Wednesday, March 13, 2013

Two versions of soft start

UPDATE: 08.02.2019
There is upgraded version of this project: Push button ON/OFF + Soft start v.3
 My new project is something that is useful when creating power amplifier with toroidal transformer. Usually the starting (inrush) current is very high for a brief moment until the smoothing capacitors are charged. This is stressful for the capacitors, bridge rectifiers and the transformer itself. Also the fuse may blow.
The soft start circuit is used to limit the starting current to acceptable level. This is achieved by connecting the transformer to the mains via a resistor which is shorted with relay contacts a few moments later.

I decided to combine a soft start schematic with push button toggle switch schematic to create a module ready to be used in a power amplifier or some other power consumer.

I experimented with two different schematics - one with CMOS logic 4027 and other with dual timer NE556.

The first schematic uses J-K trigger (flip-flop) connected as T-trigger (flip-flop). When the button is pressed the state of the trigger is changed. When going from OFF to ON state the signal is passed via a resistor and a capacitor to the second part of the schematic.
There the second J-K trigger is wired in a very unusual way with the reset pin connected high and the set pin used as input. Here is the datasheet of the 4027. In the truth table we see that when the reset pin is high all other inputs are ignored except the set pin.When the set pin is high, the output is also high and when the set pin is low the output is low.
The resistor R6 and capacitor C6 are used to delay the signal from OFF to ON state. With the values used in the schematic there is about 1 second delay. With different values it is possible to change the delay time if needed. The diode D2 shunts R6 when schematics goes from ON to OFF, so the second relay to be switched off without delay.

The second version uses a dual timer NE556. This one is more trivial. The first timer is wired as push button toggle switch and the second is an ordinary switch connected again with delay circuit R5, D2 and C6.

Both schematics share the same power supply circuit and soft start switching is the same.
The resistors R8-R10 are 150Ohm/10W and are connected in parallel resulting a 50 Ohm / 30W resistor. On the PCB two of them are side by side and the third is on top of them in the middle.
T1 is small 2-5W transformer with 12-15 VAC secondary wiring.
J1 connector is used if 12VDC is needed  for some external schematic, for example speaker protection or IR remote control module or even temperature controlled fan.
J2 is for connecting the momentary push button.
J3 is for connecting the LED, indicating power state.
K1 and K2 are 12V relays capable of switching 230VAC/16A.
F1 must be chosen according the device that would be connected to the soft start module.

Personally I prefer the first schematic. I tested both schematics on breadboard and both worked, but the second is prone to interferences - when the cable connecting the push button to the board is long and some powerful appliance is switching on or off nearby, there is a chance for false switching of the schematic.

There must be great care when working with mains wiring or else there may be fatal consequences.

Links for downloading of the schematics and PCBs in PDF format: 
Soft start with 4027
Soft start with NE556
Use them on your own responsibility.

Most of the resistors, capacitors and diodes in the PCBs are SMD. Lately I am using more and more SMD elements in my PCB designs because this saves drilling holes which is very annoying :)
If you decide to use any of these two PCBs, check them thoroughly, because they are not tested by me yet.

P. S. I found a mistake in PCB - wrong spacing of the relay pins, so I temporarily removed the download links until I fix the issue.

P. P. S. The error in the PCB was corrected and the download links are  back again :)
Here are pictures of the finished module:

This short clip shows first schematic in action on the breadboard.
The C6 in the video is 10uF so that the delay is clearly visible.


  1. Thanks for your work!!!
    Can I replace the resistor 50 ohm 30W to 62 ohm 30W ?

  2. Здравей, Христо.
    Направих варианта с 4027, но не работеше. Пробвах с чипове от различни производители - същият резултат. Проверих чиповете - всичките са си здрави.
    Оказа се, че причината е в С4 - явно прекалено омекотява фронта. Сигурен ли си в стойността му? Да не би да трябва да е 220 pF?
    Защото без него схемата си работи без проблем. Предполагам, че ще е добре да го има, за да гаси разни бързи преходни процеси, но със значително по-малък капацитет.

    1. В екземпляра на снимките съм ползвал 220n, даже сега го измерих на платката за всеки случай. При мен работи без проблеми. Останалите елементи точно по схемата ли са? Какво точно се случва като натиснеш бутона?
      Тъй като схемата е тригер, няма никакво значение колко е полегат фронта - в един момент трябва да превключи.
      Провери си резисторите R1, R2 и R3. Измери напрежението на пин 4 - трябва да е почти 0 V. На пин 3 трябва да е 0V и при натискане на бутона да става 12V.

      Предполагам и с по-малка стойност на C4 може да се мине. Важното е, като се натисне бутона да превключва състоянието само веднъж.
      На този адрес има кратка статия по въпроса за изчистването на сигнала от превключвател:

    2. Всичко си е точно по схемата. На пин 4 напрежението е 0.25V. Всички напрежения са си точни, елементите - също. Проверих всичко някаколко пъти, защото не можех да си обясня, защо не работи.
      Като натисна бутона, всичко работи както трябва - първото реле включва веднага, второто - след 1 секунда. Но това става, ако държа бутона натиснат. В момента, когато отпусна бутона, релетата изключват.
      Направих на пробна платка схемата в опростен вид и така успях да хвана, че причината е кондензатора. Но не мога да си обясня защо става така.

    3. Не знам защо се получава така :(
      Пробвай с различни стойности - 100nF, 10nF за да провериш какво става.
      Ето тук едно малко по различно свързване на бутона през диод:
      В тази схема ползват 100nF.
      Там също се споменава, че ако размениш местата на ключа и съпротивлението (R1 в моята схема) схемата ще превключва при отпускане на бутона, а не при натискане.

    4. Днес пробвах с различни стойности на C4 - до 10 uF. Същото поведение. Странното беше, че и с малка стойност (330 pF) пак се държи така. Ще го оставя без кондензатор, но така е на границата на поносимост :)))) В смисъл, че прибл. 10% от включванията се опорочават от преходните процеси.
      За идеята пробвах с кондензатор паралелно на бутона, вместо на R1. Резултатът е същия, което се очакваше, т.к. по променлив ток веригата е аналогична.
      Утре ще опитам с размяната на ключа и R1. Ако се получи, ще търся бутон с НЗК.
      Свързването с диод - мисля, че няма да има ефект. Диодът помага в момента на включване на бутона, а при мен проблемът се проявява при изключването му.

    5. The problem is the CD4027. It works well with NXP chip, I encountered the same problem with Texas Instruments chip and ST

  3. Perfect work ! I'll use it for my dual-mono amplifier DPA 223 :-) Thank you very much !
    There is the screenshot of the one channel
    It's designed in Czech Republic by Pavel Dudek. I'am also from Czech Republic :-)


    1. Wow, dude, this is some nice work! Very impressive!

    2. There is his "bigger brother" DPA 222 (about 120W to 4ohm per channel).
      Also Pavel Dudek's work :-)

    3. This is really pleasure for the eyes! Such nice design!

  4. Thanks a lot!
    Got my PCB soldered and tested! Works like a sharm!

  5. This looks great! I wonder if I could use this circuit to control two individual transformers of 500va each for a dual mono amplifier. I was thinking of a DPST relay but I am not sure if the circuit is capable of controlling two outputs. Thanks in advance!

    1. Two transformer primaries connected in parallel and switched like one. I don't see the reason to switch them individually.

  6. There is one fuse that "controls" both the small transformer as well as the 220V output. I guess that means that the fuse must be rated at the max amperage of both the output transformets, say about 8A. But, if one of them failed, the current might be less than what's needed to blow the fuse, so the output would be practically unprotected. I was wondering if I could use the fuse just for the 12V transformer, use a DPST relay to provide two outputs which would then be controlled by a 4A fuse each.

    1. You can still connect the two transformers in parallel and each of them with it's own fuse. In this case the fuse on the board will be redundant. You can also modify the schematic - the pin of the relay that is connected after the fuse must be connected before the fuse. That way the fuse will protect only the schematic and can be small value - 0.5A or even smaller.

    2. This is great! Thank you very much, I will go with this version!

    3. Sorry for bugging you, but this is an interesting circuit and I need to know one more thing. Would it work with 5v transformer instead of 12v? I am asking because I am planning to also use a microcontroller which operates at 5v and it would be great if I could use the same power rail. Of course I understand that the relay coils would have to be 4.5V or 5V to operate, but what about the rest of the circuit? Thanks in advance!

    4. The schematic work without problems with 5V supply. You should decrease the two base resistors R5 and R7 to 4.7k or 3.3k to make sure there is enough current to switch the relays.
      Also try to find HEF4027 - Someone in the comments pointed out that the schematic is not working well with J-K flip-flop from other manufacturers. I tested this and indeed the schematic is not working as it should with different chips. It work (kind of) without capacitor C4, but because of bouncing of the button contacts sometimes it registers two clicks instead of one.
      I am making an improved version, so if you are not in a hurry, you should wait for it. I will publish it in couple of days.

  7. I will wait for your improved version then. Thanks again!

    1. The new post is here :