Transformer for Studio 200 amplifier

Can you tell me if it is possible to use a 40-0-40V 300VA transformer with the Studio 200 Amplifier published in February 1988 and is it possible to bridge it? The original article specifies 35-0-35V 300VA. My brother has one and I’ve got a 40V tranny which I was thinking of using. (E. Z., via email).

Yes, you can use your 40-0-40V transformer. And yes you could use the amplifier in bridge mode to drive 8-ohm speakers (NOT 4-ohm!).

Converting TV sets to oscilloscopes

I have had an oscilloscope on my bench for many years – a BWD 509B – the CRT for which has sadly died on me. I have been trying to locate a source of supply for a new CRT without any luck. A 130BUB series type is required. If you know of a source of supply I’d be grateful for the information.

Alternatively, I remember that, many years ago, Radio, Television & Hobbies, brought out a circuit which converted a small TV to an oscilloscope. I don’t know whether it was ever produced in kit form. Do you have any information on this? (L. N., Queenbeyan, NSW).

EA magazine published a CRO adaptor for TV in May 1980. This was useful for audio frequencies only.

These days, a better approach is to build a sound card interface for your PC and then use oscilloscope software from the internet, as detailed in our August 2002 issue.

Car subwoofer amplifier not economic

Any chance of a car subwoofer amplifier? Yes we all hate them but my senior students keep asking for it. (D. K., via email).

A car subwoofer is not an economic project when you take into account the need for a high power inverter. You are better off buying a built system from Jaycar. On the other hand, if you use an efficient subwoofer, you can use the 70W Mighty Midget class-H amplifier featured in the March 2002 issue. This was also used in the PortaPal PA Amplifier featured in February & March 2003.

Smoke Gets In Your Eyes (Especially If The Zener Diode Is Wrong) I am having a problem with the Keypad Alarm circuit, from the April 2003 issue. I have triple checked all the components and solder joints and all appear to be fine. The problem is that the first three components on the power input circuit get extremely hot and smoke is visible. The 10Ω resistor (first on the circuit) has burned out a number of times and each time I have replaced the part with a higher rated part, I am currently running a 10Ω 5W ceramic resistor. So far this appears to be OK, however this part and the first two diodes still get very hot and smoke can still be seen! Other than this the alarm functions correctly and is fully functional. The power source I am using is a 12V SLA 7.0Ah battery as recommended in the documentation supplied. Would you please be able to advise me on any ways to correct this problem? (A. P., via email). Heat and smoke are symptoms of a serious fault so you can’t just keep on installing a bigger resistor. That is akin to "using a bigger fuse". Our guess is that the zener diode is the wrong voltage rating. If it was in the wrong way around, the circuit would not work at all. To check this, first measure the output voltage from the 78L05 regulator – it should be 5V. If it is 5V and the regulator is not getting hot, either the zener diode (ZD1) is not 16V or the 100µF 16V capacitor is in the wrong way around. On the other hand, if the 78L05 is getting hot, or has the wrong output voltage, then you have a short or a faulty component elsewhere on the PC board. Try pulling out the zener diode. If that solves the problem, it was the wrong voltage rating.

USB power for Micromitter

How would I go about powering the Micromitter FM transmitter (SILICON CHIP, December 2002) via USB? What is the range on this unit? (S. D., via email).

You can power the Micromitter via a USB port by connecting up power as per the battery power overlay. The 5V can be obtained from pin 1 (5V) and pin 4 (0V) on the USB port. Transmission range should be around 15m.

CFL driver will not work with 110V CFLs

The circuit in the September 2004 issue entitled "Oh no! Not Another CFL Inverter" is very interesting but I’m not sure what it’s supposed to power. 370V DC? Is this for a tube type CFL and not the kind that normally fits into a light socket (with the built in ballast)? In America, this obviously won’t power light socket CFLs at 12VDC input but possibly would power CFLs from 6V?

Is this for powering just the CFL plug-in tubes? (K. C., Cypress, USA).

In countries like Australia which have domestic mains power supplies of 220-240VAC, CFLs have an internal bridge rectifier to produce about 320V DC, the same as delivered by the CFL driver circuit. Because 240VAC CFLs have this internal bridge rectifier, they can run on DC/DC converter circuits like the CFL Driver.

In countries like the USA which have mains supplies of 110-120VAC, CFLs have an internal voltage doubler rectifier (typically two 22µF 200V electrolytic capacitors and two 1N4007 diodes) to again develop around 320V DC. Because of their different rectifier arrangement, these 110V CFLs cannot run from a DC/DC converter. Nor can they run from a high frequency DC/AC inverter because their internal rectifier diodes and capacitors are not designed to cope with high switching frequencies.

XP reports USB overload

I recently built the USB Power Injector described in the October 2004 issue of SILICON CHIP. It works as described but despite using it ahead of a 4-port unpowered hub I still get messages from Windows XP stating that the device exceeds the power capability of the USB port. Do you have any suggestions as to how I might overcome this problem?

The unpowered hub will be unaware of the fact that additional power is available to it and will therefore continue to report an overload condition to Windows. This is completely normal.

Mods to PowerUp to use with PC

I’ve just finished building the Power-Up project (kit sourced from Jaycar) from the July 2003 issue. I thought you might be interested in a slight mod I had to make.

I am using a PC of moderate specification (400MHz Pentium II with two hard drives) and found that the kit was too sensitive with this on the master outlet. It would activate the slave appliance outlet with trimpot VR1 almost fully anti-clockwise but it would not shut this outlet down when the PC was turned off.

The reason for this was that the standby current was too large. The output of IC1a was just under 10V p-p when the PC was in standby. By decreasing the gain of the first amplifier (IC1a) from 471 to 69, I was able to get the unit working as planned. This required replacement of the 470kΩ resistor in the feedback loop around IC1a with a 68kΩ resistor. (S. H., via email).

Valve Preamplifier Power Supply Problem I am querying the construction details of the power supply section of the Valve Preamplifier project (SILICON CHIP, November 2003). I have followed the instructions exactly or would have the first time if the kit had a long enough section of the 0.2mm wire! My second attempt at winding the secondary WAS right. First, the kit came with an IRF-820 instead of the schematic’s STP-6NB50 Mosfet. I did a quick web comparison and this seems to be a valid substitution. All the rest of the power supply is straightforward and went together easily. On initial switch-on (being fed from my bench supply current limited to about 400mA), the output only rose to about 10V (DC) and the Mosfet started warming faster than I thought it should. I progressively upped the current limit only to see that the Mosfet just got warmer and the output stayed at about 10V. So I stopped that test before the melting started! I hooked up my scope and check-ed the switching output waveforms from the TL494 IC and all looks good at a frequency of about 29kHz. To reduce the dissipation on the Fet, I substituted a resistor for the primary coil and the output switching is all going rail to rail on the Fet Drain. Refitting the transformer and checking with the scope for the fly-back pulse – it looks correct on the primary but I get nothing at all on the Secondary (Finish); no AC whatsoever. I can’t help thinking it might be phasing on the transformer (but I forgot that theory ages ago!). If there was a project "revision" in a later magazine, please refer me to it. (N. P. via email). Since the Mosfet appears to switching the primary correctly, it sounds as though you might have a shorted turn in the secondary. You might be wise to rewind the transformer, following the instructions very carefully!

High & low side driver explained

I am experiencing difficulties with technical jargon and in your website it says that I could ask. What is the meaning of the terms " high side driver" and "low side driver" for a solenoid driver? (P. C., Sao Paulo, Brazil).

A high side driver is like a switch in the positive line to the solenoid while a low side driver is in the negative (ground) side of the solenoid.

Polarity explained

I have read the article entitled "Shut That Mutt" in the April 2004 edition and I have big hopes that my 5:00am early calls from the neighbour’s dog may be nearing an end.

I am a raw novice and I have a question. On page 37 of the April 2004 edition, left-most column, second last paragraph, you say "watch the polarity". In the next column paragraph 2, referring to the LED, quote "Again watch the polarity".

Polarity is my issue. I know there is positive and negative but would you have a magazine issue that will show me how to recognize how to get the polarity correct on electrolytic capacitors, LEDs, and it seems, the tweeters in this case? (B. C., via email).

If you have a look at the PC overlay diagram (Fig.3), you will see that LEDs and some capacitors have plus (+) signs near or on them. This shows how these components must be installed. That is what we mean by polarity.

As far as components like capacitors diodes and LEDs are concerned, the polarity must be correct otherwise the components themselves may be damaged or the circuit won’t work correctly.

As far as the tweeters are concerned, no damage will result if they are not connected correctly but their phasing will be incorrect so that their diaphragms will not both move in the same direction. Hence their outputs will tend to cancel rather than reinforce each other.

Light Bulbs In Microwave Ovens My teacher told me that if you put the metal part of a normal (mains-powered (household) light bulb into a bowl or glass of water (the glass part is out of the water) and put it in the microwave, the bulb will light up. Is this true? If it is, how does it work? (D. H., via email). This could be the basis of an urban myth and could be a good way of blowing your microwave oven if you don’t know what you are doing. What happens is this. You must have a glass of water in the microwave to provide the magnetron with a safe load. Then, when you turn on the microwave oven, the RF energy in the oven compartment excites the filaments and what typically happens is that the sections of the filament are progressively blown out as they heat up and become white-hot. So yes the light bulb does light up as each section of the filament glows and lights but then you are left with a defective bulb. It is probably a good idea to have the metal section of the bulb immersed in the water because otherwise it may constitute a "shorted turn" in the microwave energy field.

Remote control extender for multiple units

Can the Remote Control Extender Kit Mk.II (SILICON CHIP, July 1996) be extended to link up to four or five units, for example DVD, Video, Satellite Foxtel, Satellite Digital set-top and hifi amplifier, by simply adding the additional three of four infrared LEDs for each unit and having only one single IR unit in the line of sight of the remotes for each device. (H. C., via email).

You can use several infrared LEDs instead of a single one. You can connect three in series and change the 220Ω resistor to 100Ω. For even more LEDs, you can connect another series string in parallel with the first. Connect the second string between the collector of Q1 and the 9V supply.

Jumbo LED clock has link options

I have built the "Compact Jumbo LED Clock" from the March 2001 issue of SILICON CHIP but I can’t get it to work at all. Instead of the colon flashing, a "1" flashes instead. Are there any faults with the design? (D. D., Blenheim, NZ).

The Jumbo LED Clock had link options to suit both versions of the large displays specified. These are marked "S" and "V" on the display board. You must install either the "S" or "V" links, not both. It would appear that you have selected the wrong link option for the displays you are using. Try connecting with the alternative linking.

TV signal strength/quality meter wanted

Our channel 10 signal seems to have another faint signal with it. Can you recommend a meter and information so I can test for this and any advice on filtering it out? I’m an electrician so I’m OK using meters but I just need a bit of educating about TV signals. (S. K., Toronto, NSW).

There is no simple meter for measuring TV signal strength – you would be looking at an instrument costing several thousand dollars. Even then, it would not show "signal purity" or the strength of an interfering signal on the same channel.

In summer time (on hot summer evenings) there is often co-channel interference between TV channels on the same frequency – it could be from a station on the channel 10 allocation hundreds or even thousands of kilometres away.

The signal is bounced off the troposphere and causes a characteristic noisy "venetian blind" effect on the picture. This can happen with any VHF channel but is not common on UHF channels. Apart from the possibility of using a more directional antenna to discriminate to the wanted signal, there is nothing you can do about it.

WARNING! SILICON CHIP magazine regularly describes projects which employ a mains power supply or produce high voltage. All such projects should be considered dangerous or even lethal if not used safely. Readers are warned that high voltage wiring should be carried out according to the instructions in the articles. When working on these projects use extreme care to ensure that you do not accidentally come into contact with mains AC voltages or high voltage DC. If you are not confident about working with projects employing mains voltages or other high voltages, you are advised not to attempt work on them. Silicon Chip Publications Pty Ltd disclaims any liability for damages should anyone be killed or injured while working on a project or circuit described in any issue of SILICON CHIP magazine. Devices or circuits described in SILICON CHIP may be covered by patents. SILICON CHIP disclaims any liability for the infringement of such patents by the manufacturing or selling of any such equipment. SILICON CHIP also disclaims any liability for projects which are used in such a way as to infringe relevant government regulations and by-laws. Advertisers are warned that they are responsible for the content of all advertisements and that they must conform to the Trade Practices Act 1974 or as subsequently amended and to any governmental regulations which are applicable.