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Wednesday, March 13, 2013

Two versions of soft start

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.

в
19:18

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Етикети: 4027, NE555,
NE556, soft
start, toggle
switch

12 comments:

Ivaylo KostadinovApril 1, 2013 at 3:56 PM

Great work!
ReplyDelete

Igor SpasokukockiyNovember 23, 2013 at 4:09 PM

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

ReplyDelete

Replies

ХристоNovember 23, 2013 at 5:09 PM

Yes, no problem.
Delete

Reply

Александър АтанасовMarch 1, 2014 at 12:44 AM

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.
ReplyDelete

Replies

ХристоMarch 1, 2014 at 1:18 AM

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:
http://www.labbookpages.co.uk/electronics/debounce.html
Delete

Александър АтанасовMarch 1, 2014 at 1:53 PM

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.
Delete

ХристоMarch 1, 2014 at 5:09 PM

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:
http://www.circuit-innovations.co.uk/push_switch.html
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.
Delete

Александър АтанасовMarch 2, 2014 at 12:29 AM

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.
Delete

Reply

Tomik723April 24, 2014 at 9:33 PM

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
http://img686.imageshack.us/img686/5293/dpa223letmo.jpg
It's designed in
Czech Republic by Pavel Dudek. I'am also from Czech Republic
:-)

Tomas
ReplyDelete

Replies

ХристоApril 25, 2014 at 1:51 PM

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

Delete

Tomik723April 27, 2014 at 8:22 PM

There is his "bigger brother" DPA
222 (about 120W to 4ohm per
channel).
http://reebaudio.webnode.sk/images/200000775-04980059a4-public/DPA222.jpg
Also
Pavel Dudek's work :-)
Delete

ХристоApril 28, 2014 at 12:28 AM

This is really pleasure for the
eyes! Such nice design!
Delete

Reply

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