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Got a technical problem? Can't understand a piece of jargon or some technical principle? Drop us a line and we?ll answer your question. Write to: Ask Silicon Chip, PO Box 139, Collaroy Beach, NSW 2097 or send an email to

Very low speed motor-control wanted

I have a couple of questions about the 10A Full-Wave Motor Speed Controller (SILICON CHIP, May 2009). Would it be able to smoothly control an electric drill spinning at between 10 and 20 RPM for between four and six hours and if so, would this cause any damage to the drill, like overheating the windings of the motor due to the switch-on current?

What I want to use it for is rotating a fishing rod in a cradle, via an adaptor in the chuck that is being built/repaired for a few hours so that the epoxy that is applied to the bindings around the guides does not sag. There are some demonstrations of how to achieve the speed control on YouTube but shall we say that they don’t look all that safe. Or would I be better off using a DC cordless drill with a DC speed controller such as the high current variable frequency controller you published awhile ago.

There are specialist motors and rigs for this job available but the cost isn’t worth it unless you are doing it professionally. (D. G., via email).

• The simplest approach would be to use a barbecue rotisserie motor. These are cheap, run at low speed all day and don’t require a speed control. You can buy cheap 240VAC rotisserie motors on-line, at Bunnings or your local barbecue retailer.

Running an electric drill at low speed is impractical. It will overheat because its inbuilt fan is ineffective at low speed and produces no cooling. In any case, 10 RPM is extremely slow; it’s only one revolution every six seconds.

It is also very doubtful whether a cordless DC drill could do the job for that length time. The rotisserie motor is the best and simplest solution.

High-frequency ferrite transformer query

I am investigating the possibility of having a variable output from a ferrite transformer working on a variable frequency to alter the voltage. The wattage has to be around 200W and the transformer has to be small.

One application I already know of in the work field is the inverter welders where the throughput is about 3600W and the transformer is only about a 60mm cube. The object of the exercise is to increase the frequency to obtain a higher voltage output and working around a frequency of about 5kHz. Can you help? (D. B., Warwick, Qld).

• We are not sure of the output voltage you require. We did publish a 200W 12V-to-240VAC inverter in February 1994. Its output voltage can be adjusted by altering the pulse width applied to the transformer. Altering the frequency does not alter the voltage.

Note that 5kHz is a relatively low frequency and ferrite transformers generally operate above 20kHz and up to 1MHz.

UHF remote control antenna problems

I would appreciate some information regarding the Deluxe 3-Channel UHF Rolling Code Remote (SILICON CHIP, July & August 2009). I have built the receiver, with two transmitters.

I am getting intermittently poor operation from both transmitters. I suspect the 433MHz receiver module. A CRO reveals continuous “digital looking” noise at the data output pin and pin 1 of the PIC, with or without the receiver antenna which changes as a transmitter operates. As I am aware of no nearby RF sources I would expect this pin to be quiet except when the transmitter is working.

Direct wiring between the transmitter PIC and the receiver PIC gives perfect operation. Is it likely the receiver module is faulty? On both remotes, I have tried a new UHF module and swapped PIC chips and re-registered with the receiver, noting the warning about distance, to no avail. The receiver acknowledge LED flashes but relay operation is intermittent.

I believe the noise is corrupting the data which the PIC interprets as an incorrect code. (I. S., Wollongong, NSW).

• It is normal for the receiver to continuously output noise or data. This is because the receiver has an automatic gain control that increases the gain when no signal is present. The output is then just noise. Upon reception of a signal, the AGC alters the gain for best reception and the output is the data stream from the transmitter.

Poor operation from the transmitters is possibly an antenna problem. Make sure that both the antenna wire link and the coiled antenna wires are included on each transmitter and that each coiled antenna wire is the correct length (138mm) before spiralling. Make sure also that the spiralled wire ends are stripped of insulation before soldering the antenna in place. The receiver antenna must also be the correct length (157mm) and the enamel insulation must be stripped from each end before soldering it to the PCB.

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