Soft start using LM555

[portreathbeach]

I need a soft start for a PSU and decided that a LM555 may do the trick.

My idea is to have my pre-amp turn on my power amp via a relay. When the relay is energised it will apply voltage to a high power resistor that is in series with my transformer. After 1 second, the 555 timing circuit will energise a second relay which shorts the start-up resistor.

I was thinking of using a small 240 - 12 volt (or 5 volt) transformer which is powered when the amp is first powered. The output of this smaller transformer will be rectified and feed a 12v (or 5v) regulator which then powers the LM555. The output of the LM555 will energise the by-pass (resistor shorting) relay.

I'm trying to find the circuit that I need and found this

Power On Delay Circuits by 555 | Circuit Project Electronic

(second circuit down) and was wondering if this would do the trick and why 3 of the pins have been left floating?


My original idea for the soft-start was to make use of the PIC microprocessor that I'm using for IR decoding and input switching in my pre-amp to switch the bypass relay, but I want to be able to use the amp separately sometimes, so that's why I thought a small timer circuit would do.)

So then, will this timer schematic work for what I want?

Thanks in advance

[Th3 uN1Qu3]

Circuit 2 sounds like what you need. As for why those pins were left floating - they aren't needed in this circuit. The 555 can do a lot of things, and not all pins are needed for every application. One thing you'll want though is add a 10n capacitor from pin 5 to ground, to prevent the timer from false triggering.

[portreathbeach]

Thanks for the quick reply. I'll get experimenting right away....

[nightanole]

Dumb question, You can run a coil directly from a 555 timer?

[Th3 uN1Qu3]

Yes you can. But it's wise to add a pair of protection diodes before the relay so the 555 is not damaged by inductive kickback. See here: 555 and 556 Timer Circuits

[nigelwright7557]

It might be worth considering a small cheap PIC for this then you can add DC protection to the amp.

A PIC input triggers at 2 volts so you can just add a couple of resistors from the amp output to the PIC to check for DC being there too long.

I did this on my amplifier test rig and it has saved me a fortune in speakers.