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Soft Start Circuit

This purpose of a "soft start" circuit is to prevent a power surge when the component, such as an amplifier, is first turned on. Amplifiers with large capacitance in the power supply draw huge amounts of current upon initial turn-on, and this circuit is designed to softly charge the capacitors, which prevents fuses from blowing and prevents damage to the capacitors themselves. This particular circuit is designed to use a relay. When the power is initially turned on the relay will foward the AC voltage to resistors and/or thermisters in order to gradually charge up the large capacitors in the power supply. After a preset time delay, the capacitors will be mostly charged, and the relay will switch to provide power directly from the wall to the power supply. The circuit is based on Figure 2 from Elliot Sound Projects #39, Figure 2. Click on the link to see the entire project, and learn more about soft starts.

Here are the features of this little circuit:

Features

This is up and working, after I made the boards I discovered some errors, but those have been overcome. This will go into a future amplifier.

Click on Graphic/ Link For Zoom/Download Click on Graphic/ Link For Zoom/Download Description
Populated Board#1
Schematic

Here is shown the PCB populated with components. Just feed in some AC and you are ready to go.

Also shown is the schematic. To understand how it works, check the Elliot Sound Projects web site, project #39, figure 2.
Old Layout- Corrections to Make
New Corrected Layout

My 1st PCB layout had some mistakes, grounding out the power supply and swapping the drain and source on the MOSFET! Shown are the old layout with the appropriate corrections to make the layout work as well as a corrected layout. I've not actually built the corrected one, but there are only a few changes so it should work. I have verified the corrections to the old one make it work as it should. See the movies. Not shown in the old layout are the jumpers (they are shown in the corrected layout). You need to add jumper wires as shown in red if you want to use the undelayed DC output terminals (top and bottom left of board) in either of the layouts.

Movie, Operation overview

Zipped QuickTime Movie #1

Movie, Closer Up

Zipped QuickTime Movie#2

Here are some ".mov" movies in QuickTime, zipped. You need to UNZIP them prior to playing, though some operating systems have this built in. Try it and See.

The red light goes on when power is applied, and the green one comes on after the delay. Simple. Pretty poor resolution from my digital camera.