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necplusultra 1st February 2011 10:21 AM

where is soft start relay inserted in PSU circuit

I would like to know where the relay of a soft start is inserted in a PSU circuit.

After the recifier?
after the regulator?
before amp supply?

I've built a 5v time delay circuit to energize the relay but I don't know where the best place to insert it.


DF96 1st February 2011 05:23 PM

Somewhere between the incoming mains supply and the first (reservoir) capacitor, but if you have to ask this question is it safe for you to build a PSU?

alexcp 1st February 2011 06:56 PM

Normally softstart goes before the transformer's primary - see, for instance, the reference power supply design from National. Be careful with the mains voltage, though.

Or, if you want to know where the relay should be powered from, there are different views on that. One way is as per the app note above; another is from a separate "housekeeping" power supply.

necplusultra 1st February 2011 10:13 PM

I must admit that I am a newby but this is why I am asking these questions. Rough learning curve. Thanks for the danger concern though. So I can assume that this relay is best right after the rectifier and before the reservoir capacitor.

I am using a seperate "housekeeping" transfo for this purpose.

necplusultra 2nd February 2011 03:36 AM


been doing more reading and the relay is used to short the 3 parallel 5W resistors on the main AC before the transfo. The resistors are there to absorb the initial power surge and the relay kicks in after about 200 ms delay. At least this is the way I understand the soft start circuit.

I originally thought this was more of a "mute" type circuit protection.


DF96 2nd February 2011 09:25 AM

It depends on why you want a soft start: to reduce the DC inrush current to the transformer (useful with toroidals, as they have low DC resistance), or just to reduce the initial charging pulses for the reservoir capacitor.

AndrewT 2nd February 2011 11:21 AM

we all want you as a Member.
Learn about the dangers of working with mains electricity before working with mains electricity.
Get someone who is skilled to show you, or take a course at night school, or do a lot of reading and asking.

Soft start is different from slow charge.

To save money and space and time, many try to combine these two functions into one device.

A soft start is to start up a transformer, but to limit the start up current so that it does not blow the fuse, nor trip the breaker.

A slow charge is to charge up the smoothing capacitors slowly enough so that they do not heat up excessively internally and do not exceed their maximum charge rate (V/us)

The soft start is fitted in the mains primary circuit. This is where the biggest danger is.

The slow charge should preferably be placed in the secondary before the capacitors, but it can be either side of the rectifier.

Before you do anything else, build a mains bulb tester, bulb ballast, dim bulb. There are hundreds of references in this Forum and elsewhere. The Dim bulb is in google from wayyyy back. This will be your first mains powered project, get it checked before you plug it in !!!!

Read what it is, what it does to save you blowing up your projects and your self and how cheap and simple it is to assemble.
Learn to use it for starting up every mains powered project. And use it again every time you modify a mains power project.
I recommend you bulb test the wiring to your mains transformer with nothing else attached as the first stage of keeping your self alive.

necplusultra 3rd February 2011 08:51 AM


Thanks for the danger tips AndrewT. I am quite aware of mains danger. I've redone all of my home electricity from breaker panel to outlets. Our outlets are mostly 110V but some large appliances are running off 220V. It was a shocking experience (no pun intended). So I am testing my gear from 110V. I'll use your advice testing a lightbulb for my soft start circuit. My PSU is already built and using a 150VA toroidal. I am hearing a thump in my speakers when I turn on the switch. This is what I would like to eliminate. I built the timer circuit on a breadboard and now I am ready to test it through the relay on 110V mains. I will try the lightbulb first.

Nonetheless, I do have another question. Our 110V has one hot, one neutral and one ground wire. So When I connect to the outlet, we are never sure about the hot or neutral wire. Ground is always the middle but sometimes the hot and neutral are reversed from within the outlet.

Does it make a difference on which wire to connect the 3 resistors to main?
Does it always have to be on the hot wire and if so, how do we make sure we are tapping the + side?

Some threads mention to use three 33 Ohm 5W resistors and others go as high as 68 Ohm. What would be the ideal rating for 150VA off of 100V main?


MJL21193 3rd February 2011 10:27 AM


Originally Posted by necplusultra (
My PSU is already built and using a 150VA toroidal. I am hearing a thump in my speakers when I turn on the switch. This is what I would like to eliminate.

A soft start will not get rid of a turn on thump (that is amp related, not power supply related). Also, you really don't need a soft start on a 150VA transformer, the inrush current draw is low enough.

It's always better to put all of the switches, fuses, and other current limiting devices on the hot mains wire. Standard outlets here have 2 slots and the ground pin. The wider slot is neutral - trace it back to the IEC socket (if you are using one).

AndrewT 3rd February 2011 10:43 AM


Originally Posted by MJL21193 (
Standard outlets here have 2 slots and the ground pin. The wider slot is neutral - trace it back to the IEC socket (if you are using one).

Does that equate to polarised plugs and sockets?
One cannot insert the plug top the "wrong" way around.

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