# Constant current circuit

#### gjoo

May 22, 2013
63
Can someone explain how the constant current circuit works in this circuit. I understand parts of it, but I am a little unsure of exactly the whole process of that part of the circuit. For instance, how is the electromagnet kept at constant voltage?

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#### Alec_t

Jul 7, 2015
3,455
R14 is a current sense shunt. The voltage across it (proportional to current) is buffered by U2B and sensed by the micro at pin AN11. In response, the micro adjusts its output voltage on DAC1OUT. This is conditioned by U2A which drives the MOSFET gate to whatever voltage is needed to keep the drain current at the required value.

#### crutschow

May 7, 2021
762
how is the electromagnet kept at constant voltage?
It's not necessarily.
It is kept at a constant current independent of the coil resistance.

#### gjoo

May 22, 2013
63
This is a novice question, which I am, but I look at MOSFETs as switches, but how in this circuit does it directly change the electromagnet voltage?

#### Bluejets

Oct 5, 2014
6,581
but how in this circuit does it directly change the electromagnet voltage?

#### AnalogKid

Jun 10, 2015
2,804
I look at MOSFETs as switches
Don't. A switch is a small part of the full range of MOSFET operations.

While a power MOSFET certainly can perform as a switch, that is actually a special case of its actual operation. Conceptually, think of it more as a variable resistor like a volume control, except that the resistance is varied as a function of the voltage applied between the gate and source, as opposed to the rotation of the wiper against the resistance element.

A theoretically perfect MOSFET acts at the two endpoints of a pot - it is either an infinite resistance, or zero ohms. A real world MOSFET can have a minimum resistance, Rdson (resistance from drain to source in the "on" state) of milliohms, and an off state in the megohms (defined by its leakage current down in the microamps or nanoamps) but it also can have all of the resistance values in between. There are charts on the datasheet showing the relationship between the gate-source voltage (Vgs) and the drain current or drain-source resistance.

In your circuit, U2A varies the voltage at the Q1 gate (see how convenient reference designators are?) based on the voltage across R14. R14 is in series with the coil, and that's how the circuit "knows" what the coil current is.

ak

#### gjoo

May 22, 2013
63
Don't. A switch is a small part of the full range of MOSFET operations.

While a power MOSFET certainly can perform as a switch, that is actually a special case of its actual operation. Conceptually, think of it more as a variable resistor like a volume control, except that the resistance is varied as a function of the voltage applied between the gate and source, as opposed to the rotation of the wiper against the resistance element.

A theoretically perfect MOSFET acts at the two endpoints of a pot - it is either an infinite resistance, or zero ohms. A real world MOSFET can have a minimum resistance, Rdson (resistance from drain to source in the "on" state) of milliohms, and an off state in the megohms (defined by its leakage current down in the microamps or nanoamps) but it also can have all of the resistance values in between. There are charts on the datasheet showing the relationship between the gate-source voltage (Vgs) and the drain current or drain-source resistance.

In your circuit, U2A varies the voltage at the Q1 gate (see how convenient reference designators are?) based on the voltage across R14. R14 is in series with the coil, and that's how the circuit "knows" what the coil current is.

ak
Really appreciate the explanation, now I get it.

Feb 19, 2021
617
How a N channel MOSFET typically behaves :

Regards, Dana.

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