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Old 11th March 2004, 10:30 AM   #1
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Default zehner regulator

ok,

i want to have ~5V. And i don´t want to use 7805. All the circuits with LM317, TL431 etc. use more parts, and so its not really less to use them as to build it discret --> first circuit ?

Whats the purpose of R2, i found it in a circuit on the net.


Now i want 2 times same voltage. As FETs and zehners have not really the same VGS and zehner voltage it would be a lot of matching and i need to order more parts to get 2 identical regulators. Could i do it like in the second ciruit? Disadvantages? and what value for the pot to choose?

Should i use other values for components?

thanks.
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Old 11th March 2004, 11:04 AM   #2
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Default R to D too?

The 100 ohm resistor provides a lower corner frequency for the 220uF filter cap since the impedance of the zener is usually in the range of 10 to 30 ohms. Unfortunately the power supply rejection of the voltage divided formed by 4.7K resistor and the zener has been made about 10 times worse by it's inclusion. Take the resistor out and connect the zener to ground. R2 is now connected between the positive side of the zener and the postive terminal of the 220uF cap. Same filter frequency without decreasing the regulation for the zener. You can make R2 larger and go up to a 1K ohms without any problem. The pot as in the second circuit will work fine. If you put a fixed resistor of 50k ohms between the pot and ground the circuit will be easier to adjust due to the smaller voltage change for a given rotation amount.
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Old 11th March 2004, 11:20 AM   #3
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Thanks. I also changed The 4,7k to 1,5 as i read there is less noise from the zehner with more current. Is this better?
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Old 15th March 2004, 03:50 PM   #4
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What about a current source instead of R8. Then you could get away with the 100ohm resistor and still have PSRR?
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Old 23rd April 2004, 04:30 PM   #5
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I built the circuit in #3, and now please don´t lough at me - discovered Vds seems to depend much on current. Between no load and 30mA, the voltage decreases 0,9V.

So should i better build instead of the FET a NPN, MJE15030 i have, and low ohm connected a 5,6V zehner, no pot?

The supply should feed a DAC , needs ~ 60mA , additional filter inductor/caps at the DAC.
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Old 23rd April 2004, 04:38 PM   #6
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Is it going to power digital logic?
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Old 23rd April 2004, 04:46 PM   #7
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No, to the analog +-5V supply of AD 1865
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Old 23rd April 2004, 05:29 PM   #8
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Default 7805

No really, you need to use the 7805. Or the 317 or some other IC regulator. It will be a lot more accurate and it will be cheaper. The series pass transistors in any of these circuits costs as much as a 7805.
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Old 23rd April 2004, 05:37 PM   #9
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but the transistors i have on hand, the ICs not.

Now i´m at 5.05V unloaded, 4,99V at 30mA. My cheap multimeter says 13mV AC independdnd of load. Will this be sufficient for analog DAC supply? (additional filtering direct at the DAC)
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Old 23rd April 2004, 06:35 PM   #10
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Default 13mV AC

This 13mV will go straight into your audio. I would not consider it anywhere near acceptable in the DACs I build. You will end up with maybe a 50db S/N ratio if you're lucky. The regulation sucks too and if there is nay load variation, that too will go straight into your sound. The 5V supply for the analog portion of a DAC id very critical.

You should go up to a 6.2V zener and use a darlington connected transistor. Also use a 2 stage RC filter to drive the zener. Either that or a 3-4 transistor feedback type regulator like we made in the 60s. The design of a linear regulator is really quite a complex affair that you are going through to save less than a dollar on a 7805.

The power supply is the foundation of every electronic circuit and if you cut corners there, the rest will never work as well as intended.
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