autir

I have been studying the principles of audio microphone preamps (example: http://www.ee.washington.edu/circuit_archive/circuits/micamp.html and transistor amplifiers in general. Often I see the same circuit in two different sites, in one site the capacitors are not polarised and in the other site they are. -> When do we need our capacitor to be polarised and when not? -> How do we determine polarity? I am talking especially for the "from previous stage" and "to next stage" capacitors, but not just for them. Thank you in advance.

Someone told me that each of the 8 clones of a standard cat.5 UTP cable is certified for a maximum current of 0.6 Amperes. In search of a copper wire able to withstand 1 Amp with ease (and be able to fit in a veroboard hole) I visited an electronics store. The salesman proposed that I use UTP, he said that it is 22 gauge and so can withstand up to 3 Amperes ( :o :o :o ) of current. What is your opinion regarding all this?

In need of a tool to cut the tracks of my veroboards, I have bought this pen. It accepts noses composed of many glass fibers and works as a brush. I have been unable to break a single track, with much brushing I have managed to break two or three adjacent tracks. And by that time everything in a 5 dot radius was badly scratched. Not to mention the loose fibers piercing my fingertips, adding to my frustration. 1) What is the "proper/formal" name of this devilish device? So that I may perform searches in Google. 2) Any suggestions on how to use it? Any clues? Links to tutorials? 3) Does anybody have another tool to suggest? Thank you very much.

Thank you for your reply. -> Regarding fuses: Since the transformer is 9.6 VA, the input current will be 9.6/220 = 43.6 mA. I understand that the input current will be higher than this because no device is 100% efficient, but isn't 80 mA big? In general, how do we calculate a value that is neither too small (many burnt fuses) nor too big (lack of protection)? -> Why do we place capacitors in parallel with the rectifier diodes? Why 4.7 to 10 nF? If I use a bridge rectifier IC will I need capacitors? -> There are no resistors connected to the outputs of the LMs, this was just a test circuit for simulation ;D -> You mean a 1N4001 with its P end connected to the IC's output and the N end connected to the IC's input? I have seen such a technique in the LM317 datasheet, and it was supposed to protect the IC in case there was a short-circuit in the input area. A similar pattern is the Figure 33 in the LM7800 series datasheet from ST. In general, how can the input voltage be lower than the output? If we have a short circuit, for example, won't the output also drop to zero? -> why do we need a 10 uF cap in parallel with the 100 nF in the output? -> The transformer is indeed too small - but just for the 12V area of the circuit. In the LM7800 datasheet if you study the IC current vs. input-output voltage difference (Figure 5) it shows that for 500 mA the Vi-Vo can be lower than 3 Volt. But this isn't the case, as both in simulation and in a breadboard prototype I've constructed the 12V value slowly starts to lower above 280 mAh. What would be the ideal transformer for this circuit? If I'm not mistaken, it should be rated as 0.5*squareroot(2)=0.7 A and [12+3(for the Vi-Vo)+2(for the bridges)]*squareroot(2)/2=12 Volt. What have I calculated incorrectly? Thanks again :D

A PIC is a microcontroller. They are manufactured by Microchip. Just follow the links

Your silence implies that nobody knows where to put a fuse in a voltage regulator circuit? ???

My first circuit.... isn't she a beauty?

I have a transformer rated as 12Vac / 800mAh. I measured the voltage rating of the transformer with no load as Vmax=14.3 Ohm. If Rint is the internal resistor of the transformer, Rint= (14.3 - 12)/0.8 Ohm <=> Rint=2.875 Ohm. (I) When connecting an R=4.9 Ohm resistor directly to the transformer, the voltage at the ends of the resistor was Vr = 8 Volt. I=Vr/R, but I=Vrint/Rint. So Vr/R=Vrint/rint (where Vrint the voltage drop at the Rint, and Vrint=Vmax-Vr). So Rint=((Vmax-Vr)*R)/Vr <=> Rint=((14.3-8)*4.9)/8 <=> Rint=3.85875 Ohm. (II) The cases (I) and (II) differ by far. Why? note: R was not a power resistor and it was connected for less than a second because it started to heat up really fast.

Forgive me for raising this topic from the dead... :-\ What is the voltage difference between the Ground pin of the LM and the ground of the power socket on my wall? What is the voltage difference between the ground output of bridge-rectified AC and the ground of the power socket on my wall? What is the difference between Neutral and Ground? (generally speaking, not related to LMs) Thank you very much.

Six. Three on one side and three on the other. The AC voltage of the devices in my country is 220-230V, if that helps. Are the two transformers - the one from the backups 400 and the one from backups 600 - the same? How many Amperes can it support? And how do I measure it?

It is a matter of physics. I believe that you are looking for this? http://www.wonderquest.com/radio-waves.htm

I have read that electrolytic capacitors age within 3 to 30 years, depending on quality. After that they gradually dry out; their capacitance is reduced and their resistance is increased, making them useless. Can't we substitute electrolytic caps with something else if we want a long-lasting circuit?

Ok Mihali. I attach the service manuals. Note that they mention several transformers; Most of them are soldered directly on the PCB except this huge heavy transformer which is screwed on the metal surface of the bottom of the box. Thank you very much. apc.tar.zip

AM waves reflect on the ionosphere and so can travel by bouncing: ionosphere -> earth -> ionosphere etc. Higher frequencies cannot do that. So you need a line-of-sight with the other end of the link.

Hello all I have two UPSs from APC. One is model APC Back-Ups 400 and the other is APC Back-Ups 600. Inside each box there is the PCB, the battery and a big heavy transformer. Can anyone give me details regarding what type of transformer this is? I have downloaded the service manuals but could not understand which transformer it was.

I always thought that electronics are indestructible by time, and the only way to damage them is improper voltage. So, for example, a computer motherboard will become unusable when its electrolytic capacitors dry out? What happens when the cap dries out? Is its capacitance reduced, or its maximum voltage ?

I suppose that the current rating is Irms as well?

Before opening this thread I had already glued such a circuit using hot melt glue which smelled like vinegar ;D Unlucky me... I guess I have to throw the circuit away and create a new one? What other "ammunition" can a hot melt glue gun receive, besides glue and silicone?

I beg your pardon ?!? :o ???

On several PCBs I have observed that some components are glued together. I want to do the same in a circuit of mine which will be installed outdoors: nothing but four RJ-45 female connectors soldered on a veroboard. I want to do this in order to insulate the wires from moisture and weather effects in general. I own a 11mm hot melt glue gun. Which substance should I use, glue or silicone? Why?

1) What measurements can we make in an AC/AC transformer with nothing but a digital multimeter (and diodes, capacitors, resistances etc.)? 2) Suppose we have an AC/AC transformer rated 12V/800mA. Do the values refer to rms, peak or p-p? 3) After rectifying the AC with a bridge and connecting the output to an electrolytic capacitor, we can see that the voltage of the capacitor is greater that the V(peak) of the AC transformer. Why does it happen? Shouldn't it be V(peak)-V(d)? (Where V(d) is the voltage drop in one of the four diodes of the bridge).

Problems problems problems. I visited two respectable stores. The salesman in the first shop told me that there are no 50- and 75- ohm BNC connectors. There is one type of connector which accepts all types of cable (regarding resistance values). The salesman in the second shop told me that the 75-ohm BNC connector exists only in the crimp model, in order to allow crimping of 75 ohm cables with thick wiring. The screw/solder model of the BNC has no such problem, so comes in one model. I am all confused...

I went to the store to buy some .33

gsmaster thanks ;D What are the losses of these devices?

The impedance of the cable is 75 Ohm. By the way, if I am not mistaken, this is the impedance needed for satellite LNBs. Both televisions have good receptance. The point is that I am concerned regarding their receptance once water passes through the tape and into the cables. I would like to connect one, maybe two additional televisions on the antenna without installing any form of power-demanding device on my roof (amplifier etc.). What are the losses of this "impedance matching juction box" (how do I ask for this in a greek shop? Any translations? :)) ?