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Speed controller causes low-speed "cogging"

I have recently completed the ‘Improved Speed Controller For Universal Motors" as featured in the October 2002 issue. It is driving a brush motor rated at 2A; well within the design’s 5A rating.

The unit controls OK at the higher speeds. However, when turned down, it "hunts" from a high speed to virtually off and then accelerates back to the high speed and so on. I have had one of the ANU’s technicians check the unit but he cannot identify the problem.

Years ago I built the controller’s lower-tech predecessor and that controls the motor quite well. Any ideas as to why the newer unit is causing problems? (A. D., via email).

  • It is possible that diode D3 is faulty and is causing disruption with the back-EMF and hence the speed control. Note, however, that the minimum speed for an appliance is dependent on the onset of "cogging", where the motor tends to run in short bursts. So VR2 needs to be set so that the minimum speed is high enough so this cogging does not happen.

    The slowest minimum speed varies from appliance to appliance.

    Simple mixer circuit wanted

    I want to build a preamp which will combine the left and right signals from my digital set-top box, to enable me to drive a centre speaker (such as that sold by Dick Smith Electronics). I do not require ultra hifi, as the idea is simply to improve the clarity of speech. What kit(s) can you suggest, please? (T. S., Claremont, WA).

  • The easiest way to do this is to make up a mixing lead that connects to the left and right outputs from the digital set-top box and combines the left and right signals using a 10kW resistor in each signal wire. The junction of each resistor then becomes the centre channel signal for a power amplifier.

    A commercial power amplifier can be used or there are many kit amplifiers available such as the SC480 (Jaycar kit KC-5345) and the accompanying power supply (KC-5347). The required transformer is the MM-1095 from Jaycar. Note that the amplifier and power supply must be built into a suitable case.

    If you build your own amplifier, the two 10kW mixing resistors for the left and right channel signals can be placed at the signal input to the amplifier rather than using a mixing lead as mentioned above.

    Software for Smart Card Programmer

    I cannot find files for IC-PROG105a on for your Smart Card Programmer (SILICON CHIP, July 2003). I can only find IC-PROG105e. Is this file sufficient? Can I also program the PIC877 smartcard with the same software? (T. T., Westmeadows, Vic).

  • The later "e" version of IC-PROG 105 can be used. For the PIC16F877 you need a loader. The GSM a3 gold and silver wafer card loader should be able to be used. More information is available on the net – see

    Speech filter to reduce background noise

    I am wondering if you have published a circuit for an audio filter? I am designing some equipment that needs a filter to remove background noise and focus on the frequencies of speech. (N. H., via email).

  • Strangely enough, we have not produced an audio "speech" filter although it would be easy enough to do. Basically, you need a combination of a high-pass filter with a low-pass filter. To see examples of high-pass and low-pass filters, have a look at the Subwoofer Controller in the August 2007 issue, specifically at the filter stages involving IC1b (low-pass) and IC5a (high-pass).

    You would need to scale the capacitors or resistors in the filter networks to provide your speech filter function.

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