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31  Using Arduino / General Electronics / Re: Why are there so many more N-Channel Fets available then P-Channel? on: August 18, 2014, 09:15:55 am
MarkT nailed it in response #7, electron charge carrier mobility versus hole charge carrier mobility.

http://forum.arduino.cc/index.php?topic=261456.msg1846103#msg1846103
32  Using Arduino / General Electronics / Re: Old Computer Power Supply on: August 17, 2014, 10:45:04 pm
eoto88, did you try that with a 10 ohm resistor on the 3.3V line?
33  Using Arduino / General Electronics / Re: Why are there so many more N-Channel Fets available then P-Channel? on: August 17, 2014, 12:35:19 pm
It is also simpler to provide power to things like solenoids, LEDs, motors, etc. that are running on much higher voltages than the microcontroller that is switching them, using low side switching.
34  Using Arduino / General Electronics / Re: Old Computer Power Supply on: August 17, 2014, 11:38:16 am
Don't go by the color, go by the position on the connector.

But with a non-standard supply from Compaq, all bets are off. I see PSON, the question is does it get pulled low, or high to turn the supply on?

Older computer power supplies only watched the 5V line for out-of-range. Newer ones watch all the lines, so without a load they may shut down.
35  Using Arduino / General Electronics / Re: Switch de-bounce circuit. on: August 17, 2014, 12:01:15 am
And that's why I'm pedantic.
36  Using Arduino / General Electronics / Re: Old Computer Power Supply on: August 16, 2014, 11:58:42 pm
The load resistor is there because the power supply regulates the 3.3V line (5V on older AT power supplies) and the other supplies are just along for the ride. IE, they are in the range because of the relative number of windings on the secondary.

But without sufficient loading, ringing occurs and while 3.3V will be regulated, the other voltages may be incorrect.

So a load resistor on the 3.3V line on an ATX supply, and a jumper on PS_ON# to turn it on.
37  Using Arduino / General Electronics / Re: Best PCB degign for beginner. on: August 16, 2014, 04:39:49 pm
Click on Download Freeware:

http://diptrace.com/downloads/download-diptrace/

I don't know if they offer it any more, but for a while they'd send you a key to unlock double the number of pins if you said you'd heard about Diptrace on Homebrew_PCBs or Electronics_101 (yahoogroups), and a variety of other forums.

The 3D library is a separate download for the Windows/Linux version, included in the Mac version.
38  Using Arduino / General Electronics / Re: Old Computer Power Supply on: August 16, 2014, 02:59:23 pm
Hang on, the computer power supply is still plugged into the motherboard? Well, there is your 5V load.
39  Using Arduino / General Electronics / Re: Transistor Selection and Wiring to Drive LEDs with PWM control on: August 15, 2014, 04:16:27 pm
MOSFET, better idea. 20mOhm for a good one, at 750mA that is only I^2 x R = 11.25mW, only 15mV dropped across it. No heat sink required. Just make sure it is logic level.
40  Using Arduino / General Electronics / Re: Transistor Selection and Wiring to Drive LEDs with PWM control on: August 15, 2014, 11:17:39 am
Oh, and the 150 ohm resistor that fungus suggested would work, too.

2.5V/150 ohm = 14.7mA
2V/150 ohm = 13.3mA

I just figured an explanation of how and why a particular resistor is used, would be more important for you than just giving you a number.

Also, there is not only a maximum amount of current you can draw from an individual Arduino pin, there is also a maximum current that can flow through the Vcc and ground pins. So you could be well within the maximum current limit on each pin, but go over on the Vcc/ground max current. No sense in using more current from a pin than you really need, then.
41  Using Arduino / General Electronics / Re: Transistor Selection and Wiring to Drive LEDs with PWM control on: August 15, 2014, 11:13:58 am
From the datasheet:

Quote
Saturation voltages
IC = 3.0 A; IB = 12 mA VCEsat* max. 2.0 V

It also says that the HFE (current gain) is about 1000. As you can see from above, they are overdriving it by about 4x. This is typical for a darlington pair transistor. The rather large VCEsat voltage is typical of a darlington pair, too, but may be more on the order of 1.5V at the 750mA your LEDs draw.

So using the datasheet's preferred ratio of 1:250 base to collector current, 750mA/250 = 3mA.

About 5V will be provided by the Arduino (actually a slight bit less as their is internal resistance) and the VBE drop must be taken into account when calculating the base resistor. VBE at 3A is rated at 2.5V. So about 2.5V will be dropped across the Arduino to Base resistor.

2.5V/3mA = 667 ohms
For the sake of taking care of the slightly less than 5V out of the Arduino, variations amongst transistors and resistors, let's use 470 ohms.
2.5V/470 ohms = 5.3mA    well under the maximum base current.

Say worst case, the Arduino manages to put out only 4.5V because of a depressed Vcc, leaving only 2V across the resistor:
2V/470 ohms = 4.3mA     well over the 3mA required to drive the TIP120 into saturation.

As for heat dissipation in the transistor, we'll use the 3A rating of VCEsat of 2V, as we don't know exactly how much lower it would be at 750mA because it isn't linear. And we should err on the side of caution, or risk burning up the transistor:

750mA x 2V = 1.5W

A TO220 package will burn up with more than 1W, and even at 1W you are taking a risk. You should also double that number to take into account changes in ambient temperature, objects near the heat sink that interfere with air flow, dust, etc.

But 3W is still not bad. Simply bolting it to an aluminum case with the proper insulation and heat sink paste (more is NOT better) or to a reasonably sized heat sink should suffice. If you use a silpad, do NOT be tempted to add heat sink paste.
42  Using Arduino / General Electronics / Re: Best PCB degign for beginner. on: August 15, 2014, 10:41:54 am
There is a plug-in for Eagle that will convert Eagle boards and schematics to DipTrace format.
43  Using Arduino / General Electronics / Re: Ultrasonic sensor circuit design on: August 14, 2014, 05:43:48 pm
Yes, a lot of gain in a single stage is a bad idea. You lose frequency response as compared to spreading the gain over 2 or more stages.

GBW of an Op Amp is Gain-Bandwidth Product. Called that because if you multiply the gain times the bandwidth, it equals 1.

So a GBW of 1MHz means that if you wire up the Op Amp as a buffer with gain of 1, the -3dB bandwidth is 1MHz because 1 x 1MHz equals the GBW of 1MHz.

But if the gain is 100, the -3dB bandwidth is 1MHz / 100 = 10kHz. So the gain of 100 x 10kHz equals the GBW of 1MHz.

With a gain of 5000, 1MHz / 5000 = 200Hz. Ouch. That is in addition to the possibility of oscillation, continuous or intermittent, as Magician points out.

However, all this is a bit moot, as you are using a comparator, not an Op Amp. Or is U1 a different IC?
44  Using Arduino / Sensors / Re: need sensor which can detect human presence on: August 14, 2014, 11:54:29 am
How about just a big shiny red button? No instructions needed. Put a blinking light above it, and you'll get nearly 100% detection.

I built a jacob's ladder (rising arc) in a mad scientist looking wooden box with blinking lights and two big rotary knobs on it. Even in an artshow with signs saying "don't touch", and a sign next to it that said "Hazardous voltage! Do not touch!", people would still turn the knobs and put their hand on the plexi over the arc.
45  Using Arduino / General Electronics / Re: 4x AA Ni-Mh voltage drop on load on: August 14, 2014, 11:48:25 am
If you are measuring voltage at the input of the regulator, then that includes all the voltage drops along the way. I thought it would be unusual for a NiMH cell to drop that far in voltage.
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