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Ask Silicon Chip

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 silchip@siliconchip.com.au

Wiring two SC480 modules for stereo

I am building a stereo amplifier using two SC480 amplifier modules (plastic version) and was wondering what is the best way to position the two modules and their associated wiring, so as to minimise interference, distortion and radiated noise.

I am using a single heavy-duty power supply module and a larger transformer. Clearly, both modules need to be more or less next to each other, since they have to be attached to the heatsinks at the rear of the case. This means that the power leads to the module further away from the power supply board have to run past/over/under/around the first module and its audio input leads. (J. S., via email).

You really have no choice but to put the modules side by side with the output transistors mounted to a common heatsink. You will have more problems with radiation from the power supply than from each module. Keep the power transformer’s secondary leads as far away as possible from the input leads to the modules.

Suppressed zero voltmeter needed

I am having a spot of bother in that I haven’t done any electronics for about 20 years and now have the need to nut out a circuit. I have a bank of batteries in my motor home (2 x 6V in series and two sets in parallel giving 12V @ 175Ah) which run a 12V-to-240VAC inverter.

I have reclaimed a 250mA meter from an old cassette recorder and placed a 62kW resistor in series with it to give a 0-16V FSD meter. But measuring the batteries gives only about two degrees of deflection between 12V and 14.6V which I am finding hard to observe.

Is there some way I can make a circuit that gives a full scale deflection of 15V but which doesn’t kick in until it reaches 10V; ie, reading from only 10V to 15V?

I have thought of using zeners and voltage dividers, etc but as I don’t want to blow the guts out of my meter, I thought I’d ask for help. (P. F., via email).

All you need to do is to put a 10V zener in series with the meter and then calibrate the circuit for full-scale deflection at say, 15V. You will need to adjust the series resistor to achieve this.

The result will be that for voltages below 10V, there will be no pointer deflection on your meter. As the voltage is increased above 10V, the zener diode will conduct and the meter will indicate a linear range up to 15V or whatever you set it to. This is called a "suppressed zero" meter.

Test circuit for a silicon bilateral switch

How do you test an SBS (silicon bilateral switch)? Do you have a test circuit? (A. W., via email).

We do not have a test circuit. All you need to test an SBS is a variable DC supply and a 4.7kW resistor. At any voltage below its breakover rating, no current will flow through the SBS. When you wind the supply above the breakover rating, the SBS will "break down" to a low voltage with the current limited to a safe value by the 4.7kW resistor.

Just connect up the suggested test circuit, wind up the voltage and you will have a graphic demonstration of how an SBS works.

Energy Meter will not run at 9V

I have just completed the Energy Meter from the July & August 2004 issues of SILICON CHIP and while it seems to work satisfactorily there are a couple of problems. Also, to make the unit more readable in some locations, I have replaced the supplied LCD with one with a LED backlight, with a separate pushbutton to charge a capacitor to drive a transistor to operate the LED for a short period after the button is pressed, rather then drive the backlight all the time (and save the backup battery) when not powered.

I cannot get the meter to zero regardless of OFFSET setting. When mains-powered but with no load connected, the POWER reads between -13.7W and -15.2W with a variety of OFFSET settings from 0 to 150. There appears no correlation between OFFSET parameter and Watts.

Under battery power, the Watts reading does shift with OFFSET as I would expect and a setting of 7 seems to zero the meter successfully. The POWER parameter also works as expected, allowing me to calibrate the meter according to your directions. Watts up?

When I first constructed the meter, I installed a partially charged rechargeable 9V battery and when powered up, the lower line of the LCD was not visible, nor could I get it to restore by cycling power or holding CLEAR to reset the device. This is not the case when the battery was charged (by external charger). I presume its charging current is preventing full voltage on the 5V rail and/or somehow inhibiting the correct initialising of the LCD module on power up.

Is this something I need not be con-
cerned about or is it a symptom of some other problem? (B. G., via email).

The Energy Meter is not designed to run on a 9V battery as a standard power source. This is because the 9V battery cannot supply sufficient current at start-up.

This is why the LCD does not start up correctly on battery power. It will operate correctly when powered via the mains power supply and then by battery. The battery is there to maintain operation if power goes off.

The zeroing problem is probably due to signal entering via the transformer. In other words, it is coupled in via the transformer laminations. Make sure the metal case is earthed correctly and that the mains leads kept away from the PC board.

A Studio 350 on steroids

I have a number of questions with regard to the Studio 350 amplifier module described in the January & February 2004 issues.

First, the power supply design shows a massive 6 x 8000mF 80V capacitor bank just for one module. I assume the large capacitor bank is intended so that two modules can be used from the one power supply with the addition of an extra 500VA transformer. I just want to verify with you that this was correct.

I was very interested in modifying the amplifier for more output power. I was interested firstly in the maths behind how you calculated and plotted the power curves for a complex load, so I could do some plots of my own with different speakers. I wanted to increase the number of output devices from eight to 16 so an inductive 2-ohm load can be driven without damage.

The issues, as far as I know, are that the MJE15030/15031 driver transistors probably will not have enough grunt. If I modified the module to have 16 output devices, changed Q8 and Q9 to MJW1302A/MJW3281A and provided the power supply has double the 8000mF filter capacitors and a 1kVA transformer, would 700W of power into a 2-ohm load be achievable at low distortion?

I was also interested in increasing the power rails from 70V to 95V so that 500W was available into a 4-ohm load. If all the filter capacitors were 100V and I increased the number of output devices so the SOA (Safe Operating Area) was not exceeded, including upgrading Q8 and Q9, and Q2 and Q3 (2SA1084 90V) were increased to 2SA1085 (120V devices) and Q1 (BC556) was also upgraded to 2SA1085 due to the higher voltages, will this work? Perhaps my bias current may be a bit high? Can you see any other issues?

Finally, if I went to the trouble of gain-matching Q2 and Q3 and all the output transistors, would it improve THD performance? Would you agree that matching Q2 and Q3 (the long-tail differential input pair) will make a bigger improvement with distortion figures than going to all the trouble of matching the output devices? (B. T., via email).

The answers to your questions are as follows:

(1). The amount of capacitance used for the supply is about right if the module is to be operated at full power and with low distortion. However, a stereo pair could be driven from the same supply with little perceived reduction in output power.

(2). We assumed a "typical" reactance for our calculations, allowing us to plot single load lines for the 4-ohm and 8-ohm cases. An easier method is to plot the worst case for all reactive loads, which is just a straight line. For the 4-ohm case shown in Fig.1 of the article, this line would extend from the same maximum current point as the 4-ohm resistive plot to a point at twice the rail voltage (ie, 140V). To understand how this works, we recommend a good textbook such as Douglas Self’s "Audio Power Amplifier Design Handbook" (see the review on page 65 of this issue).

(3). We cannot recommend that this amplifier be used to drive 2-ohm loads – even with modifications. At best, distortion will be higher due to beta loss in the output stages and at worst, it may be unstable.

(4). Matching will not make a noticeable difference to performance, especially considering that we’ve provided offset adjustment. Just make sure that the output transistors are genuine On Semiconductor (Motorola) devices.

We described a 500W amplifier in the August, September & October 1997 issues of SILICON CHIP.

Running a 6V car radio on 12V

I am restoring my 1953 Pontiac convertible here in California. I have converted the car to 12V with a GM alternator and have a perfectly good 6V radio that I want to keep as is.

I was told that if I change the vibrator to a Delco 12V unit, then I can use the radio as is. Is this so? I did not try it but I have the vibrator. I could not find a voltage step-down device to match the current requirements of the radio.

Is this the right fix? (M. S., via email).

You cannot simply change the vibrator as other parts of the radio circuit work at 6V as well, principally the heaters of the valves. The safest way to run the radio is to use a 12V to 6V regulator. You would need to find out the current requirement first.

Problem with Deluxe Battery Zapper

I have been struggling to get the Deluxe Battery Zapper (SILICON CHIP, May 2006) to work. In the condition checker side of things, I had to change the value of the two resistors connected to Q7; the 10kW to 5.6kW and the 4.7kW to 22kW. This was done to make Q7 function properly (before, it was on all the time). And since there was a high amount of positive voltage at the gates of the Mosfets, why wasn’t there a constant high current running through the 0.22W resistors?

I can now detect the pulses from Q7 at the gates with my logic probe but my scope is not working properly, so I can’t test whether I’m getting a pulse of current across the 0.22W resistors. The logic probe detects no pulse at the drain or sources of the Mosfets. I am detecting a battery positive voltage at the sources. Is this normal? Are my Mosfets shot?

On the Zapper side of things, although I have seen a spike on my scope, the meter outputs only reveal the battery voltage. (C. B., via email).

It should not have been necessary to change the resistor values in the base circuit of Q7. If this transistor was "on" all the time, this must have been due to the output of IC2d being low all the time (instead of high and low only during the pulses).

This suggests that there is a problem earlier in the pulse generating circuit – ie, around D7-D9, IC3 or IC2, etc. You may have a faulty component or one that is reversed, or perhaps even a solder bridge between pads somewhere in this area.

If you can measure battery voltage at the sources of switching FETs Q3-Q6, this does suggest that one or more of the FETs may have developed a short.

You don’t mention what kind of meter you are using to measure the voltage at the "meter" terminals. In order to measure here properly you need to use a DMM, or at least a 20,000 ohms per volt meter, to provide a high-resistance load.

Touch switch for LED control

I am interested in a low voltage touch-sensing switch to turn on a LED drawing 350mA. Is this possible and if so can you give me details on how do it? (A. P., via email).

Have a look at the Body Detector in the October 2001 issue and the Proximity Switch circuit in the January 2002 issue.

Uprating the versatile electronic load

In your March 2006 issue in Circuit Notebook, Jim Rowe contributed a circuit for a Versatile Electronic Load. Is it possible to replace the Mosfet so as to increase the SOA (safe operating area) to 10A at 20-24V or is there another alternative? I’ll appreciate very much your help. (R. R., via email).

If you don’t need to use the electronic load at voltages above 100V, substituting an IRF540N device for the STP6NK60Z MOSFET shown in the March 2006 issue will allow it to cope with currents beyond 10A at 20-24V. On the other hand, if you still need to use the unit at voltages up to about 400V, you would need to keep the STP6NK60Z and add a second one connected in parallel with it, on a second heatsink.

Tacho For 2-Stroke Outboard Motors

I am looking for a portable tachometer/rev counter suitable for 2 & 4-stroke outboards. Your digital tacho (SILICON CHIP, April 2000) as sold by Jaycar (Cat. KC5290) is claimed to do the job.

We have tried other tachometers whose makers claim that they will do the job but upon testing, these claims have proved false. Can you give me an assurance that this kit will, in fact, read revs from 2-stroke outboard motors? (G. G. Taupo, NZ).

The Digital Tachometer is designed to operate with 2 or 4-stroke engines with up to 12 cylinders.

There are two inputs – a digital input and an ignition coil input. However, with some ignition systems, there is no connection that can be used to trigger the tachometer since there is no digital tacho signal or suitable high-voltage ignition coil signal. That’s because some outboard motors use a capacitive discharge ignition with a magneto style trigger and a coil that develops a high voltage via the spinning magneto magnets. This ignition system requires no extra supply and so there is no 12V supply available. However, a 12V supply is required for the tachometer.

Because of this we cannot guarantee that the tachometer will work with all outboard motors. The tachometer will work on outboards that have a 12V supply and that operate with a standard Kettering ignition system.

Some readers have successfully used the tachometer on outboard motors with capacitor discharge ignition by building an optical trigger using a rotating vane and a photo interrupter such as the Jaycar ZD-1901.

Battery Zapper Not Suitable For Gel Cell Batteries

I recently built the Deluxe Lead-Acid Battery Zapper & Condition Checker (SILICON CHIP, May 2006) but have the following problems:

(1). The Zapper mode works for 6V, 12V and 24V batteries but the Condition Checker only works on 12V and 24V units. When using the 6V mode, all five of the LEDs light up and remain lit.

(2) When in the 6V position, putting the voltmeter on pins 14 & 1 of IC2 (4093B) shows that the voltage is down below the working voltage of the IC (3-15V). The red high LED (pins 2 & 4) on the logic probe will not light up because the voltage is not high enough. Pins 3 & 5 are lighting up, indicating green on the logic probe.

I purchased this kit to use on SLA Gel Acid batteries (both 6V and 12V) and also lead acid batteries in ride-on mowers and motorbikes. Could you please tell me how to fix these problems? (G. K., via email).

First of all, you mention that you purchased the kit for use on SLA gel cell batteries. We assume that you will mainly be using the tester to check the condition of these batteries, because it is generally accepted that SLA batteries do not respond to zapping and apparently can sometimes explode if they are connected to a zapper for a significant period of time.

If the condition-checking LEDs remain on when you are trying to check 6V batteries, this may be because pulse oscillator IC2 is not functioning. This in turn may be due to a very low supply voltage for IC2, which would correspond to the very low reading you are apparently getting for the voltage between pins 14 & 7 of that IC. The voltage at pin 1 is not relevant, because this is not a supply voltage pin.

When the circuit is idling, pins 5 & 3 of IC2 should be at logic low while pins 2 & 4 should be at logic high. These two sets of pins only switch to their opposite logic levels briefly after you have pressed S4, the Check button.

It’s not easy to suggest what may be causing the very low supply voltage for IC2 and IC3 in the 6V position. We suggest that you check the polarity of all diodes, including ZD4, in case you have fitted one of them with reverse polarity.

Frequency Readout For Radios

(1) The October 2003 frequency counter was a great design but many people would love to be able to use this unit as a digital dial attached to older communications receivers, etc. This would involve IF frequency offsets, both above and below the received frequency.

An obvious and popular choice would be the counter reading 455kHz low. Perhaps you could consider revealing the programming secrets of this design to those of us who have yet to venture into PIC programming. A small amplifier to interface between the counter and RX oscillator would also be a useful addition. (M. K., Jandowae, SA).

(2) I wonder if would be possible to design a small indicator, that when placed next to a radio, would indicate its tuned frequency, maybe via induction with the local oscillator. I have a clock radio that only displays the time – the dial indicator is not illuminated and just about impossible to see with my aging eyes. So, I find myself tuning up and down to try to get a station that I want to listen to. This takes a while as I invariably tune in the wrong direction, then have to wait for a station ID to be broadcast, etc. Anyway, just thought it may be a challenging but useful little project. (G. T., New Farm, Qld).

Both the above requests are looking for the same solution – ie, frequency readout with an input offset to compensate for the receiver’s intermediate frequency. In fact, the approach is to measure the receiver’s local oscillator instead of the tuned frequency and then offset the frequency reading by the value of the intermediate frequency.

SILICON CHIP has not produced a project along these lines and we feel that few people would build one if we did. However, a suitable design was published in the October 1982 issue of "Electronics Australia", entitled "Digital Readout for Shortwave Receivers", by John Clarke. That design is still valid although it uses a large number of 4000 series CMOS and 74LS series chips.

We can provide a photocopy of the article for $8.80 including postage.

Notes & Errata

StarPower Luxeon Star LED Power Supply, May 2004: several constructors have reported that the sense voltage (set with VR1) could not be adjusted high enough when driving 3W and 5W Stars, resulting in insufficient LED current.

This problem was resolved by
replacing the MC34063A switchmode controller IC with an On Semiconductor (Motorola) branded part.

Smart Card Reader / Programmer, July 2003: the plastic 3.5-inch to 5.25-inch disk drive adapter shown in the photos is available from PC Case Gear, on the web at www.pccasegear.com.au (look in the "Accessories" section) or phone (03) 9584 7266.

Automatic Daytime Running Lights (Circuit Notebook), August 2006: on the circuit diagram, transistor Q2 should be identified as a BC327 not a BC337. Also, the resistor in the base circuit of Q1 and resistor R1 should both be 4.7kW in value, not 10kW as shown.

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.

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