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Soft power on circuit

sealdogfish

Dec 13, 2016
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Hi,

I'm trying to implement a soft power on and not sure if it's quite right.

When the button is pressed it should enable Q1 and then Q2, the current will flow to REG_INPUT and through R20 to keep Q1/Q2 enabled.
The P_OFF is connected to a GPIO which will be pulled low to power off Q1/Q2.

I'm wondering if there will be a voltage drop from VIN to REG_INPUT and if the resistor values are correct?

Thanks in advance!
 

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Harald Kapp

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Nov 17, 2011
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Your circuit basically looks workable, but it has one big flaw: When connecting the collector of Q1 to the base of Q2 as shown without current limiting resistor, you will effectively short circuit V_in to ground via the base-emitter diode of Q2 and the emitter-collector circuit of Q1. Insert a series resistor between teh collector of Q1 and the base of Q2

I'm wondering if there will be a voltage drop from VIN to REG_INPUT
Yes, depending on the transistor used ~0.1V...0.2V (look up Vcesat in the transistor's datasheet).
The resistor values are not critical as long as the base current into Q2 (see above) is high enough to drive Q2 into saturation at the load current drawn.
 

pgib8

Jul 26, 2015
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I would let the gate be charged from VIN but add extra capacitance and a resistor. Once power is applied, the gate will charge slowly.
 

Harald Kapp

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@pgib8 : Which gate? You're talking about another circuit. The op's circuit has bipolar transistors with emitter, base and collector. No gate.
 

pgib8

Jul 26, 2015
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Yeah, I guess I would just use one mosfet, with extra capacitance at the gate and a resistor to limit the charging speed of the gate. it was just a thought, maybe i'm wrong
 

sealdogfish

Dec 13, 2016
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I have updated the schematic.

This should be a lot better now I think.

R19/R20 are 1k which should be more than enough to saturate Q1 (< 0.01mA Ic)

R24 I have chosen 470R which should be enough to saturate Q2 up to 400mA or so (Circuit will not draw more than 150mA)

R21 I believe is necessary to prevent a floating input at the base of Q1, is that right?

P_OFF is connected to an ATMega gpio which I believe can safely source 20mA so the 1k R20 resistor should be a high enough value.

Do I need a capacitor for debounce or something like that?

Thanks a lot!
 

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sealdogfish

Dec 13, 2016
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Here is the complete power circuit with charger, regulator if you're interested. It's an RF remote control with one of those cheap 1.3" OLED i2c displays and CC1101 SPI rf module.

I've just been learning at home, never went to school for electronics or anything, want to make sure it's good before getting a pcb made.
 

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pgib8

Jul 26, 2015
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Best way to make sure it's good is to build a prototype.
 
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