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PS28

Low power dual power supply.

Contents


Introduction.

PS28 universal dual power supply for light loads.
The output voltage range is ±1.25 V to around ±20 V.
Maximum output current is ±100 mA, but this can be increased by using a heatsink on the regulators.
With some combinations of filter components and input/output voltages, the circuit is short-circuit proof, but in general expect that it is not.


Voltage regulators.

This is an overview of the different IC voltage regulators that can be used with the PS28.
These are typical values that do vary between manufacturers.

Table 1: Overwiev of different regulators.
TypeLM78xxLM78LxxLM79xxLM79LxxLM317TLM317LLM337TLM337L
CaseTO-220TO-92TO-220TO-92TO-220TO-92TO-220TO-92
Output voltage5 V to 24 V
fixed
5 V to 15 V
fixed
-5 V to -15 V
fixed
-5 V to -15 V
fixed
1.25 V to 35 V
adjustable
1.25 V to 35 V
adjustable
-1.25 V to -35 V
adjustable
-1.25 V to -35 V
adjustable
Maximum input voltage35 V (1)35 V-35 V-35 V37 V (2)37 V (2)-37 V (2)-37 V (2)
Output current1.5 A100 mA1.5 A100 mA1.5 A100 mA1.5 A100 mA
Current limit2.4 A140 mA2.2A140 mA2.2 A300 mA3.7 A320 mA
Ripple rejection, 120 Hz80 dB62 dB66 dB50 dB64 dB65 dB77 dB80 dB
Output noise voltage,
10 Hz-10 kHz (3)
13 µV13 µV40 µV40 µV38 µV38 µV38 µV38 µV
Output noise voltage,
10 Hz-10 kHz (4)
28 µV28 µV120 µV120 µV38 µV38 µV38 µV38 µV

(1)40 V for LM7824.
(2)This is specified as input-output voltage differential, but in this application the output capacitor will normally be discharged when power is applied so it is the maximum input voltage.
(3)At 5 V output voltage. LM3x7 adjust pin decoupled to GND.
4)At 15 V output voltage. LM3x7 adjust pin decoupled to GND.

There are a few voltage regulators available for higher voltages that will fit in the PCB (LM317HV, LM2936, TL783), but I have not tested the circuit with these.
Note that the PCB is designed for a maximum voltage between any track and GND of ≤100 V.


Schematic.

Schematic with LM7xxx regulators.
Fig.1: Schematic with LM7xxx regulators.

AC0..AC2 connects to a the mains transformer.
I10 and I11 are fuses.
D10..D13 is a bridge rectifier.
C20, C30 are reservoir capacitors for the rectifier.
C21, C31, R20, R30 are filter components that are very efficient in reducing the harmonics from the rectification. With the values shown, the 100 Hz component is reduced by more than 20 dB and the voltage drop over the resistor is 1 V at 100 mA.
If you do not need this, R20, R30 can be shorted and the reservoir capacitance doubled.
C26, C27, C36, C37 are HF-decoupling for the voltage regulators.
D20, D30 are reverse-voltage protection for the regulators. If the circuit is built as shown (where the only discharge path for the reservoir capacitors is through the regulators) D20 and D30 can be omitted.
C25, C35 provides some regulation for load-spikes. They are quite small, but enough for most linear circuits.
D22, D32 prevents the outputs to go below/above the ground. If this happens, the regulator may go into latch-up or die.
This can happen in circuits where a large part of the load is between VO+ and VO- (rather than to GND - like many OP-AMP circuits) during power-on if one of the fuses is open, if you use a voltage doubling rectifier or if there is a short-circuit between VO+ and VO-.
D14 is an LED to indicate power-on. It can be placed on the PCB or wired between the LED and VO- or GND terminals.

Schematic with LM3x7 regulators.
Fig.2: Schematic with LM3x7 regulators.

Same as fig.1, but:
C22, C24, C32, C34 are HF decoupling for the regulators. These are different locations than for fig.1 in order to have them close to the regulators.
C23, C33 are decoupling for the regulator's adjust pins.
R21, R22, R31, R32 sets the output voltage.
The regulator maintains 1.25 V across R21 and the minimum load current guaranteed to maintain regulation is 10 mA giving a maximum value of 125 Ω for R21. It is common to use a value of 240 Ω for R21 and this rarely causes any problems. It is possible to increase R21 if you have a minimum, known load current to GND (not to VO-).
The output voltage is set by the value of R22 according to:
R22 = R21 * (Vout - 1.25) / 1.25 (ignoring the current in the Adj pin).
R22a allows the output voltage to be reduced for trimming and is normally not required.
LM3x7_Calc.ods is a spread-sheet that calculates the resistors and output voltage (including most component tolerances). Download (as zip file).
D21, D31 prevents C23, C33 to be discharged through the regulator's adjust pins in case of a shorted output.


Transformer connections.

Normal transformer connection schematic.
Fig.3: Normal transformer connection.
V+ ≈ Vt * √2
Capacitor voltage rating > V+
Diode voltage rating > 2 * V+
Ripple frequency is 100 Hz / 120 Hz.

Single output transformer connection schematic.
Fig.4: Single output transformer connection. Fuse I11 and capacitor C30 are replaced with wire-jumpers.
V+ ≈ 2 * Vt * √2
Capacitor voltage rating > V+
Diode voltage rating > V+
Ripple frequency is 100 Hz / 120 Hz.

Voltage doubler transformer connection schematic.
Fig.5: Voltage doubler transformer connection.
V+ ≈ 2 * Vt * √2
Capacitor voltage rating > V+
Diode voltage rating > 2 * V+
Ripple frequency is 100 Hz / 120 Hz between V+ and V- and 50 Hz / 60 Hz between V+ and GND or V- and GND.
If there is a difference in the load current from V+ to GND and V- to GND, there will be a DC current in the transformer that may cause it to saturate. With normal audio OP-AMP circuits, this is generally not a problem.


Rectifier calculation.

Transformer and rectifier calculation is a very complex subject and is covered in (many) books.
PS28_Rectifiers.asc is a LT Spice model for simulating the rectifier.
The input to the model is:
Minimum, nominal and maximum mains voltage.
Transformer VA rating, primary voltage, full load output voltage and no load voltage.
Capacitor and resistor values.
Load current to GND or between VO+ and VO-.
Normal, single output or voltage doubler circuit.
The model assumes all transformer losses are resistive, so it is only reasonable accurate for transformers with low core losses (toroids).

PS28_Rectifiers.ods contains a list of output voltages and capacitor currents with some standard transformers.
Download PS28_Rectifiers.* (as zip file).

Plot of typical rectifier waveforms.
Fig.6: Typical rectifier waveforms.
Transformer: 30 VA, 2 x 15 V loaded, 2 x 17.4 V unloaded. C20, C21: 1000 µF / 50 V. R20: 10 Ω.

The lime graph is the voltage on C20.
The blue graph is the voltage on C21.

Fig.6 shows the voltages on the 2 capacitors at low, nominal and high mains voltages.
The upper, blue line is the voltage on C20 and C21 with no load. This is the voltage (at high mains voltage) that the capacitors and regulator must be able to handle.
The middle graph shows the voltage on C20. Vp is the peak voltage, Vv is the valley voltage and the ripple voltage is the difference between the two.
The lower graph is the voltage on C21, showing the effect of the R20/C21 filter. The valley voltage on C21 is the lowest voltage where the regulator must maintain regulation.

The only value shown in Fig.6 that is easy to calculate is the maximum capacitor voltage at no load:
Vmn is the nominal mains voltage.
Vmr is mains regulation. You normally use a value of 10%, but for some power sources like generators you may need to use a higher value.
Vmh is the highest mains voltage.
Vmh = Vmn * (1 + Vmr / 100)
Vsl is the loaded output voltage for each secondary at an input voltage of Vmn. This is normally stated in the transformer data-sheet.
Vsu is the unloaded output voltage for each secondary. This is normally stated in the data-sheet as a voltage. In some cases it is stated as transformer regulation in %. In this case:
Vsu = Vsl * ( 1 + Regulation / 100 )
The maximum peak output voltage from the transformer is:
Vsp = Vsu * Vmh / Vmn * √2
Example:
Vmn = 230
Vmh = 253
Vsu = 13.6
Vsp = 13.6 * 253 / 230 * √2 = 21.2
This is the maximum voltage across C20 in Fig.3. For the circuits in Fig.4 and Fig.5 this voltage must be multiplied by 2 as the 2 secondaries are in series.
You can subtract a diode drop ( 2 for Fig.4 ) if you want to take this to the limit.

Table 1: Maximum unloaded voltage per secondary for some values of DC voltage. Numbers in () are with a 0.5V diode drop.
VspVsu (Fig.3)Vsu (Fig.4)Vsu (Fig.5)
25 V16.1 V (16.4 V)8.0 V (8.4 V)8.0 V (8.2 V)
35 V22.5 V (22.8 V)11.2 V (11.6 V)11.2 V (11.4 V)
37 V23.8 V (24.1 V)11.9 V (12.2 V)11.9 V (12.1 V)

In some cases it can be difficult to obtain the required output voltage under load without exceeding the maximum unloaded voltage. A solution can be to take a transformer with better regulation (a larger transformer).


Component selection.

Capacitors


For most applications of this circuit, general-purpose electrolytics is the best choice.
It serves no purpose to use special low-impedance capacitors or "audio-grade" capacitors (unless you fancy their colors).
Contrary to common belief, general-purpose capacitors do have a very low ESR.
The ESR can be calculated from the dissipation factor ( DF or tan Φ ) [1]:
ESR = ( DF / 100 ) / ( 2 * π * f * C ) where DF is the dissipation factor (in %), C is the capacitance value and f is the frequency where DF is specified.
For a 1000 µF / 50 V capacitor with a DF of 0.12% this is 1.6 mΩ (this was the cheapest capacitor I could find a data-sheet for).

C20, C21, C30, C31


For most applications with up to 200 mA load 1000 µF, 35 V or 50 V capacitors is a good choice.
If you decide to use a smaller value, do check that their rms current rating is high enough.
The board accepts capacitors of up to 16 mm diameter with 2.5, 5 or 7.5 mm pin-spacing.

C22, C24, C32, C34 or C26, C27, C36, C37


These must be ceramic capacitors. 100 nF / 50 V will work.
Capacitors with a pin-spacing of 2.5 or 5 mm can be used.

C23, C33


These are 10 µF, 25 V..50 V with 2 mm pin-spacing.

C25, C35


These are 100 µF, 25 V..50 V with 2.5 mm pin-spacing.
This value may be too small for some applications, but if lower impedance is required, it must be placed near the point-of-load to be effective.
A 100 µF / 25 V capacitor has an ESR of around 20 mΩ. This is the same as 1 m of 0.75 mm2 (AVG22) wire.

Diodes


All diodes are 1 A rectifier diodes with sufficient voltage rating. I specify 1N4007 as I use these everywhere (and they were cheaper than 1N4001 the last time I bought).

Fuses


The board has universal footprints for fuse-clips for 5 * 20 mm fuses.
If this regulator is the only load on a reasonable small transformer it may be safe to use it without the secondary fuses (the primary fuse must be there).
I you run the board from an auxiliary winding on a large transformer, the secondary fuses are required.
A suitable value is normally T500mAL/250V. The T means slow or delayed, 500mA is the current rating and the L means "low breaking capacity" (a plain glass fuse), 250V means it is a mains fuse (not a 32V automobile one).

Resistors


The resistors for the LM3x7 regulators are 0.25 W..0.6 W 1% metal-film types.
If you use the worst-case minimum load current (10 mA) for LM3x7T, R21 and R31 are 120 Ω.
It is common practice to use a value around 250 Ω and this works in most cases.

R20, R30


The choice is a compromise between voltage drop over the resistors and the ripple voltage on C21/C31. Use PS28_Rectifiers.asc or PS28_Rectifiers.ods to find a suitable value.
These resistors must be mounted 5..10 mm above the PCB as they will get very hot or burn in case of a short-circuit on the output.
With TO-92 regulators and up to 4.7 Ω / 0.5 W resistors the supply is short-circuit proof.
With TO-220 regulators the circuit is not short-circuit proof with metal-film resistors.
Wire-wound resistors can be used (mounted on one end). These will tolerate a very high pulse-power load (typically ten times their rated power for 0.5..1 s) and should be able to survive opening the fuses. I have not tested this.

Specification.

Specification are shown for 2 versions of the PS28.
PS28AAA is with LM317/LM337 regulators, PS28ACA is with LM7815/LM7915 regulators.
The transformer is 15 VA, 2 * 18 V ( 2 * 20.9 V unloaded ).
Unless noted, mains voltage is 230 VAC.
The unit dBu is dB referred to 0.775 V.
More detailed measurements are in the file PS28A_Measurements.ods (part of the download later).

Table 2: Specification for PS28AAA and PS28ACA.
 PS28AAAPS28ACA
 V+V-V+V-
RegulatorLM317TLM337TLM7805CTLM7905CT
Output voltage, 198 VAC, no load (1)14.94 V-15.19 V15.04 V-14.99 V
Output voltage, 198 VAC, 50 mA load (1)19.94 V-15.18 V15.03 V-14.98 V
Output voltage, 198 VAC, 100 mA load (1) (2)14.94 V-15.18 V15.02 V-14.96 V
Output voltage, 198 VAC, 200 mA load (1) (3)14.93 V-15.17 V15.00 V-14.96 V
Output voltage, 253 VAC, no load (1)14.95 V-15.19 V15.02 V-14.98 V
Output voltage, no load (1)14.95 V-15.18 V15.06 V-15.03 V
Output voltage, 50 mA load (1)19.94 V-15.18 V15.01 V-14.96 V
Output voltage, 100 mA load (1) (2)14.93 V-15.18 V14.96 V-14.94 V
Output voltage, 200 mA load (1) (3)14.92 V-15.17 V14.89 V-14.89 V
Output noise voltage, 22 Hz - 22 kHz, 50 mA load-87.4 dBu ( 33 µV )-86.8 dBu ( 35 µV )-83.4 dBu ( 52 µV )-76.0 dBu ( 120 µV )
Output noise voltage, 400 Hz - 22 kHz, 50 mA load (4)-87.6 dBu ( 32 µV )-86.9 dBu ( 35 µV )-83.5 dBu ( 52 µV )-76.5 dBu ( 120 µV )
Output noise voltage, 22 Hz - 80 kHz, 50 mA load-86.2 dBu ( 38 µV )-82.6 dBu ( 57 µV )-83.0 dBu ( 55 µV )-75.9 dBu ( 120 µV )
Output noise voltage, 22 Hz - 22 kHz, 100 mA load-87.3 dBu ( 33 µV )-86.6 dBu ( 36 µV )-82.7 dBu ( 57 µV )-75.4 dBu ( 130 µV )
Output noise voltage, 400 Hz - 22 kHz, 100 mA load (4)-87.4 dBu ( 33 µV )-86.8 dBu ( 35 µV )-83.1 dBu ( 54 µV )-76.1 dBu ( 120 µV )
Output noise voltage, 22 Hz - 80 kHz, 100 mA load-85.8 dBu ( 40 µV )-82.3 dBu ( 59 µV )-82.4 dBu ( 59 µV )-75.2 dBu ( 140 µV )
Output noise voltage, 10 Hz - 10 kHz, 50 mA load (5)23 µV25 µV37 µV87 µV
Output impedance, 10 Hz, 50 mA load (6)14 mΩ45 mΩ50 mΩ42 mΩ
Output impedance, 1 kHz, 50 mA load26 mΩ38 mΩ25 mΩ60 mΩ
Output impedance, 20 kHz - 20 kHz, 50 mA load177 mΩ47 mΩ298 mΩ441 mΩ
Board size (length / width / height)96.5 mm / 45.5 mm / 33.5 mm

(1)Ripple is below wide-band noise.
(2)Regulators get hot - should have a heat-sink.
(3)Regulators get very hot - must have a heat-sink.
(4)This measurement checks for low-frequency noise. It is <0.1 dB lower than the 22 Hz - 22 kHz value for white noise. If it is higher than the 22 Hz - 22 kHz noise, it indicates that there is some mains hum present.
(5)Calculated from 22 Hz - 22 kHz, 50 mA noise for comparison with data-sheet values.
(6)This is specified at 10 Hz as I can not measure the DC output resistance due to thermal effects.

PCB.

PS28AAA photo.
Fig.7: Photo of mounted board.

Download PS28A design files.

I have boards available for this project. See the PCBs page.


Known Issues / updates.

PS28A:
No known issues.


References.

[1] Illinois Capacitor "Impedance, Dissipation Factor and ESR"

Poul Petersen, C/Faya 14, 35120 Arguineguín, Las Palmas, Spain.
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