# Basic Switching

#### eckcotech

Joined Jan 17, 2012
28
Hey all,

I am having trouble sorting through the thousands of transistor choices for making a simple switch. I will be driving the transistor with a PIC Micro controller.

Transistor will be controlling a solenoid on/off at 12VDC 29W for full on state.

Vs = 12VDC
Chip Output = 0/5VDC 20ma(Max)

Any advice on choosing the correct transistor for this application would be a great help. I just don't understand all the calculations involved.

Better yet, a brief explanation of HOW to choose the correct transistor may serve me better.

Thanks

#### Audioguru

Joined Dec 20, 2007
11,248
Your 29W solenoid draws 29/12= 2.4A. Nearly all transistors saturate with a base current that is 1/10th the collector current. But your microcontroller does not have an output of 2.4A/10= 0.24A.
You could use a darlington power transistor like a TIP120 that saturates with a base current that is 1/250th of its collector current (9.6mA for your circuit) but its saturation voltage loss is a max of about 1.9V.

A Mosfet input current is nothing when it switches occasionally. Its saturation voltage loss is almost nothing.

#### PaulEE

Joined Dec 23, 2011
474
Your 29W solenoid draws 29/12= 2.4A. Nearly all transistors saturate with a base current that is 1/10th the collector current. But your microcontroller does not have an output of 2.4A/10= 0.24A.
You could use a darlington power transistor like a TIP120 that saturates with a base current that is 1/250th of its collector current (9.6mA for your circuit) but its saturation voltage loss is a max of about 1.9V.

A Mosfet input current is nothing when it switches occasionally. Its saturation voltage loss is almost nothing.
I second audio's MOSFET recommendation. IRF510 is a popular medium power MOSFET...and something RadioShack usually has, if you have access and are in a hurry.

#### eckcotech

Joined Jan 17, 2012
28
I have some IRL530 Power MOSFETs sitting here but I cant seem to make them work as described.

I have it hooked up like this......

I'm trying to test it by adding/removing 5vdc from a power supply to the gate pin but it does not seem to switch.

Any thoughts?

#### eckcotech

Joined Jan 17, 2012
28
Not sure why, but a simple pull down resister on the Gate pin seems to have made this work correctly.

#### Audioguru

Joined Dec 20, 2007
11,248
The gate of a Mosfet is an insulator that draws almost no current. It has a fairly high capacitance that charges to +5V when you connect it to +5V and the charge remains for a long time when you remove the +5V source. So it needs a pulldown resistor to discharge the capacitance.
The output of a PIC actively goes high and low to charge and discharge the gate capacitance.

#### crutschow

Joined Mar 14, 2008
33,317
I second audio's MOSFET recommendation. IRF510 is a popular medium power MOSFET...and something RadioShack usually has, if you have access and are in a hurry.
That device requires 10V to fully turn on. It only partially turns on a gate voltage of 5V. The circuit needs a logic level type MOSFET such as the IRL520.

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#### eckcotech

Joined Jan 17, 2012
28
This is the one i'm using at the moment ...

IRL530

It says logic level but reading the datasheet it also says 10v

#### crutschow

Joined Mar 14, 2008
33,317
This is the one i'm using at the moment ...

IRL530

It says logic level but reading the datasheet it also says 10v
You aren't reading the table headings. The data sheet shows a gate voltage of 5V for its rated "Drain-Source On-State Resistance". The 10V is the "ABSOLUTE MAXIMUM RATING". Don't confuse the two.