Basic Switching

Thread Starter

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
11,249
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
423
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.
 

Thread Starter

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?
 

Audioguru

Joined Dec 20, 2007
11,249
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
25,417
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.
 
Last edited:

crutschow

Joined Mar 14, 2008
25,417
This is the one i'm using at the moment ...

IRL530

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