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Questions on the Playmaster Pro series 3

As a complete novice, I hope you can help before I start building the Playmaster Pro Series 3 ("Electronics Australia", February & March 1994).

My reading recommends snubbing capacitors for the bridge rectifier. Is this a good thing and needed? Where do I find appropriate values and what about heatsinking for this unit? What about a bypass capacitor for the AC input to remove RF? Would high frequency stability be improved with the addition of a small value choke on the speaker output?

Now for the stupid question, as I cannot find the answer in any of the books I have read: how can it be possible to force 250W of amplifier output through four 5W source resistors? How can something that is rated at 5W handle the output from a power transistor that can put out 10 times that? What stops them from melting or burning out?

Lastly, there is now a 500VA toroidal available from Jaycar. Would there be any improvement in stability and transient handling ability by upgrading the power supply side using the larger transformer (with upgraded reservoir capacitors and 5A fuses)? (T. M., via email).

There should be no need for snubbing capacitors across rectifier diodes in audio power amplifiers. We have not seen them used. Nor should there be any need for a bypass capacitor across the AC input.

The amplifier does have a Zobel RC network at the output so there should be no need for an extra choke. Nor is there much point in upgrading the power supply. The bridge rectifiers in the power supply are mounted on the chassis and this should be adequate as a heatsink. In any case, we are not able to make more detailed comments about this design.

As far as the emitter resistors are concerned, they are of very low resistance and therefore dissipate very little power themselves when the amplifier is delivering full output. To be specific, when the amplifier is delivering 185W into 8 ohms, the current through the FET source resistors will be 4.8A. Therefore the power dissipated in the four 0.22Ω resistors will be only about 2.5Ω or 635 milliwatts in each resistor.

As a final comment, the Pro Series 3 is a difficult amplifier for the novice to build and the FET output stages can be prone to oscillation problems at frequencies around 100MHz or higher. We would recommend you have a look instead at the Ultra-LD Stereo Amplifier described in the November & December 2001 and January 2002 issues and available as a kit from Altronics. It also involves a fair amount of work to assemble but has proved to be more trouble-free than the Pro Series 3.

Soft-start lamp circuit wanted

Have you published a circuit for a zero crossing, soft-start switching device for lamps that can blow at switch-on? (B. C., via email).

There are several approaches to this problem. The first was the ideal solution: a Lamp Saver circuit published in the June 1986 issue of "Electronics Australia". However, it used a 2N4992 SBS device which is very difficult to obtain now.

We also published a soft start circuit for lamps in the Circuit Notebook section of the September 1991 issue but no PC board was published.

Finally, the Touch Lamp Dimmer published in the July 2005 issue does include a soft start feature.

No zero-crossing power control circuit involving Triacs can be used for lamp control because the technique of switching blocks of 50Hz gives rise to severe flicker.

Wrong current from electronic load

I have assembled the Versatile Electronic Load from Circuit Notebook of the March 2006 edition. With a 15.4V power supply connected to this unit, I can only produce a load current of 1.35A. With 15.4V at the MOSFET drain and 7.71V at the gate, I have 7.41V at the MOSFET source.

The MOSFET Safe Operating Area diagram indicates that at 15V applied, I should theoretically be able to draw approximately 4.25A. Can you help? (P. A., via email).

If you are only achieving a total load current of 1.35A when there is 7.41V at the MOSFET source, that suggests that the resistance to ground in the "3" position of switch S1 is about 5.48Ω (7.41/1.35), rather than the 1W which should be produced with the network of resistors shown in the Electronic Load circuit. If the resistance to ground was correct (1Ω), you’d be able to measure only 1.35V at the MOSFET source for a load current of 1.35A and 4.25V for a load current of 4.25A.

So your measurements suggest that there is something wrong with the resistors making up your "1-ohm" current range resistance; ie, the four 4.7Ω/10W resistors or the 2 x 15Ω/5W resistors. It would therefore be a good idea to check these resistors carefully. For example if the four 10W resistors were really 47Ω instead of 4.7Ω, this would give a "range 3" resistance of very close to 5.48Ω, instead of the correct 1W.

Running CCTV cameras from a switching supply

I have recently acquired an uninterruptible power supply made by Tactical Technologies which has a supply output of 13.8V DC at 5A. Can this be used to supply CCTV cameras rated at 12V? (D. G., via email).

Ideally, it should be adjusted to provide 12V; an internal adjustment should be available. Be careful though – if it’s a switchmode supply, part of the circuitry may be floating at 240VAC! If there’s no adjustment, connect three diodes in series to drop the voltage to 12V. However, if it has residual switchmode hash, it might interfere with the video signal from the cameras.

Load impedance of the JV60

Some time ago I remember a speaker project that SILICON CHIP published that employed two (Vifa?) bass drivers connected in parallel for the low-end response. However, the second driver also had an inductor in series with it, to act as a low-pass filter at the -3dB point of the first. This effectively extended the overall bass response by using that of the second driver to supplement that of the first, without affecting frequencies above the -3dB point.

My question is: what impedance does the amplifier see? Does it see (assuming 8-ohm drivers) 8Ω down to the -3dB point of the first, then 4W below that? Or does it see 4W across all frequencies handled by the bass drivers? I’m assuming that this therefore has an impact on the electrical sensitivity of the system. (P. S., Lane Cove, NSW.

The system in question was probably the JV60 described in the August 1995 issue. The practical answer is in the impedance curve which was published in the article. It showed the system as having a nominal impedance of 8Ω.

The woofers are effectively never in parallel since they both have their own crossover network.

How about a chip amplifier?

I have been looking at your amplifier projects from the past few years and one thing you have never attempted is a chip amplifier. These are very simple, utilise cheap National Semiconductor ICs and give off minimal heat. Sites such as www.chipamp.com are a good reference. One using the LM4780 chip would give out 120W RMS into 8Ω or 60Ω into 8Ω depending on the configuration. (N. M., via email).

We have had many hybrid amplifiers over the years: December 1993 (LM1875); February 1994 (LM3876); March & April 1995 (LM3886); October & November 1996 (TDA1519A); May 2001 (TDA1519A); March 2002 (TDA1562Q); February & March 2003 (TDA1562Q); December 2004 (LM1875).

By the way, since all these amplifiers are essentially class-B designs they don’t give off any less heat than an equivalent amplifier design using discrete components.

We had a quick look at the specifications of the LM4780 and as far as we can see, it has little to recommend it compared to our current designs using discrete transistors. Both its distortion and noise figures are fairly average. Indeed, it is safe to say that no hybrid amplifier chip presently available gives a better performance than a carefully designed amplifier using discrete components.

The advantage of the hybrids is generally lower cost, simple assembly and no need for adjustments.

Tacho has wrong supply connection

I have built the digital tacho from the April 2000 issue of SILICON CHIP. Could you please tell me if there is a modification to correct the problem that when the car is started, often the tachometer does not register.

I have had to wire in a switch to totally disconnect it from power until the engine is running so that it reads correctly. If it is connected when the car is started, it often reads nothing. (J. G., via email).

You have probably connected the tachometer to a supply point that is switched off when the car is starting. Check that the supply from the car for the tachometer input does not drop to below about 9V when starting.

Carbon pile battery tester wanted

I am looking to build a carbon pile battery tester. Is there a kit available to make one? Are there suppliers that sell adjustable carbon piles? (S. R., via email).

While carbon pile battery testers are still being made, they are a bit old hat. We have not described one.

Newer battery testers use a switchmode circuit which pulls very heavy but short current pulses from the battery. In fact, the Condition Checker in our Battery Zapper featured in the May 2006 issue uses the same principle.

Missed Call Alert is locked up

Both myself and a friend have constructed the Phone/Fax Missed Call project and both of us are having identical problems.

When powered up, we get a constant call alert signal. The reset switch discharges the capacitor but has little change on the output from pin 10 on the flipflop, inputs from pins 5 & 8 are as per the article but there is a constant 6.48V on pins 9, 6 & 10. There is also a 5V drop on the 100Ω resistor at the input.

When the reset switch is pushed, pin 5 goes low and the voltage on pins 9, 6 and 10 increases slightly to 6.55V but no change to Q1 and the call alert. Any assistance would be appreciated as we seem to be at a dead end. (N. L. via email)

From your description, it sounds as if the pin 8 input of the IC1b/IC1c flipflop is being held down at logic low level and thus keeping the flipflop in its "set" state. You don’t provide any voltages for the circuitry involving IC1a and IC1d but we suggest you check this section carefully because the problem may be in this area.

In the absence of any calls, pin 1 of IC1a should be at almost +12V while pins 3 & 13 should be down at almost 0V, pin 12 at +0.6V and pin 11 (the output of IC1d) should be again close to +12V. These voltages should only switch to their opposite logic levels during the ring tone of an incoming call; pin 1 should go low, pins 3, 13 & 12 should go high and pin 11 should go low.

If this does not occur, you may have a problem in either this section of the circuit or in the circuitry on the "phone line" side of the optocoupler.

Just a suggestion: are you connecting the Missed Call Alerts onto phone lines carrying ADSL broadband links but before the ADSL filter? If so, your problems may be caused by almost constant triggering by the ADSL carriers on the line. In this case, the solution would be to fit an ADSL filter in the phone line ahead of the Missed Call Alert.

Possible Lightning Damage To SMS Controller

I purchased the SMS Controller kit (October & November 2005 issues) about 18 months ago and have had no problems at all with it until now.

The unit is installed on a yacht and is used to indicate an alarm trigger but is also used to turn a 12V fridge on and off via relay. Recently, I didn’t get an SMS response back to "OK" that the fridge was on and thought it to be a bit odd.

I investigated this to find the power LED was on but found that none of the other LEDs were working and the large resistor next to the fuse had burned out. Will it be a matter of replacing this component? I just need to know what could have caused this and whether the rest of the circuit is damaged.

The yacht’s power supply is 12V but through regulated solar charging and boost charging, battery levels could reach 14.5V. Could this have done it? (A. L., via email).

The burnt resistor may be the result of an indirect lightning strike. In a scenario like this, zener diodes ZD1 and/or ZD2 would typically fail short-circuit and blow the fuse. However, as the power LED is on, we must assume that there isn’t a short circuit across the power rail and the fuse must be intact. The resistor (although burnt) must also be intact, although it will probably measure high.

Start off by replacing the burnt resistor, then remove all of the ICs from their sockets. Disconnect the phone and power up. Check the output of REG1. It must measure 5V as detailed in the instructions. If not, check for overheating in all other on-board components. The regulator itself may also be faulty.

If the 5V supply measures OK, power off and plug the ICs back in. Power up again and check for normal operation. If the unit is still dead, the chances are that the high-voltage spike has damaged one or more of the ICs.

Modifications To The Big Digit Clock

I am interested in purchasing the PIC Programmer kit featured in the March 2001 issue of SILICON CHIP. I would like to know if this unit is capable of programming other PICs. Is the PIC 16F628 software and electrically compatible to the 16F84? I want to use 16F628 in place of the 16F84 in the Big Digit Clock project featured in the same magazine.

A further modification I would like to try with the clock is the use of larger digits of around 100mm to 120mm. Could the use of higher current rated transistors in place of BC328s Q1-Q8 with the same biasing be OK or would further modifications be necessary? (G. W., via email).

The 16F628A is pin-compatible with the 16F84A. However, due to increased functions within the 16F628A, assembly language programs from the 16F84A will probably need modifications before they will run on the newer device.

We suggest that you stick with the 16F84A in the clock project unless you have the skills to modify the source code appropriately. It’s available from our website.

Although the PIC Programmer & Checkerboard was not intended for use with the 16F627/8, it can be used with these new pin-compatible devices with a small modification.

You’ll need to install a resistor between pin 10 of the PIC socket (IC2) and ground. The purpose of this resistor is to ensure that the RB4/PGM pin is at logic low level during programming, so preventing inadvertent selection of the 16F627/628 LVP (Low Voltage Pro-gramming) mode.

Choose a value of about 100kΩ so that it doesn’t interfere too much with the 10kΩ pullup resistor. Also, make sure that pole 5 of DIPSW6 is open during programming.

Note also that the software described in the article is now out of date and will not run on Windows 2000/XP. We’re now recommending "WinPic", which can be obtained from http://people.freenet.de/dl4yhf/winpicpr.html. Select an interface type of "Tait, 7407 driver +PNP transistor" on the "Interface" tab for use with this programmer.

Regarding the use of larger displays in the Big Digit 12/24 Hour Clock, it all depends on the specifications of the proposed devices. It may well be that a higher voltage (rather than higher current) rail is needed, if the larger displays use more than four LEDs in series.

Running A Quartz Clock At Half Speed

Is it possible to modify a quartz clock mechanism for 24-hour operation; ie, to run at half speed? (T. B., via email).

Not only is it possible but because we were feeling extremely generous, we knocked up the circuit shown above to the do the job.

Fig.1 shows the circuit of a typical quartz clock. The clock IC uses a 38kHz crystal and divides it down to provide narrow pulses every 2s which drive the motor escapement. Our modified circuit in Fig.2 uses the 32kHz crystal in an external oscillator and feeds it to a flipflop to derive 16kHz. This is then fed to the clock chip’s oscillator input, whereupon it is divided down to narrow complementary pulses every 4s.

Note that CMOS chips IC1 & IC2 run from 3V, while the clock chip is run from the normal 1.5V supply.

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.

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