# I need a circuit to gradually incrase the voltage from 0 to 12 volts. Any idea?

My new car amp makes a pop sound when it gets the power. So i need to increase the voltage from zero, to twelve volts in 3-5 sec. I need a very simple circuit.

Update:

Like an automatic PWM

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• 2 months ago

A very large capacitor fed by a suitable high current resistor - eg. a headlamp bulb.

Add a relay across the cap so it pulls in once the voltage is high enough, and shorts out the lamp.

Or, far simpler and more reliable:

Use a changeover relay on the output of the amp with a much smaller R-C coil delay, so the amp output is temporarily connected to power resistors instead of the speakers for a second or so when power is applied, then it switches to the speakers after it has stabilised.

That's how the anti-thump circuit works in most commercial amps that have one.

• derfram
Lv 7
2 months agoReport

The 'far simpler' approach has a good possibility of working.  Pre-manufactured Speaker Protection boards are available and should work just as well on a car unit as they do on a home stereo.  Example https://www.amazon.com/Assembled-Speaker-Protection-Circuit-Electronic/dp/B00K1YXK1G

• 2 months ago

The most simple way by putting a very big variable resistor in series along the 12V positive line between +12V and amplifier positive terminal,and then turning it from maximum resistance to zero resistance with your hand .

You need to find out the current drain from your car amplifier at average normal drain that you did not provide over here.

Suppose it is a 500W car amplifier, it drains about 40A at max. and average about 20A. Buy a big variable resistor what ever you can get from market, has resistance between 0.5 ohm till 1 ohm can do, The resistor wattage rate can be chosen between 100W till 500W, The bigger wattage is better to avoid overheated in the first 3-5 seconds. No more heat is generated while it reaches to zero resistance.

Redo the calculation as the amplifier is not 500W.

• 2 months ago

These things are usually Class D (digital) and use a switch mode power supply (SMPS).  Soft start on these is generally not a good idea unless that soft start is engineered into the unit itself.

The reason is based on the characteristics of a SMPS.  A SMPS can produce its regulated output power over some finite range of input voltages.  What this implies is that it draws more current at low input voltage than at a higher input voltage.

If your unit (with SMPS) is specified to draw 20 amps at the nominal 12 VDC input, then it will draw 24 amps at 10 volts, and 30 amps at 8 volts (assuming it starts at that low a voltage).

Increasing the input voltage slowly (soft start) means that when the power supply first starts, it will start drawing a very high current - beyond what the installing may have been designed for.  The result is often a power cycling - unit tries to turn on, the current draw is too high and the applied voltage falls - the unit drops out and the cycle repeats.

On the other hand, the unit may have a built in power relay, so when you turn it on you are just actuating that relay.  A 'soft start' din the line you are using to turn it on/off will have no effect.

• Dixon
Lv 7
2 months ago

It isn't clear if you mean it pops when the ignition first connects the power to everything in the car, or just that the amp pops when individually turned on. Assuming the second, you would have to turn the amp on first (or leave it always on) and then turn on the soft-start power to the amp, which means another dashboard switch. In a perfect world you could get inside the amp and attach the mod to its own on/off switch.

The circuit below should work but it is just off the top of my head and untested. I don't bear any responsibility for failure / damage. It isn't a particularly soft start, maybe a few 10's of milliseconds at the actual turn on point. So if the amp still pops the 1uF could be increased to 10uF or 100uF.

Note it uses a p-channel MOSFET, nominal gate threshold Vgsth is between -2V and -4V and it can handle plenty of current. Take precautions to avoid static electricity damage when handling during construction.

In other applications such as powering a DC motor, this sort circuit would normally use an n-channel MOSFET to take the negative side of the load slowly to ground. However, in cars the case of an amplifier might well be earthed to the car body and also internally grounded, so we can't mess around with the amp's 0V and we have to go with slowly introducing the supply, which is what you wanted anyway.

• Dixon
Lv 7
2 months agoReport

Don't increase to 100uF, it will take too long to turn on!