Discussion in 'Power Electronics' started by boresp, Jul 8, 2016.

1. ### boresp Thread Starter New Member

Jul 8, 2016
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0
Hi guys, I need to design something like this: http://www.circuitdiagramworld.com/...start_business_unit_circuit_diagram_3269.html
The thing is that I have a 12 V Lead Acid battery and I need a soft-start, something like the circuit I linked above. The problem is that you need more than 12 V to obtain 12 V in the stationary state but I have 12 V available at the battery. Do you know any other way to obtain a soft-start like that with the same input and output voltage?

Thanks.

2. ### Sensacell Well-Known Member

Jun 19, 2012
1,184
277
Can you explain what its for?
Why do you need a "soft start"????

3. ### boresp Thread Starter New Member

Jul 8, 2016
4
0
I need to design a 12 V power source with low noise which lasts for months without turning off, so I'm using 2 lead acid batteries which I alternate, when the battery I'm using is discharged I disconnect it and I connect the other battery while charging the first one. The problem is that when I connect the second battery I obtain a voltage peak due to the difference between the level of charge of both batteries. I need to reduce that voltage peak, so I want to increase the voltage gradually for a few seconds until I reach 12 V.

I hope I explained correctly, thank you!

4. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,699
909
What you need to search for is PWM soft-start motor. The power is converted to a square wave. The % on-time (i.e., duty cycle) is increased from 0% (off) to 100% (full on) over some defined period. The motor "sees" the average, not the individual pulses, and reacts as if the voltage is simply increasing over that period. If you use mosfets for the switching elements, you can get very close to 100% of your supply voltage.

Pre-made controllers based on that principle are plentiful. You need to define the current required for your motor to find the best fit.

John

5. ### boresp Thread Starter New Member

Jul 8, 2016
4
0
Sorry I forgot to mention that I can't use Pulse Width Modulation, but thank you anyways!

6. ### jpanhalt AAC Fanatic!

Jan 18, 2008
5,699
909
Can you please tell us how much current the device draws? Why can't you use PWM? Do you realize that PWM + low pass filter/integrator = an analog voltage ramp.

John

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7. ### boresp Thread Starter New Member

Jul 8, 2016
4
0
Ok, I didn't realize that, thank so much, I'll take a deeper look on that! The device draws 0.3 Amps aproximately.

8. ### Sensacell Well-Known Member

Jun 19, 2012
1,184
277
You could use two Pchan FET's as high-side switches, connected to the load via two diodes in 'OR' configuration.
Put a large cap between G-S, with a large resistor between Gate and Ground.

When the new battery is connected, the FET is off, as the cap charges, the FET will gently turn on.
Place a resistor 10X larger across the cap to discharge it while you change batteries.

You lose 0.6 V in the diodes, so the output will always be lower, but the battery voltage varies far more than that.

Last edited: Jul 9, 2016