need soft start circuit schematic

BrianGT

Retired diyAudio Moderator
2002-01-21 2:42 pm
near Atlanta, GA
www.chipamp.com
I managed to locate the above mentioned article at:

Soft start

Aren't NTC resistors the same thing as thermistors?

I am stuck between two designs for a soft start circuit for my pair of Aleph 2 amplifiers. The two plans that I am considering are:

-Thermistors, as used in the original design.

-Soft start circuit with power resistors and a relay

I am thinking that the relay would be better, because it would be a much cleaner signal path, but I am setback by the complexity. Has anyone here gone the path of the relay? I am wondering if it would be worth the extra effort. I already have a pair of good high current relays that I picked up a while back. Anyone have advice on this choice?

--
Brian
[email protected]
 
My choice would be the relay and power resistor. You didn't mention the coil specs of your relay. This type of circuit can be made very simple. If the relay coil is rated for mains voltage just connect it parallel to the transformer primary. The power resistor should be in series with the transformer and the relay coil.

When the power is turned on the resistor in series with the transformer will limit the current flow to the transformer and reduce the primary voltage and the relay coil voltage. As the power supply caps charge and current flow in the transformer reduces the voltage drop caused by the series resistor will be reduced and the transformer primary voltage will start to rise. When the transformer primary voltage (this is the relay coil voltage too remember) gets high enough the relay will pull the contacts closed and short the resistor and provide full power to the transformer. This will normally be at about 75% of full mains voltage but depends on the relay.

Use care when picking the value of the power resistor. Too high of a value and the relay will never close to give you full power. Too low and the inrush current limiting will be less. I would pick a value that gives about 2-3 seconds delay from power on to relay close.

An added feature of this type of soft start is that if anything is wrong in the amp. (shorted PS caps, blown diodes, shored finals) the amp will not go to full power, but the resistor will get very hot fast so use a high wattage part or a mains voltage incandesant light bulb and pick a wattage that works (100 watt, 75 watt or what ever).

I have built this up as a seperate unit and just plugged the amp into it and used a switch on the delay unit as the power switch for the amp. This works best if light bulbs are used. You can hide them in a box and keep the bright flash hidden when the amp is turned on.
 
I use relay and resistor but not the same way. I go with a 48v dc
relay (coil) on my power supply can and I use 20-50 ohm 50 watts
on AC primary,when can voltage hit about 36vdc the relay go on and the resistor is out.With power supply of +/- 60dc I ad 1k 5w
in series with coil relay. If your power supply is lower than 45vdc
use a 24v coil relay. Always use my variac for testing from 105v-
130v. Never one come back in over 25 years.I use tripple contacts
relay in //.