I think I have a neat, simple and easily implemented idea for a soft start circuit - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 3rd September 2010, 12:56 PM   #1
diyAudio Member
 
hifinerd's Avatar
 
Join Date: Aug 2010
Default I think I have a neat, simple and easily implemented idea for a soft start circuit

Hi All,

I have been trying to come up with a simple soft start circuit that routes power initially through a resistor or thermistor and then activates a shorting relay after a few seconds. I think I have come up with a workable circuit that is easier to implement than most discreet designs and should be relatively immune to noise and switch on hard.

A rough conceptual schematic can be seen attached below, but it will need refinement. Essentially I use a 556 timer, which is a dual 555 timer IC in the one package, as a high voltage, high current schmitt trigger. It activates sharply when the charge in the cap reaches 2/3 of the supply voltage, or 8v in this case, and won't change state again until the voltage drops to 4v. This makes it ideal for our application and hopefully makes it less prone to false triggering.

It is necessary to use two 555s (or one 556), as the 555 schmitt trigger is inverting and the first 555 will initially be in the high state, which is no good as this would turn the relay on from the outset. However, if we feed it directly to the second, the second will initially be in the low state. When the cap reaches 8v the first 555 will go low, which in turn will make the second 555 go high and activate the realy..

The 555 can source/sink 200mA and should be able to easily drive most relays directly.

I think this is a neat design that should be pretty foolproof and easily thrown together on a breadboard. Obviously some supply bypassing is needed for the 555 and the RC network needs to be tuned to suit your turn on delay. Oh and the 555 might need some zener protection to protect it from the inductive kick of the relay.

For more info on the 555 schmitt trigger have a look at this:

555 and 556 Timer Circuits

So what do you think?
Attached Files
File Type: pdf soft start.pdf (9.8 KB, 371 views)

Last edited by hifinerd; 3rd September 2010 at 01:03 PM.
  Reply With Quote
Old 3rd September 2010, 01:33 PM   #2
AndrewT is online now AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
if the relay is between V+ and 555 out, does that remove the need for the second inverter?
Add a Zener + diode across the relay coil for back emf suppression.
__________________
regards Andrew T.
  Reply With Quote
Old 3rd September 2010, 01:39 PM   #3
diyAudio Member
 
hifinerd's Avatar
 
Join Date: Aug 2010
Quote:
Originally Posted by AndrewT View Post
if the relay is between V+ and 555 out, does that remove the need for the second inverter?
Add a Zener + diode across the relay coil for back emf suppression.
1) Actually, yes i think you are right. I found a schematic for a single 555 circuit on the web here:

http://www.eleccircuit.com/wp-conten...powerdelay.gif

2) Already mentioned that in the last paragraph of my post

Thanks for simplifying things.

Last edited by hifinerd; 3rd September 2010 at 01:47 PM.
  Reply With Quote
Old 3rd September 2010, 01:53 PM   #4
AndrewT is online now AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Quote:
Originally Posted by hifinerd View Post
2) Already mentioned that in the last paragraph of my post
show the pair in your schematic along with the re-located relay.
__________________
regards Andrew T.
  Reply With Quote
Old 3rd September 2010, 01:55 PM   #5
lineup is offline lineup  Sweden
diyAudio Member
 
lineup's Avatar
 
Join Date: Dec 2005
Location: the north
This was my thinking, too.
This delay thing needs only 1 timer.

You take the output to relay from DISCHARGE Pin.
I should take it from OUT.
Even if most relay could be triggered by DIS,
we have the OUT Pin for SOURCE and SINK a lot of current.

The formula for calculate TIME, as I have got from my SPICE Sim
seems to be:
Quote:
T(sec) = RC x 1.11

RC = T/ 1.11
Here you can see the result of my testrun,
with RC set to 8 seconds ( 47uF and 153k )
Attached Images
File Type: png LM556_delay_1.png (11.3 KB, 645 views)
__________________
lineup
  Reply With Quote
Old 3rd September 2010, 01:59 PM   #6
diyAudio Member
 
hifinerd's Avatar
 
Join Date: Aug 2010
Quote:
Originally Posted by lineup View Post
You take the output to relay from DISCHARGE Pin.
I should take it from OUT.
Oops, yes you are correct. That is what I meant to do but I accidentally attached the output to the wrong pin in my haste to make the schematic. And yes, you can use only 1 timer as linked in my above post.
  Reply With Quote
Old 3rd September 2010, 02:06 PM   #7
AndrewT is online now AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Hi,
your final circuit proposal is good. It is worth posting for all to see.

Far better than the RC time delay that we see so many recommending after getting the circuit all wrong and fiddling with values to try to achieve some semblance of timer control.
I wonder how wide the timing tolerance becomes with variations in mains supply voltage?
I'll bet some of them never activate. Just like MyRefC when component tolerances go skiwhiff.
__________________
regards Andrew T.

Last edited by AndrewT; 3rd September 2010 at 02:08 PM.
  Reply With Quote
Old 3rd September 2010, 02:14 PM   #8
diyAudio Member
 
hifinerd's Avatar
 
Join Date: Aug 2010
Quote:
Originally Posted by AndrewT View Post
Hi,
your final circuit proposal is good. It is worth posting for all to see.
Ok, working on it.
  Reply With Quote
Old 3rd September 2010, 02:15 PM   #9
diyAudio Member
 
hifinerd's Avatar
 
Join Date: Aug 2010
Quote:
Originally Posted by AndrewT View Post
H
I wonder how wide the timing tolerance becomes with variations in mains supply voltage?.
Won't vary at all if you use a regulated supply...I think.
  Reply With Quote
Old 3rd September 2010, 02:50 PM   #10
diyAudio Member
 
hifinerd's Avatar
 
Join Date: Aug 2010
Default OK, here is the final circuit pending your suggested improvements

For your perusal...

EDIT: Ah crap! forgot the supply decoupling. Will add it later after hearing your additional feedback.

Also, one might want to use 15v as most people will be dealing with 12v relay coils.

Attached Files
File Type: pdf soft start.pdf (11.0 KB, 172 views)

Last edited by hifinerd; 3rd September 2010 at 02:53 PM.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Wanted: Simple soft start circuit MtBiker Solid State 14 4th August 2010 08:27 PM
simple soft start slr 5000 Solid State 30 31st July 2010 09:11 PM
Soft start & Soft Switch circuit: can anybody help? m.parigi Solid State 95 22nd August 2005 03:32 PM


New To Site? Need Help?

All times are GMT. The time now is 05:33 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2