Turning LEDs on/off

[rsisk]

When I buy the FET, is there anything specific I need to look for? Probably going to Radio Shack to find one tomorrow. If not available then will probably go to mouser.

[EricVanWyk]

Originally Posted by rsisk

When I buy the FET, is there anything specific I need to look for? Probably going to Radio Shack to find one tomorrow. If not available then will probably go to mouser.

Your application is really light load and low voltage, so most FETs will work fine. Here are the general things you look for:

Type: Your options are "N" or "P" channel. Either can be made to work, but Ether drew the schematic with an N channel. N is generally preferred because they have better electron mobility.

Vds: Maximum voltage from Drain to Source, or how much voltage you can block. You need at least 5V, but I've never seen a normal discrete FET with a Vds rating less than 5V.

Vgs: Maximum voltage from Gate to Source, or how much signal voltage you can apply. Again, most FETs can handle 5V just fine.

Vgs_thresh: Minimum voltage from Gate to Source to guarantee turning on the FET. Most FETs will be in the 1 to 3 V range, so you'll be fine.

Rds: Resistance from Drain to Source while on. Subtract this from the 82 ohm series resistor you calculated. It'll be relatively small, you can likely ignore it.

Ids: Maximum current from Drain to Source while on. It needs to be more than 20mA. Just about any FET that is big enough to be in its own package can handle this.

[rsisk]

OK, just got back from Radio Shack trying to get the right components. The transistors were hard to match up, so I got a couple.

One is a MOSFET N-channel transistor and the other is a bag of NPN-Type transistors. I've included pictures below. My guess is the MOSFET is the closest match and where I am starting to build the circuit.

Am I correct in assuming the following:

Source - The +5V/82ohm/LED side of Ether's diagram
Drain - The ground side of the diagram
Gate - The DIO Signal/100Kohm side of the diagram

Here are the components I purchased, 100K-ohm resistors, MOSFET transistor, and NPN transistors



And the back of the MOSFET transistor and NPN transistor bag

[Ether]

Originally Posted by rsisk

Am I correct in assuming the following:

Source - The +5V/82ohm/LED side of Ether's diagram
Drain - The ground side of the diagram
Gate - The DIO Signal/100Kohm side of the diagram

No. For an N-channel MOSFET, the source is connected to ground, and the drain is connected to the load, which is connected to the power.

The gate is connected to the signal, as you said.

So in the sketch I posted earlier, the source is on the bottom of the FET, and the drain is at the top. See annotated sketch attached.

[rsisk]

Thanks for that timely information, just starting to put this thing together.

[Ether]

Originally Posted by rsisk

Here are the components I purchased, 100K-ohm resistors, MOSFET transistor, and NPN transistors



And the back of the MOSFET transistor and NPN transistor bag

Maybe it's just my connection, but my DNS can't resolve rsisk.com:

[EricVanWyk]

It resolves for me, that FET will work fine. It is a beefy fellow.

An easy way to figure out which way to orient a FET is to look at the arrow in its schematic symbol, as it (sort of) represents the FET's bonus parallel diode. You want this diode to oppose normal current flow, or else it will always allow it and you won't be able to turn the flow off.

In your symbol it points from the source, so you want normal current flow to go the other direction (from the drain).

Sometimes they draw the diode more explicitly, I've seen it several ways.

[Ether]

Originally Posted by EricVanWyk

It resolves for me...

I just ran tracert from a web server and it found it:

216.86.200.206

I'll try that...

edit: got it, using http://216.86.200.206/electronics1s.jpg weird.

[DonRotolo]

The DSC Digital I/O does not appear to be able to sink 20 mA.
Ether's transistor switch should work fine. A FET (especially an IFF510) may be overkill, a plain NPN switching transistor will be fine.
All these parts (resistors, 2n2222 or 2n3906 transistors) are available at Radio Shack.

[Ether]

Originally Posted by DonRotolo

The DSC Digital I/O does not appear to be able to sink 20 mA

source (in this case)

[Ether]

Originally Posted by DonRotolo

A FET (especially an IFF510) may be overkill, a plain NPN switching transistor will be fine.

But you know, as soon as he gets that little 0.1 watt LED working, he's gonna want to swap it out for an auto headlight and run it straight off the battery.

[rsisk]

Originally Posted by Ether

But you know, as soon as he gets that little 0.1 watt LED working, he's gonna want to swap it out for an auto headlight and run it straight off the battery.

Mind reader!

So, I have too admit I am feeling pretty foolish, and I want to get this in the thread for accuracy, as I was working on the wiring for the circuit, I pulled up the specs on the DSC and figured out I had the LED plugged into the PWM instead of the DIO. Moving the LED to the DIO power/ground and turning in on/off via the Digital Output works just fine.

Sorry for the diversion and thanks for the info n making the switch. Now I want to try out the switch to see if I can mAke it work.

Question: why was a 100K ohm resistor used on the gate?

[Ether]

Originally Posted by rsisk

Question: why was a 100K ohm resistor used on the gate?

Credit for the following answer goes to Eric VanWyk:

The purpose of the 100K resistor is to dampen electrical oscillations. The gate of the FET has some capacitance. The wiring leading to the gate pin may have some inductance. Under certain circumstances, together these may form an LC oscillator. The resistance is there to prevent those oscillations. The 100K is not critical. It can probably range over an entire order of magnitude. The higher the resistance, the better the damping, but the slower the FET's response to a step input signal (because of the gate capacitance). The lower the resistance, the faster the FET responds to a step input signal, but you increase the chances of oscillations.

[Ether]

Originally Posted by rsisk

http://www.rsisk.com/npn.jpg

Rick, just a heads-up: be careful with the part number 276-1617 package of 15 NPN transistors.

According the the reviews here (scroll down), the package may contain PNPs instead of NPNs. And if they are NPNs, the pin labeling on the package may be wrong. Check the datasheets.

... and let us know when you get that MOSFET working.

[EricVanWyk]

Originally Posted by rsisk

Question: why was a 100K ohm resistor used on the gate?

100K is extremely conservative. I generally put as large an R in series as I can get away with for a given switching speed, others will put as small an R in as necessary to damp gate ringing. For reference, I'm putting ~5 ohms in gate series for a 400kHz switcher for a hobby project.

So, the PM Ether quoted explains why a resistor is necessary. The reason it is so much larger than you might expect is just to over-protect the gate. It costs the same and lets you rest a bit easier.