# predictablity of change in Vgs vs change in Ids?

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I am using a P-channel mosfet to act as a voltage controlled resistor to soft-start a small motor, which draws a couple amps while running with load. My circuit quickly (

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** Makes you a complete idiot.

Drive the same mosfet with a PWM wave at about 5 kHz - bring the motor voltage up to full from zero that way.

....... Phil

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Fuck you asshole.

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....... Phil

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The value you want is called the transconductance of the device. In a given MOSFET, it will vary by a factor of about 1.5:1 over temperature.

Although the maker doesn't give you a maximum spec for this, they do give you the minimum of 20 and a typical value on the graph called "transfer characteristics". Picking points off the graph, I see a typical value of about 40 or 50. Based on this I'd expect the maximum to be something like 100.

You may want to change your circuit so that it controls the voltage ramp on the motor instead of the current.

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If you want better control, try negative feedback: a resistor from source to ground will act as a nicely linear current monitor for feedback purposes.

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Is it intrinsically better to ramp voltage instead of current? Arent I doing that by using the mosfet as a voltage controlled resistor?

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I'm not following you. Do you mean if I were using opamps? I am using an NPN transistor whose base is connected, via 1k, to the drain of the P-ch power mosfet which controls power to the motor. When the P-ch starts to turn on, current goes to both the motor, and the NPN transistor base, which then slowly discharges a cap connected to the P- ch gate, slowly turning it on further.

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Ramping the voltage is a little better. The voltage more directly controls the speed of the motor. The back EMF of a motor is generated by its spinning. The back EMF is proportional to the RPM. The motor's current is more closely tied to its torque.

If you don't need anything too exact, a simple circuit may do what you need. Consider this simplified circuit:

Vcc ! \\ / \\ ! +---- Vout ----------+ ! ! --- ! --- !!-- In -\\/\\/--+------!! !!--- Gnd

I've drawn this as an N channel and with a resistor in place of the motor. When the In signal goes high, the capacitor slows the change in gate voltage. When the transistor starts to conduct, the change in gate voltage slows down greatly. The negitive feedback via the capacitor prevents the fet from swithing more quickly.

I real life, you would want a resistor and zener to protect the gate of the MOSFET.

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