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ESR Meter / Low Resistance Meter
Posted on Tuesday, May 17, 2011   •   Category: Test and Measurement

This ESR Meter is perfect for any electronics repair technicians, engineers or hobbyist. This handy ESR meter measures electrolytic capacitor equivalent series resistance (ESR) in the circuit. ESR is a very important characteristic of capacitors greater than 1 microfarad. This meter makes measurements which are often impossible to check with standard digital capacitance meters. This ESR meter is based around ICL7107, 4049, NE555 and TLC274 operational amplifier and can measure resistance from 0.01 Ohm up to 19.99 Ohm. ESR value is displayed in Ohm on four digit LED display . The power consumption is only 8mA using 12V battery. ESR Meter offers very simple design and is easy to assemble.

FM Radio Receiver using TDA7088
Posted on Tuesday, May 17, 2011   •   Category: FM Radio / Receivers

This tiny receiver is not much bigger than an AA cell. It is powered off two LR44 button cells, which are expensive and I assume wouldn't last terribly long. I'll be on the lookout for LR44's at the markets and $2 shops now that I've got this radio! As with all these sorts of radios, the headphone lead functions as the aerial. Supplied with this receiver were a pair of those awful "in-the-ear" type of miniature type earphones. Apart from the appalling sound quality, they are insensitive, unhygenic and dirty, fragile, and do not block out external sounds. So, I use the normal kind of headphones instead. The enclosure is all clipped together, and once I'd opened it, sure enough, a TDA7088T was visible. The audio amp appears to be one transistor; ie. single ended class A. I don't know what current it's drawing so I can't say whether it's consuming much more battery current than a class B amp would. In any case I would prefer AAA cells rather than the LR44's.

Soft Start for Power Supply
Posted on Tuesday, May 17, 2011   •   Category: Power Supplies

The circuit will limit the current through the supply wires to 5.5A for about 1.5sec. After that time the relay will close and the current flow won't be restricted anymore. This is a very interesting circuit if you have a large toroid with big electrolytic caps connected to the power supply, since these will act like short circuits for a small amount of time if they start charging. This unit is a delay unit that can be connected directly to the mains power supply. Itīs not obligatory to use one but it is a good idea, specially if you have a big toroidal transformer larger than 300 VA. This unit has a delay circuit and for the delayed time the mains power is supplied through power resistors minimizing in this way the big inrush current due to big capacitors and big toroidal transformers in the power supply. When everything is stable it shorts the power resistors and supplies the mains power directly.

FM Radio with TDA7000
Posted on Tuesday, May 17, 2011   •   Category: FM Radio / Receivers

This project is a FM Radio based on TDA7000 and LM386 integrated circuits. What is unusual about TDA7000 IC is how it operates. It is a proper FM superhet receiver, with the usual local oscillator, mixer, IF amplifier, limiter, and phase detector. The difference is that there's only one tuned circuit; the local oscillator. Like the Pulse Counting Receiver, the TDA7000 relies on a low IF so that ordinary Op Amp circuitry can take care of the gain and bandpass characteristics. Only 70Kc/s is used with the TDA7000. Now, you might remember that the deviation of a broadcast FM signal is +/- 75Kc/s. A fully modulated signal would therefore sound rather distorted. So, how can this IC work? It's quite simple in that there is what Philips call a Frequency Locked Loop. Basically, the local oscillator is shifted in response to detector output so that the bandwidth of the mixer output is never more than +/- 15Kc/s. It is actually compressing the frequency range of the modulated signal. The muting or squelch feature is novel to say the least. Although it performs as any other muting circuit does, the TDA7000 provides an artificial noise generator so that the receiver still sounds alive while tuned off station. If you don't need that feature, just remove the .022uF condenser at pin 3. Not all Philips data sheets show it, but connecting a 10K resistor from the supply to pin 1 will disable the squelch.

Class A Amplifier 8W
Posted on Monday, May 16, 2011   •   Category: Amplifiers

This is 8W Class A Amplifier I recently built. I am very pleased with the sonic results of this amplifier. It really does not disappoint. Even using fairly standard 3 way speakers in a large room, surprisingly there is ample power. What strikes me the most is the ability of this amplifier to differentiate between instruments and noises in the sound stage. This clarity is what I like most and I think this is achieved by deceptively simple and pure circuit topology. I used the original board layout, transistors and JFETs, and made some modifications. Heat sinking was increased to approximately triple the amount recommended. Instead of using the standard bridge rectifier, capacitor bank and battery setup, I opted for a fully regulated supply with a total of 127,0000 uF capacitance per channel and a 500 VA toroid transformer.

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