NUTS/Volts function generator in Sept.2006 has mistakes

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bodgy
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Re: function generator mistakes

Post by bodgy » Sat Sep 16, 2006 1:25 am

Timothy Rasch wrote:Hi, about the preceeding letter I don't get why you are talking about a"led having a 5-15ma range is sufficient".
As I mentioned I haven't read this article. I mentioned the LED in response to jollyrgr who wrote about the LED.
The LED D2 and R1 (the related 2K resistor) see 12VDC. Assume that the LED is a standard red unit that drops a standard 1.5V. This leaves 10.5V for the 2K resistor. Taking the 10.5V/2000 Ohms gives 5.25mA. An LED MIGHT glow at that current level; the "usual" range is 20mA.
JPKNHTP Elektor Electronics (UK version), Electronics World (UK) Circuit Cellar (US) EPE Mag(UK) .

I no longer subscribe to EPE as with the loss of their main contributor they now publish articles from the Australian magazine Silicon Chip - apart from the fact that living in Australia I have direct access to SC and it would be silly to pay twice for the same articles plus the UK news, I don't like the SC format.

Other magazines (from the UK) that are still around are - Practical Wireless, Shortwave Magazine and Practical Television (I think they've dropped the Practical from the name). All in all the UK still has 6 monthly magazines, though I think Electronics World (used to be Wireless World) will disappear in the next few years and sadly EPE as well. Practical Wireless and Shortwave Mag are nowadays quite similar in content and I'd expect one of them to fall by the wayside by amalgamation.

There are though, many trade based magazines still in existence.

Colin
On a clear disk you can seek forever.

bodgy
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Other magazines

Post by bodgy » Sat Sep 16, 2006 1:41 am

I see I didn't quite answer the question.

Elektor - has high end projects on the whole - is now difficult to get for you US'ers other than by subscription. Unusually they actually build all the circuits sent to them and re-engineer them as well as redesigning the PCB's to their house style. Elektor they also have French, Dutch and German editions that have their own individual projects.

EW doesn't really have a website - EW . A magazine for EE's really
Practical Wireless
Television now covers computers and other electronic equipment as well as television

Shortwave Magazine has amalgamated with Radio Active and can be found as Radio User

Nostalgically I can recall Radio Constructor, Hobby Electronics, ETI, Maplin Magazine, Practical Electronics (now part of EPEmag) and the old Wireless World my very first introduction to electronics at age 13. All these magazines were going at the same time as those now still in business - a veritable cornucopia of electrons methinks

Colin
On a clear disk you can seek forever.

JPKNHTP
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Post by JPKNHTP » Sat Sep 16, 2006 8:01 am

-JPKNHTP
-God Bless

bodgy
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Post by bodgy » Sat Sep 16, 2006 2:57 pm

They do have a downloadable version U$9.95 for a year. (EPE)



Colin
On a clear disk you can seek forever.

Timothy Rasch
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function generator has almost all mistakes

Post by Timothy Rasch » Sat Sep 16, 2006 7:13 pm

Hi, to answer the preceeding letter one magazine I get locally in Green Bay,Wisconsin from United Kingdom is Electronics World about $10 . It's a Electronic Engineering magazine.Another two is Circuit Cellar and Audioxpress . I enjoy these three every Month. There is always something interesting in them. I didn't try out the red LEDS instead of the lamp in Nationals LM386 datasheet for the wien bridge oscillator yet. I think the LED does not have the thermal range of resistance as the lamp does and the lamp has bidirectional resistance independant of polarity so the distortion with the Led could be higher.
Tim Rasch [email protected]

dyarker
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Post by dyarker » Sun Sep 17, 2006 12:26 am

jollyrgr - "Doesn't a full wave bridge take 12VAC and convert it to 12VAC*1.414 = 17VDC?" Yes, the supply voltage would be almost 15.5V at peak of ripple (using 0.7V for diode drop) if the filter cap was connected across the full wave bridge. But in the article there is an inductor in series before the cap. This adds impedance ahead of the cap, which drops voltage. (LM386N-1 has a 12V limit)

In my copy of magazine C5 values in schematic and parts list agree. In text "p" got changed to "u".

Check the value of C1, 0.15uF. If true to as built, the ripple would be horrendous! This may partly explain the distortion mentioned.

hole_puncher - "What is 9 volt about it?" The article doesn't say, so assume he means output level.

In article - "Since the circuitry is based on AC, I would prefer using electrolytic capacitors." ?????????? Anybody have a guess which characteristics of capacitors he's talking about? Lack of understanding? To me it is a nonsense statement because there's no explanation. Capacitor value selection is mostly based on frequency. Pick type based on value needed. With overlap in ranges, electrolytics are high capacitance, mylar/poly/paper medium capacitance, and ceramic small capacitance. Electolytics have the additional complication of polarity (some non-polar electolytics exist).

In article - "The amplitude of the sine wave is is varied by R7, a 100K ...". In schematic R7 is 4.7K in 555 section. "... fed back to pin 2 ..." is R2, a 5K pot. (Does a "2" look like "7" when retyping? But where did 100K come from?) Besides, pins 1 and 8 are gain control, so R6 probably should be the amplitude control; R7 is negative feedback control which will change the ouput level, but also affects the amount of distortion. R6 not mentioned in text.

In article - first reference to LM386 is "486". (1.5A 555 ????)

The rest of the sine wave distortion comes from not including the lamp shown in the LM386 spec sheet. The lamp acts as a temperature adjusted variable resistor in the negative feedback. The temperature changes very slowly compared to the output frequency. Tim, you are right, an LED would be worse than no lamp. An LED acts more like fixed voltage drop in series with a small fixed resistor, and is much too fast responding. Two LEDs paralleled with opposite polarity would solve the polarity problem. But before about 1.5V, there would be too much negative feedback (high resistance), at around 1.5V a rapid change in resistance, above 1.5V a low resistance. And, the resistance would follow the output wave form, instead of being averaged over many cycles (no change in amount of feedback during a cycle.

----------------------------------------------------------------
I've never written a magazine article, and don't plan to any time soon; so I feel a little bad about shredding this one. But, I agree with the rest of you; RUBBISH!

Cheers,
Dale Y

Robert Reed
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Post by Robert Reed » Sun Sep 17, 2006 8:48 am

Dyarker
"But in the article there is an inductor in series before the cap. This adds impedance ahead of the cap, which drops voltage."

The inductor L1 as shown is 500 microhenry. This would have an XL of 0.38 ohms at 120 Hz and a DC resistance of practically nothing. It would offer no filtering value at all and be all but invisible in this circuit. To accomplish any useful voltage reduction / filtering at all it would have to be increased by a factor of at least 100 and be of a wire size as to have very low DC resistance. This would be a heavy and expensive item, so even if a misprint occurred here ( micro henry to millihenry) it probably would not be a desirable feature for this supply.

I too, do not like to take potshots from the side lines, but I did find this article very confusing. An audio sinewave oscillator needs an oscillator that requires a very delicate balance of POSITIVE and NEGATIVE feedback that is maintained throughout its range. I have built several widerange units in the past and my most successful was based on a Wien bridge design using a dual air variable capacitor for tuning and appropiate switched resistors in the range switch. This kept the required feedback ratio constant.
Some day I would like to try replacing the air var. cap. (very expensive) with large value varicap diodes and try tuninig with a pure DC bias voltage to these devices. Anybody out there tried this yet ?

dyarker
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Post by dyarker » Sun Sep 17, 2006 11:34 am

My bad on XL. All the worse for the article.

On replacing air variable cap - bilateral CMOS switches (some can handle VHF) to switch in/out fixed caps (1, 2, 4, 8, etc sequence of values), varicap to adjust between steps. Yeah, I know, caps don't come in values that are multiples of 2; but it lets you use a varicap with a smaller range.

C U L,
Dale Y

Timothy Rasch
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sine and function generator mistakes

Post by Timothy Rasch » Sun Sep 17, 2006 6:19 pm

Hi Again , I forgot about the led that indicates power on. A person can use 1k ohms in the 17voltsdc supply and it will work fine. Also getting back to the power supply I agree that the inductor is not needed and doesn't do much, so don't use it and connect the electrolytic to the bridge +- and use an adjustable positive regulator LM317 from it ,and adjust regulator for 12v output. BTW 12 volt transformers are hard to find so use a 12.6v 500ma or 12.6 1.2amp transformer. The math works out to 12.6*1.414=17.8vdc unloaded.
I built the LM386 wien bridge circuit from Nationals datasheet . Keep the resistor values 4.7k,47k the same ,vary the capacitors by 2x or whatever in the wien network but match each capacitor set[use a switch to switch the pairs],use [email protected]5ma lamp[[email protected] minilamp from Radio Shack is ok] ,make sure the feedback resistor has a value of 390 ohms,and the 10uf electrolytic is connected to pins 1,8 of LM386. You can put a 10k variable resistor from output cap to ground and cap couple from the wiper to a panel phono jack. Use a .47uf 400v cap . Also bypass the vcc to ground with .1uf ceramic cap[Z5U type] IS VERY Important. If the person building this follows the National datasheet they will be rewarded with a low distortion [about .05%] and 4.8vp-p "flat" output responce across the entire audio range. For the function generator circuit I would NOT build it , the Sine circuit I talked about is a great circuit!!
It can be operated on a 9v battery because the measured current for 12v is 4.8ma and the maximum output stays at 4.8vp-p with a 9volt battery.
Tim Rasch [email protected]

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