Default on power switch

Question

I want to design an easy power switch that is on by default, but I need the ability to turn the power off. 
 Is this posible with a few mosfet's? 
 thanks for any help

KJBob · Accepted Answer

Hello Zpike, 
 The circuit below remains on unless actively driven. Q1 and Q2 are P-channel MOSFETs. Q1 is biased on by R2 . Q2 is held off by R1 . When N-channel MOSFET Q3 is driven on by a positive logic input, it turns Q2 on , which shorts out the drive to Q1 which in turn, switches off the load. 
 
 The control waveform V2 (yellow) and the load voltage (red) are shown below: 
 
 Use a logic-level N-channel MOSFET for Q3, and ordinary P-channel MOSFETs for Q1 and Q2. You didn't mention what supply voltage and load current you need to switch -- this circuit will work with a supply voltage between 5 and 15 volts DC, and you must choose Q1's current and Rds (ON) ratings for your maximum load current. If your requirements call for higher voltage, more current or AC operation, you'll need a different solution. You could look into using a "high-side switch" IC, or a solid-state relay as a switch. A quick look at ADI's product index did not turn up either of these, but perhaps I missed something. 
 Best regards, 
 Bob

LucaV · Answer

zpike, 
 at these current and voltage levels you may want to consider the ADP197. 
 Connecting the EN pin high through a pull up resistor (10k, 100k) will keep it on but allow you to turn it off by shorting the EN to GND. 
 www.analog.com/ADP197 
 Other parts in the ADP19x family may be useful. 
 LucaV

KJBob · Answer

Hello LucaV, 
 The ADP197 is an excellent choice and a lot easier than three MOSFETs. Thank you for recommending it. I had looked at ADI's product list under Power Management (reproduced below), and concluded that ADI just doesn't make high-side switch ICs. Turns out they're under Hot-Swap -- what? High-side switches have lots more uses than just hot-swap. 
 
 A search for "high-side switch" would have worked but I didn't think of it at the time. 
 Best regards, 
 Bob

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Default on power switch

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