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31st December 2019, 10:03 #1
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Low Noise Adjustable Linear ACDC Power Supply
Features:
AC – DC Conversion
Double output voltages (Positive – Ground – Negative)
Adjustable positive and negative rails
Just a SingleOutput AC transformer
Output noise (20MHzBWL, no load): Around 1.12mVpp
Low noise and stable outputs (ideal to power Opamps)
Output Voltage: +/1.25V to +/25V
Maximum output current: 300mA to 500mA
Cheap and easy to solder (all component packages are DIP)
A double output low noise power supply is an essential tool for any electronics enthusiast. There are many circumstances that a doubleoutput power supply is necessary such as designing preamplifiers and powering OPAMPs. In this article, we are going to build a linear power supply that a user can adjust its positive and negative rails independently. Moreover, just an ordinary singleoutput AC transformer is used at the input.
References
Source: http://bit.ly/2Qc9idO
[1] LM317 Datasheet: http://www.ti.com/lit/ds/slvs044x/slvs044x.pdf
[2] LM337 Datasheet: http://www.ti.com/lit/ds/symlink/lm137.pdf
[3]: Schematic Symbol and PCB Footprint for LM317: https://componentsearchengine.com/pa...?partID=248007
[4]: Schematic Symbol and PCB Footprint for LM337: https://componentsearchengine.com/pa...?partID=290650
[5]: Altium Plugin: https://www.samacsys.com/altiumdesi...yinstructions

31st December 2019, 12:11 #2
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Re: Low Noise Adjustable Linear ACDC Power Supply
1.2mVpp is high for a Low Noise Grade Power Supply.It will presumably be higher at loaded condition.
Look at that..
https://www.analog.com/en/products/l...oductoverview
In additional to, a Filter has to be used at the Input with appropriate High Quality Capacitors.

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31st December 2019, 13:28 #3
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Re: Low Noise Adjustable Linear ACDC Power Supply
Why the 10 Ohm resistors in series with the input? The additional filtering they provide is minimal. If it is to reduce input surge current they would be better placed before the first reservoir capacitors.
Brian.PLEASE  no friends requests or private emails, I simply don't have time to reply to them all.
It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.

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31st December 2019, 13:53 #4
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Re: Low Noise Adjustable Linear ACDC Power Supply
1mV is high?! Really it is difficult to understand if it is the noise floor of the oscilloscope or not. Besides, in ALL power supplies when we increase the output current, then the noise will increase. even many manufactures don't mention the noise figure! in this design I tried to address many applications with cheap and easy to use components. of course, there are more advanced expensive options.
   Updated   
They are necessary for filtering

31st December 2019, 16:10 #5
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Re: Low Noise Adjustable Linear ACDC Power Supply
Any thermal shutdown protection?

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31st December 2019, 20:36 #6
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1st January 2020, 02:24 #7
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Re: Low Noise Adjustable Linear ACDC Power Supply
They both do.
Brian.PLEASE  no friends requests or private emails, I simply don't have time to reply to them all.
It's better to share your questions and answers on Edaboard so we can all benefit from each others experiences.

1st January 2020, 09:49 #8
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1st January 2020, 11:52 #9
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Re: Low Noise Adjustable Linear ACDC Power Supply
Hi,
I think this is a standard design for a linear power supply.
Not bad.
There's nothing wrong with it. It will work ... for most cases satisfactory
*****
I'd like to comment the voltage specifications. (From the view of an electronics engineer)
In the schematic the input capacitors are rated 25V.
So if one wants to chose a transformer...it's nominal output voltage should not be higher than (25V  0.6V) / (1.2 × sqrt(2)) = about 14V.
(A transformer rated with 14V (@ nominal load) may have 1.2x higher voltage when not loaded, the capacitors will charge to peak of sine = sqrt(2) × RMS, minus the voltage drop of the diode of 0.6V @ low load).
In detail it depends on transformer specifications, but also in mains voltage stability.
One should also consider some headroom for the capacitor voltage rating.
So let's use this 14V transformer....but now at full load of 500mA output current.
The nominal output voltage now is 14V RMS. The capacitors now are pulse charged once per fullwave and maybe just in 20% of time.
This means every 20ms (@50Hz) there need to be a pulse current of 2.5A during 4ms. (Estimated).
This causes a higher voltage drop in the diode, let's assume 1.2V.
And additionally the sinewave gets a flattened top. It won't go up to 14V x sqrt(2). Let's assume a 2.5V reduced top (assumed 1 Ohms source impedance).
The peak value at the first capacitor will then be 14V x sqrt(2)  1.2V  2.5V = 16V.
The capacitor will be discharged (100% 20%) of 20ms with 500mA, this gives a ripple of 16ms x 500mA / 2200uF = 3.6Vpp
The second capacitor will reduce this peaktopeak ripple, but let's assume it will be about 2.0Vpp at the second capacitor.
Additionally there will be an average voltage drop of 10 Ohms x 500mA = 5V at the resistor. (@2.5W power dissipation!!)
Thus the minimum voltage at the second capacitor will be 16V  2.0V  5V = 9V at full 500mA output current.
If one now considers a 2V voltage drop at the voltage regulators one can expect just 7V of clean, regulated output voltage at 500mA load.
Note: All the voltages are estimated, they may differ +/ several volts in the real circuit.
How to improve the situation:
* using input capacitors with higher voltage rating
* then you are able to use a transformer with higher output voltage
* using diodes with less voltage drop
* using full bridge rectifier
* using a transformer with more stable output voltage vs load current
* using larger capacitors
* reducing lower ohmic series resistor (now 10 Ohms), or replacing it with an inductor
* reducing specification for output voltage
(When you buy a power supply with rated 25V output voltage, then it should be able to supply this at full specified load current)
KlausPlease don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.

1st January 2020, 13:25 #10
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Re: Low Noise Adjustable Linear ACDC Power Supply
Another point to consider. Asymmetrical output load may cause transformer saturation due to the single wave rectifier configuration.

3rd January 2020, 10:51 #11
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Re: Low Noise Adjustable Linear ACDC Power Supply
In the schematic the input capacitors are rated 25V
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