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Yet Another Soft Start Circuit
Yet Another Soft Start Circuit
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Old 13th June 2019, 07:44 PM   #1
johnhenryharris is offline johnhenryharris  United States
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Default Yet Another Soft Start Circuit

After looking at a bunch of soft start circuits I decided to design one according to my beliefs as to what is the best way to do it. It may be completely different from your beliefs but there it is.

I wanted a separate power supply for the soft start circuit so it could be used to turn the amp on and off and the powering up of the amps large transformer would not interact with the circuit. The transformer has a dual primary so it can be used for 115 and 230 VAC. There is a surge suppressor, R1, across the transformer input as well as a small high voltage capacitor to filter out RFI on the line from coming into the amp. The power supply always draws a few milliamps, it powers a multicolor LED that emits red with the amp in standby.

The on/off switch only switches 15 volts DC at very low current so you don’t need a switch that can handle large AC current in-rushes. When switched on, the DC power is sent through a regulator for consistent voltage and time of circuit operation. Relay K1, a small current relay, pulls in and turns on a triac, AC current flows through a couple thermistors in series to limit in-rush current, two 5 Ohm parts for 115 volts to limit the current to no more than 12 amps, or two 10 Ohm parts for 230 volts also limiting to 12 amps. There is a current limiting resistor, R2, at the triac gate and a RC network,R3/C2, over the triac for some protection of the triac.

The circuit uses a voltage comparator, U2, to delay turn on of a relay, K2, which bypasses the thermistors, I do not like to have a current limiter staying in the circuit or the heat they generate inside the amp. The voltage comparator has a voltage divider, R8/R9, to set the voltage at 10v as the reference voltage on the positive input. That voltage is compared to the negative input which has an adjustable resistor, R7, which limits the current charging the capacitor, C6, and the rate of charge therefore voltage ramps up quickly, a couple seconds to more than 10 seconds, for the delay. When the voltage on the negative input matches the positive input, the output is turned on. There is a diode, D4, across R7 to give a quick discharge path for C6 so the circuit can reactivate quickly, as in the case of a temporary power loss. R2 limits the current through the comparator which drives the gate of a small MOSFET through a gate resistor, R11. The MOSFET pulls current through a large relay K2 to bypass the thermistors and another small relay, K3, which changes the LED from red to blue to show status ON. There is a Zener diode, D5 and a small capacitor,C7, at the gate of the MOSFET to protect and stabilize it, probably not needed but cheap insurance.


So far, the circuit has been very reliable in a couple power amps with 2400VA and 4000VA toroid transformers with .78 farads to 1.2 farads of storage caps. In small part quantities the soft start board costs around $34 USD.
Attached is the Schematic, BOM and Gerbers. Enjoy.
Attached Files
File Type: zip softstart.zip (15.3 KB, 56 views)
File Type: pdf softstart_Schem.pdf (175.6 KB, 418 views)
File Type: zip SoftStartV3.zip (14.7 KB, 46 views)

Last edited by johnhenryharris; 28th June 2019 at 06:05 AM.
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Old 13th June 2019, 07:52 PM   #2
lgreen is offline lgreen  United States
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I have used a lot of soft starts with bypassed thermisters- have you measured the current/ voltage across those thermisters over time as this starts up and they are removed from the circuit?

I only ask because I have not seen measurements of this before in a soft start application.
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Old 13th June 2019, 08:37 PM   #3
nigelwright7557 is offline nigelwright7557  United Kingdom
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I did a soft start circuit based on an 8 pin PIC micro, an opto-coupler and a triac.
Just dropped the PIC power supply off a resistor off the mains.
Worked a treat and no relays etc to burn out.
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Old 13th June 2019, 09:25 PM   #4
rsavas is offline rsavas  Canada
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In any soft start you have to consider the condition of what happens if the bypass device ckt fails to operate properly?
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Old 14th June 2019, 03:52 AM   #5
johnhenryharris is offline johnhenryharris  United States
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To Lgreen: the thermistors heat up with the current through them and they drop in resistance with the heat, when the relay pulls in and bypasses them the current through them drops to almost zero and they cool off. Since they are only in the circuit for 3 to 10 seconds they don't get that hot.


If the relay to bypass the thermistors were to fail to pull in, the thermistors will stay warm and remain at a lower resistance and since they are sized for much greater current nothing will be hurt.
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Old 14th June 2019, 04:52 AM   #6
wiseoldtech is offline wiseoldtech  United States
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Way too much complexity for me, I like to keep things simple, reliable, and smooth.
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Old 15th June 2019, 05:33 AM   #7
johnhenryharris is offline johnhenryharris  United States
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Actually, if you take it by section it is very simple. The power supply is nothing special.
The voltage comparator as a timer/switch is one of the simplest way to power up. And a relay is a simple mechanical device. The circuit has been very reliable in every amp produced for the past six months since I designed this soft start. It is really the best of many designs combined into one.
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Old 15th June 2019, 05:51 AM   #8
wiseoldtech is offline wiseoldtech  United States
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The amount of components are not "simple", sorry.
When a delayed turn-on can be accomplished with much less fuss.
Asides from all that, a properly designed amp, or any equipment, for that matter, doesn't need such things.
Tubes, for instance, have their own "delay" due to controlled filament heating.
Solid state has capacitor-related surge suppression.
If.... they are designed properly.
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Old 15th June 2019, 06:12 AM   #9
johnhenryharris is offline johnhenryharris  United States
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We're not talking about a delayed turn on here but a circuit to slow the in-rush current to a power amp with a large VA transformer and/or a very large capacitor bank.
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Old 15th June 2019, 06:42 AM   #10
wiseoldtech is offline wiseoldtech  United States
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The simple circuit used in Macintosh amps like the MC2300 works fine, and those amps are still around - I've serviced/restored one just last year for a customer.
However I don't feel the need for 600W RMS, 128 pound amp in a typical home setting.
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