Philips PE 1535 power supply modification - Page 1
Forum section: electronics / beginners

author: Zapata
author_rank: Newbie
timestamp: June 01, 2020, 01:29:41 pm
content: Hello everyoneHere I am with a "first power supply project". I hope I can finally finish it with some help and input from you.The project:I have been looking for a variable voltage - constant current power supply circuit with maximum output voltage of 30v and maximum current of 1A.I first started experimenting with LM317 on a breadboard and learning about increasing current capacity using pass transistors. After destroying two 2n3055 and a LM317 I decided to improve the design and add a constant current functionality to my power supply project.I came across the good old Philips PE 1535 power supply:http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Philips/PE1535-PE1545_Operating_Manual.pdf(schematic on page 20)It seemed a good choose since it uses cheap and available parts and the design was understandable for me. I started with modeling it in LTSpice and gain more insight on how it works.Now at this stage I want to make some modifications to the original design.1. Add a constant current mode LED indicator2. Replace the long tailed pair with an OP AMP (if possible)3. Increase the maximum output current to 1AMy solutions:1. Constant current mode LED indicator:As the first attempt, I added a NPN, LED and a resistor to the circuit (see the attachment).The base of the NPN is connected to the output of the long pair. At the constant current mode, VBE become 0.6 and the transistor conducts. But the base current is too low to drive the LED. Is there a way to increase the base current without messing up with the constant current function?2. Replace the long tailed pair with an OP AMP:My idea is to learn about the OP AMPs in differential amplifier configuration in practice so I thought about replacing the long pair with an OP AMP. Is it possible or is it a good idea to begin with?3. Increase the maximum output current to 1A:The original design is for maximum current output of 0.5A.Provided that my heat sink can handle the additional power that need to be dissipated, what would be the best way to increase the current output?I thought of decreasing the shunt resistor (R137) to the half of its original resistance or alternatively, change the value of the input resistor of the long pair. What do you think of each option?Your inputs are very much appreciated

author: Zapata
author_rank: Newbie
timestamp: June 06, 2020, 01:33:26 pm
content: no one?

author: xavier60
author_rank: Super Contributor
timestamp: June 06, 2020, 01:59:40 pm
content: It might not be obvious, the Fig 1000 schematic is functionally similar to a design I have been working with for a year,https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/msg3089254/#msg3089254

author: Zapata
author_rank: Newbie
timestamp: June 14, 2020, 01:58:49 pm
content: Thanks xavier60.I do see some similarities with your design, with the difference that some of the components in your circuit are built in 723 voltage regulator.I still have to figure out how to add the CC LED.

author: Zapata
author_rank: Newbie
timestamp: June 14, 2020, 02:07:49 pm
content: Could someone explain what is the function of the negative voltage rail in this schematic?What would be the consequence of removing it from the circuit?

author: xavier60
author_rank: Super Contributor
timestamp: June 14, 2020, 02:33:42 pm
content: The negative rail is only for putting a small load on the output even when it's 0V.It can be replaced with a resistor between the output and to the left side of the CS resistor.The value will be a compromise between a useful load at low voltages and not too much dissipation at high voltages.Some sort of constant current sink is more effective and efficient although it's not possible to have the same current all the way down to zero volts without needing a negative rail again.Have a look at the Preload section in this design. It works well enough. It does impose a 10mV minimum on the output.https://www.eevblog.com/forum/projects/linear-lab-power-supply/?action=dlattach;attach=751023;image

author: xavier60
author_rank: Super Contributor
timestamp: June 14, 2020, 02:45:04 pm
content: Quote from: Zapata on June 14, 2020, 01:58:49 pmPutting the CC LED across R113 might work.EDIT: If the differential pair is replaced with an opamp. there will likely be stability problems.

author: Ian.M
author_rank: Super Contributor
timestamp: June 14, 2020, 02:49:21 pm
content: I posted a LTspice sim of the regulator section of the Philips PE 1535-00 0V - 40V, 0A - 0.5A. single output PSU three years ago. The negative bias supply is required to get the output to adjust right down to zero at no load. If, as Xavier suggests, you return R134 to ground(left side of R137, not output negative), it will have approx 0.7V minimum output at no load.You can find the sim here:https://www.eevblog.com/forum/projects/how-do-make-a-current-limiting-knob-using-lm723-for-linear-reg-power-supplies/msg1212923/#msg1212923IMHO tapping R113 for the CC LED would be too vague and 'mushy' as it starts passing significant current well before the current limit. Instead add a small signal NPN transistor with its base & emitter effectively in parallel with the LM723's internal current limit one. Use a 10K resistor in series with its base to avoid disturbing the current limit setpoint significantly. Its collector can safely sink a few hundred uA and will pull down to the 723's Vout (approx 8.6V). Use that current to switch a PNP transistor with its emitter connected to the +Unreg rail to drive the CC indicator LED.(see attached sim - copy it and paste it into the full PSU sim I linked above)N.B. be careful to choose transistors with an adequate Vceo rating!

author: xavier60
author_rank: Super Contributor
timestamp: June 14, 2020, 04:57:52 pm
content: The presence of the various Zeners in the Philips PE 1535 design limits the range of suitable input voltage.This could rule out the use of tap switching if the design is going to be adapted to higher output currents.

author: Ian.M
author_rank: Super Contributor
timestamp: June 14, 2020, 05:35:26 pm
content: Its not difficult to work around that. All you need is an auxiliary supply for the LM723 and current sense differential pair. If you replace the pair of 8.2V Zeners V106,V107 with a rail splitter I'm fairly sure you could use a single well regulated 16V supply for both.If you are going to use a single off-the-shelf transformer, you could use a center tapped secondary + a bridge rectifier to get two unregulated rails, one at twice the voltage of the other. Run the auxiliary supply regulator between CT and bridge -, and switch the supply to the main pass transistor(s) between CT and bridge + depending on the wiper voltage of the voltage set pot. When the tap is at CT it will be good for approx 60% more output current than when its at bridge +.

author: xavier60
author_rank: Super Contributor
timestamp: June 14, 2020, 11:00:42 pm
content: With my project, I used the relay to connect the transformer's 2 15VAC secondaries in parallel or series.I brought a diode out of the confusing mess that would always filter to 20V, regardless of the relay's state.In my case, it was regulated to 12V and used just for the fan and the relay coil.