by a current source to get your nominal 2 volts and not have to sort through as many BS170s. It is a good circuit if get the right fet Vgs. Temperature stability should be fine. Try it, its a pretty simple circuit. I would add a 1k gate resistor to the MOSFET as they are prone to oscillate without one.
I've used the LM334 with the LM317.
It is connected between output and adjust and feeds a string of leds. The 1.25V is within the operating range of this ccs.
With respect to the LM334's output impedance; the datasheet only shows a graph till 10kHz; I guess it's impedance at higher frequency will drop quickly and not be as good as a fet or mosfet used as ccs.
It is connected between output and adjust and feeds a string of leds. The 1.25V is within the operating range of this ccs.
With respect to the LM334's output impedance; the datasheet only shows a graph till 10kHz; I guess it's impedance at higher frequency will drop quickly and not be as good as a fet or mosfet used as ccs.
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