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Most power MOSFETs will conduct 5 amps with much less than 4.7V gate-to-source voltage. I ran the simulation with 6 different MOSFETs, and only one of them didn't work. It was an FQB11P06, which has relatively high Rds(on) of 175mΩ. I admit that, if you wanted to build a lot of these, you would probably want a higher voltage op amp, but as a one-off, a 5V op amp should work fine. I searched Linear Technology's site and found LT1677, which is an RRIO op amp which will work over the supply range of 3V to 36V, so you can eliminate the zener.Thank you all very much for all the effort you are making.
Roff: Looks nice, but what about the mostly needed 10 V for the FET ?
The opamp is hanging on 4V7 below the supply, so having a maximum output of 4,7V.
Kind regards,
Edwin
Thank you all very much for all the effort you are making.
Roff: Looks nice, but what about the mostly needed 10 V for the FET ?
The opamp is hanging on 4V7 below the supply, so having a maximum output of 4,7V.
Mr Al: Your Idea went also through my mind before, to lower each signal in the same way,
to get in a more comfortable area. I didn`t do it, bcs the voltages are already very `small`.
But it might be the better way, indeed.
I think I can have a go now, will implement the advices and let you know, abt next week I think,
how it works.
Kind regards,
Edwin
hi willbe,
IIRC most of the rail/rail amps are about 5V supplies.
I would use a power schottky to isolate the two batteries, a 400mV drop should be acceptable.
I'm baffled. Can you elaborate?When it's turned on won't the MOSFET be off anyway?
It will only turn on when the gate capacitance becomes charged up enough.
I was responding to MrAl's remark regarding a slow start mechanism. I think he was implying that it's possible that a high current could flow when the circuit is powered on. I don't think this will happen because the MOSFET will be in the off state when the circuit is first powered up.
I ran some sims, and the op amp output did slew from zero to max, causing a high current spike in simulation. I thought that a cap from the + in to Vcc would help, but the problem still existed. The circuit below solves the problem in simulation, so long as the power supply takes 200us or more to slew from zero to max.Hi Hero,
Yes, but the MOSFET gate will probably charge faster than the op amp can work into the linear mode when the feedback starts to make a difference. I think the output of the LM358 will start up at ground, and gradually ramp up during this time. As it's ramping, the MOSFET is turned on pretty hard. The LM358 is pretty slow during times when it's not in the linear mode.
Something to force the output high might help.