Electronic – Solenoid circuit current draw doesn’t match calculations

The solenoid requires a certain amount of current to generate its magnetic field. If the solenoid was a perfect inductor, the DC current would rise above all means and would most likely damage other circuit components. However, solenoids inherently have a significant amount of DC resistance used to limit the current magnitude.

Provided you place a bypass capacitor (to absorb high-frequency current pulses induced by changing the current magnitude) between GND (close to the mosfet source) and the 12 V connection solenoid, you do not have to worry about a significant overshoot. Your selected mosfet has breakdown voltage of 100 V, which is certainly an overkill.

The mosfet also has a non-zero on-state resistance Rdson (160 mOhm), which will slightly reduce the current through the solenoid. Another implication of Rds is mosfet power dissipation - which is negligible in this case (160 mOhms provided the channel is fully open).

1) Since this is semi-static application (no switching at tens of kHz), you only need to look at these parameters:

• gate voltage threshold (should be lower than you gate supply voltage)
• on-state resistance Rds (to calculate voltage drop and losses)
• allowed current (which is very much correlated to Rds)

2) One problem I see with your circuit is that the gate voltage will be 3.3 V but the MOSFETs gate voltage is specified between 2 and 4 V. In practice, it's fine because even if you get a "bad" part, the MOSFET will still partially close and allow current current to flow through its channel. An implication of low gate voltage is that the switch will work in the linear mode, where its on-state resistance is much higher than the guaranteed value.

EDIT The gate threshold voltage is the minimum voltage where the MOSFET starts conducting current; however, the channel current would most likely not be enough to turn on the solenoid. Look at Figure 1 in datasheet, which correlates gate voltage with drain current and drain-source voltage.

You could easily use this part :: FDN327N. The gate voltage is specified at 1.8 V and allowed average drain current is 2 amperes.

The value of R1 depends on:

• allowed source peak current - some PWM gate drivers can well support 30 A peak, which (with 10 Ohm gate resistor - R1) very quickly charges the gate and thus minimizes time spent in the linear mode.
• desired dv/dt, which significantly affect radiated and conducted emissions
• gate threshold voltage

I assume you drive the gate from an MCU pin - look at the datasheet on allowed pin current. That current is, however, the average current so you can drive much more on a peak basis. I would guess that 50 mA is fine -> 3.3V / 50 mA ~= 70 Ohms would be a good value for this application.

A capacitor like that is typically used to provide power for short-term current spikes in the circuit is is directly connected to.

A typical application that needs these type of capacitors (called bypass or filter capacitors) are digital integrated circuits that need a extremely short spike of power every time the state change, but are very low power as long as the state is steady. This spike is caused by internal parasitic capacitances that need to get quickly charged to a different voltage. The inductance of the connection to the power supply might be too high for the power supply to deliver the required current spike, so a capacitor is connected close to the circuit causing the spikes, so that the connection to it has less inductance.

Another use of capacitor for DC is a filter capacitor after an rectifier, to have voltage available even when the AC input is just crossing zero. In that case, I would consider the capacitor part of the power supply, not of the supplied circuit.

In your application, the load is dominated by the coil of the solenoid, which can be modelled as a series circuit of an inductor and a resistor. At turn-on time of the transistor, the inductive behaviour of the coil is dominant, causing a slow rise of the current, and afterwards the pure DC resistance of the coil determines the current, which needs to be constantly supplied while the solenoid is turned on. Neither during turn-on nor in the steady state, a short current spike is consumed, so the capacitor is pointless. Also your LiPo battery does not need a capacitor to deal with zero crossings, as it is a DC source.

Side note: A different situation occurs on AC solenoids with a significant movement of the core: The inductance of an AC solenoid with the core pulled in is significantly higher than at turn-on time. As AC current is determined by the impedance, which might be dominated by the inductance of the solenoid coil, the AC current during pulling can be many times higher than the current in steady active state, up to the destruction of an AC solenoid (due to continuous over-current) if the mechanical movement does not occur, because it is blocked. Of course, an electrolytic cap can not be used to catch the current spike of an AC circuit.