Silicon Chip Online - Ask Silicon Chip

Increased load current rating

Regarding the "Versatile Electronic Load" in "Circuit Notebook" in the March 2006 issue, how can I make a 100mH inductor (L1 in the circuit) to take as much as 17A current? (G. C., Ballarat, Vic).

First up, the circuit is not capable of sinking 17A without major modifications. Increasing the power handling capability while maintaining reliability may prove to be a difficult proposition. Multiple Mosfets in parallel with the existing device will increase power handling but it is not easy to get the devices to share the load equally.

If you want to experiment with paralleled devices, Mosfets specified for audio power amplifiers might be the best option. These are designed for use in their linear regions, so are likely to work better in this application.

In use, check the temperature of each device using an infrared thermometer or thermocouple to determine load sharing. You could also use a single, larger Mosfet (such as a "DICE" packaged device).

Regarding the 100mH inductor, you’ll probably need to custom-wind your own unit. Suitably large "Amidon" iron-powder toroids are available locally from RJ & US Imports (http://users.catchnet.com.au/~rjandusimports/index.html) and Rockby Electronics (www.rockby.com.au)

Connecting a cassette player to a computer

I have quite a few audio cassette tapes which are now longer available or have irreplaceable voices of family members no longer with us, that I would like to transfer to CD. Unfortunately I no longer have a cassette deck and suitable secondhand units at Cash Converters are still at the hundred-dollar mark.

I have looked at the headphone socket on my portable stereo but this, I believe, is unsuitable for the audio input on my computer. What do I need to do so I can use the headphone socket on my portable stereo? I’m sure I’m not the only person with irreplaceable audio cassettes decaying in a drawer; perhaps you could make it a project.

Maybe you could make it a unit that would take the audio from the headphone socket, clean the tape noise, maybe even convert it from analog to digital (though my TV card already does that) and then take it to the "Audio In" on a computer. If the headphone socket is non-optimum, perhaps you could design it to take the signal directly from the heads? (J. W., via email).

Your headphone outputs should drive your computer’s audio input without problems but you may need to load each of the outputs with a 32Ω resistor to ensure that the headphone stages work correctly.