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Everything posted by Platonas

  1. Hello people, I would like to comment on the cct a little bit. 1. Stereo can be added lated by the use of a sepparate stereo generator. 2. The same can be done about pre-emphasis using a cct from another FM transmitter and adapt it accordingly. 3. The thing I am worring about is the stability. The oscilator part consists of a single stage, T2 and T3 (BF494), and produces high levels of power. Around 100 to 200 mW. This is somewhat high for an oscillator in order to have good frequency stability. It could drift easyly (this is my opinion, I havent tested the cct at all) since I beleive at this power the transistors will heat a little bit. Anyway appart from this, it looks to be a nice cct and I hope I am wrong about the stability. Regards P.
  2. As Alun stated, Q1 is an RF amplifier for the antenna, Q2 is the tuned cct. The first coil is mainly used as an antenna impedance match that also shorts to ground the lower frequence range. Hence it must have an inductance greater or equal to that of the other coil of the tuned cct.
  3. I suggest you do something with a microcontroller. Some ideas are: - An SMS remote controller (switches on-off remote devices by sending the appropriate SMS message). - A data logger, voltage, temperature or anything else you might need to record over long durations. - A central heating temperature controller. - A vehicle possitioning system (Interface a GPS to the controller bord and every few minutes send the car's position to a remote mobile phone) Personally I like the PIC microcontrollers, you can find lot of development tools on the internet and there are good compilers for Basic, C or Pascal. Regards P.
  4. I will sugest 3 different products that look good and are cheap. 1. Proteus (w*w.labcentre.co.uk) 2. CircuitMaker (w*w.circuitmaker.com) and 3. Eagle (w*w.cadsoft.de) Proteus is supposed to be more inclusive since it has a simulation ability. Circuit maker has also simulation abilities but not for microcontrollers as Proteus has. Eagle on the other hand is (my opinion) better that the other 2 as far as schematic/PCB abilities are conserned. Its libraries are very good, has good support and it is the cheaper. The easyer to use is Circuitmaker then Eagle and then .... All of the above have schematic editor, pcb and autorouter.
  5. Certainly current limittation is only achived by the transformer capabilities. A flat battery would absorb high currents at the begining of its charging until the voltage reaches a level near the set voltage by the pot. At this level, conduction is not for the full cycle but for a smaller part than before, hence less heating. Anyway, this is not an actual current limitting technic and the charger is intended mainly for keeping the battery at a charged level. If someone wants current limittation then a series resistor could be incorporated but the heat losses increase. A 1ohm series resistor will limit the current of a flat battery (supposed a 10V across it) to about 3-5 Amps depending on transformer voltage and charger settings. The heat on the resistor would be about 25Watts which means that a resistor of about 50Watt rating must be used (due to high temperatures arising at the resistor) In this case, the cost of the resistor is almost equal to the cost of the charger :)
  6. Hello again, About the cct I posted yesterday. You made a lot of questions and I am sorry I had not said an important detail. A transformer of 13-14V a.c. is used. Then a full wave bridge rectifier is connected to the cct. So, we have d.c. cycles (positive) at he upper part of the cct and the negative is the lower part, where the batteries negative terminal is conected. Due to the fact that we have unfiltered d.c. supply, i.e. full wave rectification without electrolytic caps to smooth and keep it at high levels, the Thyristor will turn off every time the voltage reaches the zero level (100 times per second). Now, Q1 will conduct unless Q2 is on. Q2 is on as soon as the voltage across the battery is high enough and could pass through the zener. (So, we have voltage regulation) The LED lights when there is no charging, Q2 conducts. The battery is 12V, not 24 as indicated in the diagram. The cct works fine for many years in many commercial applications. It is a part of a battery charger in standby generators (24v operation). It keeps the battery charged and does not dry it out. Transformer current capabilities, Q1's size and headsink define the supplied current ability of the cct. Regards P.
  7. I made in the past a car ttery charger with thyristors. The cct is simple and works fine. It was modified to work with 24V (two batteries in series) but can be modified back for 12V operation very easyly. Just make R1=1 to 1K2 and zener diode D1 should change to 6v. For 12V operation use the red coloured values. Regards P.
  8. What size of transistor do you need? If it is a normal, small signal transistor, there are many available. U can use those described in previous post, also have in mind the working voltage, current and power. To add some more u can use: BC547C, BC550C, BC548C, BC239C, BC184, BC237B, ZTX108C.
  9. Referring to my previous post. Yes Audioguru, you was right. I was reffering to a wrong schematic, I was talking about http://www.electronics-lab.com/projects/rf/001/index.html. I don't know how do I did such a reference mistake. The transmitter you are talking about (4W with two transistors 2N2219) has certainly overheating problems and could never produce 4w of RF power, at least with those transistors. By replacing T1 with 2N4427 and T2 with 2N4427, 2N3866, 2N3553, 2N3924, BFS23A, you can achieve a power increase compare to 2N2219 and a good heatsink would be sufficient for cooling them down. 2N2219 cannot dissipate such power levels. (easyly at least) In case you increase R3 from 47 to anything between 51 to 100 Ohms, the transistor will run cooler but the power would be lower, still high for a good range. I suggest T2 be a real RF transistor like as suggested above. They have higher gain and efficiency at the desired frequencies. Regards P.
  10. Hi, For both of the initial questions. T1 can be 2n2222, 2n2222a, 2n2219, 2n2219a, bsx20 maybe 2n3904 and other simillar transistor, slide variations for the C7 capacitor's value to achive oscillation at the desire frequency. T2 is better to be 2N4427, 2N3866, 2N3924, 2N3553. For the coils use 1mm or even 0,8mm wire.
  11. The Crockcroft-Walton is simple to produce high voltages but at very low current values. The impedance of the cct increases exponentially with the number of stages, hence it is impracticable to be used when you need high current. Anyway, appart from the microwave oven transformer you may try the Neon Signs transformers. I am not sure about the power ratings but the certainly have outputs up to 10KV.
  12. I had built the cct many many years ago and worked fine. You must check first the 3 resistors if are of the correct value. R3 & R4 are both 10K. In case you made R4 = 1K then Tr1 will overheat. Is R5 = 82 Ohms? You can increase this value up to 120 Ohms without any significant reduction in performance. Check if the transistor oscillates. If not then inspect all the capacitors around it, C5,C7, C15, C6. C7 and C15 are the components that affect oscillation. Last thing, check the pollarity of the transistor, emitter-base-collector. Regards P.
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