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1861  Using Arduino / General Electronics / Re: Precision Resistors (.01% or better) - What are they good for? on: August 24, 2012, 01:44:18 am
I specified a lot of .1 and .01% resistors for op-amp feedback networks, Very frequently and not because I had any use what so ever for the accuracy of the part which was really important but the real reason was temperature stability.
Thermocouple scaling amplifiers are one case that comes to mind. I once had a problem with a Max666 in an old design where there was a voltage divider for a battery reference comparator and both values were common E12 series resistors a 6.8 Mohm and a 3.3 Mohm resistor. Until I changed to 10 PPM/Deg C resistors none would pass a temperature test without showing a bad battery. The other part of my answer was done already... hard to make 1% accuracy cheaply with 5% parts and trimming... Believe it or not but thermistors were a frequent part of a design just to compensate for temperature changes.It's Really harder to manufacture .01% accuracy test equipment with 1% parts, It can be done and has for many years BUT not on a real production basis... like cell phones or MP3 players or television receivers, Even with digital calibration done automatically with EEprommed constants and Digipots... It's still a difficult process.

Doc
1862  Using Arduino / General Electronics / Re: Free (almost) Solder Suckers on: August 24, 2012, 01:18:32 am
Yes the partner shipping is usually as fast as prime. I was told that the devices would be here no later than Sept. 9th... and they came in the mail today from a Wednesday order. If I buy from Amazon directly as I did with the solder suckers.When I go to an 'affiliate'store then the store rules are in force...
I never usually do that... the difference in cost (savings?) is eaten up by the shipping charges from the 'Affiliate' store and or the shipping time. My feeling is that the usual small change saved isn't worth giving up the Amazon guarantee.

Doc
1863  Using Arduino / General Electronics / Re: Arduino Mega, Screw (Terminal) it on: August 24, 2012, 12:59:28 am
Does anyone have a clue as to a shield for the mega that would allow a 40 pin cable, short but long enough to access the other I/O on the mega... I have an LCD (Itead) and a shield for my mega but there is NO access to anything under that shield...
I would hate to make one... 40 dollars or so??? the time isn't an issue just a one of or two of circuit board is more than I want to pay. Any Idea's?

Doc
1864  Using Arduino / General Electronics / Re: Free (almost) Solder Suckers on: August 24, 2012, 12:39:17 am
I like solder wick my self and sometimes you need 3 hands to use it as well, the only drawback is the bad stuff is really hard on PCB's and the good stuff is expensive. Personally I like the shield braid from RG316 (teflon RG174) and that wax type rosin flux you can buy at radio shack (Part # 64-022). I strip the Kevlar jacket off and just push toe center conductor out... works great and I usually always have some scrap co-ax.
That having been said a solder sucker is a great tool Especially a Vacuum commercial type.... and Now hot air rework stations are a requirement for really serious techie work anyway.
At $0.51 Ea (Yes I have Prime so shipping is free) because at that price when they become full I can leave all that toxic lead inside of it and discard it properly... and just grab a new one. Amazon Prime is great, second day shipping is free... IMO

Doc
1865  Using Arduino / General Electronics / Re: High voltage bipolar sawtooth wave generation on: August 24, 2012, 12:27:28 am
Find an old diagram for a solid state scope... having said that...
you might have "ripped" too much.
The device has a deflection circuit that might gave been used the horizontal frequency is 15,734 Hz and the vertical rate is 60 hz the color burst frequency 3.579545 MHz is the key as it is the source for the local color injection... and it is the master clock for both the horiz remember the odd 734 instead of the old 15750... turns out that 15734 is a multiple of 3.59 color burst osc.
 I digress too far really but some technical background is necessary.
The Horizontal frequency won't shift by much before high voltage regulation issues, brightness and more important Focus issues will be an issue...
Now comes the vertical and that one is much easier as it can be changed in frequency by factor of 2 both ways and in most tv sets the vertical output circuit is an oscillator and a power amp. Frequently in the same IC. It is very likely that there is some kind of Synch generator driving both the horiz and vert sweep sections of the TV and locked to the color burst frequency, So you get an inkling of what you are facing.
Last is the easiest and that is the video amplifier which is most likely an IC... back in the day it was 2 or 3 class A amplifiers with some video EQ... peaking coils to improve the high frequency response which leads to a clearer for lack of a better word... like copies of copies of old VCR tapes with out the EQ. I mention it because a video driver for a scope should be as flat as possible...
In Short, Yes it can be done, the above list is really a list of the shortcomings of attempting to convert an old tube or CRT type TV set to an O'scope. There are many Many shortcomings there.
Making a Good scope from a processor is very possible....  Great TFT LCD devices are coming available and there are some Very impressive chips available.
But the available data is limited by the processor controlling the device functions and the A/D conversions both speed, depth and quality (Noise and distortion due to Nyquist like effects)...
If you 'sweep' the sampling clock on an A/D converter around a multiple of an input being measured you will find "Aliased" products in the A/D data stream that are interactions between the sample and the sample frequency...
And Finally we get to the meat in this mess... An Arduino isn't fast or the A/D deep enough to keep track of itself let alone be a valid measuring tool for much beyond measuring voltage or time and neither is any more accurate than the Vref for the A/D converter and or the clock used by the Micro.
You could spend a lot less money and time by finding an "Oldie but Goodie" like I did, I looked for a month or so and found a Tektronics 2213, a Basic dual trace 60 Mhz scope....
Old but highly reliable, I paid $55.00 for it and $50.00 for shipping and thought I got a good deal. It's worked very well for 3 years now. In my view a good analog scope is much better for certain types of work and a digital scope is better for other things like deep event storage for those really hard to find problems. Many years ago a man I worked for wanted to rent a "storage" scope (it has two types of phosphors... and I said why... we don't need to see it, we know when the FF resets prematurely... Fix that and the issue is dead... the issue turned out to  be a snubber network and a .1u
f cap across the FF power leads. After those changes were made all was well, Permanently.

Doc
1866  Using Arduino / General Electronics / Re: LM2917 Frequency to Voltage Converter on: August 23, 2012, 11:34:20 pm
Although not the high current issue, The reason for the discrepancy in the speed is due to that series capacitor and diode... at some point probably around 2000 - 2200 rpm that capacitor is going to be charged high enough to start to 'block' the input pulses... connect a meg or so from the diode capacitor junction to ground... Not enough to affect accuracy, just enough to make sure the diode operates as a diode. What happens is that the diode is cut off by the dc generated from the input pulses. At some input condition the diode becomes reverse biased and cut off except for the diode and capacitor leakage and the load resistance which is the reason it 'sorta' works above 2 - 2.5K rpm's.

Doc
1867  Using Arduino / General Electronics / Re: Free (almost) Solder Suckers on: August 23, 2012, 10:02:49 pm
Get to Amazon and search for solder suckers... It's the first link of about 30 different items...

Doc
1868  Using Arduino / General Electronics / Re: Wheatstone bridge? on: August 23, 2012, 09:59:30 pm
There is also the problem of calibration, A Wheatstone Bridge's output is bi polar in nature and depending on the actual values on either half... Extremely sensitive, Typically used for making matched pairs of resistors... to .001% or more. A Wheatstone Bridge isn't a really great idea unless you are after 10th's of a degree accuracy over a narrow range of temperatures
 Use a current source fed from a constant voltage source... Independent of the Arduino's Vcc.
I usually use a pot and a TIL431 reference diode (read Variable Zener Diode) fed from a clean 9 to 12V source. I use a 10K 1% MF or better, use a stable one with a low tempco (10PPM/deg C.).
There is a "Table" of values for calibration by PH Anderson and a formula for doing the same thing "On the Fly" available freely on the Internet for final calibration. Don't forget 2 really important things... Bypass the regulated voltage source well and really important use a star grounding layout... 5 or 10 mV of combined ground drop voltage can destroy any real accuracy however if you pay attention to detail you can easily make a nearly "Traceable" and very accurate thermometer... for a few dollars extra... IMO

Doc
1869  Using Arduino / General Electronics / Free (almost) Solder Suckers on: August 23, 2012, 08:18:32 pm
Solder suckers... $0.51 @ Amazon, I bought 3...

Doc

Here...http://www.amazon.com/Parts-Express-Solder-Sucker--desoldering/dp/B0002KRAAG/ref=sr_1_1?ie=UTF8&qid=1345777210&sr=8-1&keywords=solder+suckers
and Still $0.51 Ea...
1870  Using Arduino / General Electronics / Re: What to use to power a Water Pump on: August 15, 2012, 05:53:28 pm
Well, Yes you could put out a low voltage to a driver transistor. But then the voltage would be continuously "dropped" across the transistor so the voltage 'dropped' times the current being drawn by the motor is the power lost in the transistor. PWM gets around this fact be lowering the time the transistor is on or off to regulate the voltage that is supplied to the load, For a 10% power out simply turn on the transistor on 10% of the time. The only power wasted in the regulator is in transition from on to off and from off to on and a small part of the energy required to flip the field off and on, similar to hysteresis losses in regular mains transformers as heat. So at higher powers PWM makes a very effective means of power control. I hope this helps to explain how it is better as well as why I had some reservations about the PWM frequency interacting with the control loop in the brushless motor controller (driver).

Doc
1871  Using Arduino / General Electronics / Re: What is the difference between a "Dev Board" and an "Evaluation Kit" on: August 15, 2012, 05:38:12 pm
IMNHSO, a Great answer too.

Doc
1872  Using Arduino / General Electronics / Re: best way to get components from pcb on: August 15, 2012, 05:36:22 pm
Con-formal or water proof coatings are rarely used any more and usually is a real Pain to remove components from (I used one for 10 years) because solder  wick or solder suckers work very poorly on  them because there is no way easily un seal the hole to get air to the top side of the PCB. Which seems to be necessary in my experience required to remove a part easily... How ever I am a great fan of a 6 mm wide chisel tipped iron or a big heatgun... Softens both the coating and the solder at the same time. Much easier to remove, Especially with a heat gun although nothing seems to work with SMT stuff except solvent and that poorly. However the board isn't likely to be worth much after you're through... Just a simple decision, Do I want to fix this thing or do I want the Goodies on it. Many times I have purchased boards at Amateur Radio flea markets simply because there were parts that were much more valuable than the dollar or so that the whole PCB  cost. My favorite is a 40 watt Hakko @ 700 to 750 deg F with a wide chisel tip. Heat them up and pop them out especially if I am in a hurry or there  are a great many parts that I need. Then I get out the heat gun...  I usually preserve the parts by preserving the PCB as it is a lot easier to find the PCB than it is to find the parts once removed. Much less work too.             

Doc
1873  Using Arduino / General Electronics / Re: What to use to power a Water Pump on: August 15, 2012, 04:59:06 pm
Quote
Yeah, but I'm thinking though, if I do that, why use PWM in the first place. It was suggested here, I actually hadn't though of it, I planned to do analog straight to the mosfet then to the motor.
the reason for using PWM instead of DC control (a variable linear regulator) is that a linear regulator or variable voltage power supply dissipates the "Unwanted" power as heat. So at low flow rates the DC power wasted will make the control element quite hot. A PWM power supply only has two states off and on, power is controlled by the period of the on signal as it gets narrower in relation to the off period the lower it's "Average voltage" is and the lower the speed the motor runs at.

Doc
1874  Using Arduino / General Electronics / Re: What to use to power a Water Pump on: August 15, 2012, 04:07:16 pm
The pump in question can possibly be affected by the pwm rate and may exhibit some strange behavior if the PWM signal is a multiple of the motor control frequency. In a brushless motor the drive to the motor is ac. I noticed that the flow rate is variable and I don't think the motor controller would "Like" to see chopped dc at a rate close to the drive signal to the motor. Other than that one minor point I see no reason why you couldn't use a PWM control for motor speed. Computers CPU fans do it quite well. I do think however that the PWM rate needs to be higher than the motor drive rate or period to avoid interactions. Try it and see... I have little experience with small brushless motors used in CPU fans beyond changing them out when they fail. Most failures are due to poor quality material used for the bearings (bushings) in the motor... But some go dead and a few I have used failed repeatedly. PWM Control or bad luck...? I never figured out which but that question has always bothered me... IMO
I don't in theory see any reason why not but I haven't tried it to see with anything I have ever built.

Doc
1875  Using Arduino / General Electronics / Re: Cannibalizing Printer and Satellite Decoder for Spares. on: August 15, 2012, 03:41:57 pm
I just scrapped a satellite receiver last week... Got about 30 electrolytics all of which measured ok, 3 crystals (One a VCO) a bunch of inductors for switchers and a whole gang of other parts including a separate Mains to 12Vdc switching PSU. Given a little patience and a good soldering iron and a free evening. Lots of really useful parts. Switching power supplies for Printers are really nice as a buck mode switching regulator will take that 24Vdc @ 500 mA output from an HP Printer and deliver 2.1 A @ 5Vdc out (the missing .3 A @ 5Vdc is the loss due to the efficiency of the switcher) and the nicest part is that it only requires a $1.95 switcher from Ebay. I always keep 3 or 4 of these parts on hand just for that purpose.

Doc
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