I think I have a neat, simple and easily implemented idea for a soft start circuit

[hifinerd]

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?

[AndrewT]

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.

[hifinerd]

Originally Posted by AndrewT

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.

[AndrewT]

Originally Posted by hifinerd

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

show the pair in your schematic along with the re-located relay.

[lineup]

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:

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 )

[hifinerd]

Originally Posted by lineup

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.

[AndrewT]

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.

[hifinerd]

Originally Posted by AndrewT

Hi,
your final circuit proposal is good. It is worth posting for all to see.

Ok, working on it.

[hifinerd]

Originally Posted by AndrewT

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.

[hifinerd]

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.