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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.

Volume control for Plastic Power amplifier

I have brought a kit from Jaycar Electronics for the Plastic Power amplifier which featured in the April 1996 issue of SILICON CHIP. It is rated at 175W into 4Ω loads. I want to put a volume control on this unit. I have been told to remove the 1kΩ input resistor from the board and replace it with a 1kΩ logarithmic pot. Can you let me know where the pot is to be installed? (G. T., via email).

 The 1kΩ resistor should be left on the PC board and you should use a 50kΩ pot (not 1kW). Connect it the same way as we have in the Ultra-LD amplifier in the May 2000 issue.

Erratic readings on PIC tachometer

I’ve built the tachometer project in the April 2000 issue. It works fine when connected to my power supply and sinewave generator but when connected to my car’s ignition coil, the reading was not accurate and the least significant digit (100s) is always jumping a few digits.

For example, the reading might be 1200 RPM but it will jump to 1700, then 1300 then 1200 RPM; not stable at all. The reading is also higher than the actual RPM. The length of the wires connected to the tachometer has been reduced to only 15cm long. Can you advise? (T. V., via email).

 You could try increasing the value of the .056μF capacitor following the 22kΩ ignition coil input resistor. This will damp down any oscillations from the ignition coil which may be causing the high count on the display. Try a 0.1μF capacitor as a first attempt. Reducing the value of the 47kΩ resistor between pins 3 of IC2a and pin 6 of IC1 to 22kΩ may also help.

Engine immobiliser flattens the battery

I bought the Engine Immobiliser kit as published in the December 1998 issue of SILICON CHIP and connected it to my 1997 Barina Swing. All voltages/currents were as per the article and the operation was OK after wiring. However, the car battery went flat overnight.

I put in a new battery and that was discharged too. Otherwise the engine immobiliser seems to work OK. On the workbench, it draws only about 150mA intermittently.

The car wiring was done by a qualified electrician (cost me $200 for the $30 kit!) and is very thoroughly done. Are there any errors in the kit? (G. K., Wollstonecraft, NSW).

 Almost certainly, your Engine Immobiliser is incorrectly wired to your vehicle. This is because the circuit should not draw current from the battery until the ignition is switched on. Your current measurements indicate that it is working correctly.

Check that the supply to the hidden switch, S1, is from the fused side of the ignition supply and not from the battery side. In this way, when you set the ignition immobiliser, current will only be drawn from the battery when the ignition is switched on.

IR remote control with 30 keys

I was reading about the remote controller for trains in the April 1992 issue (I know it’s ancient) and I would love to use the chips you used for infrared transmission/reception. These are the MV500, MV601 and the SL486 but I can’t find them anywhere. I am guessing they have been outdated, so could you please tell me the chips you would use for this same type of application today. I need to have about 30 keys on the keypad of the transmitter. (P. K., via email).

 Have you seen our more recent design from the October, November & December 1999 issues? This uses a cheap remote from Oatley Electronics and has eight buttons in all. We don’t know of any chipset which will allow you to use 30 keys.

Temperature control for vacuum forming

I want some information on your 240V 5A motor controller kit published in September & November 1992 and also on your heater controller kit published in the July 1998 issue. I want to control the temperature on the heating elements in my home-made vacuum forming machine. Or do you have something more suitable? (S. A., via email).

 We’re not sure how you wish to control the heating elements and whether they run from 240VAC or not? If they do, the heater controller described in July 1998, a zero-voltage switching design, is the right choice. If you are using a vacuum cleaner motor to produce the vacuum, the 5A motor controller is suitable; you can use it for any 240VAC brush-type motor.

Load requirements for the Ultra-LD amplifier

In your write-up on the ultra low distortion amp project in the March & May 2000 issues you recommend not using low impedance speaker loads because of increased distortion caused by current sharing imbalances in the output transistors.

One fact you can be sure of in audio amplifier design is that the output load will be highly reactive over the audio frequency range. Even good speaker designs like the Vaf DC-7 provide a widely varying load, ranging between 4-17Ω with a nominal impedance of 6Ω. Have a look at

How can we expect your amplifier design to behave when connected to such loads? Will distortion be the only issue or will there be other problems as well? (M. D., ANU, via email).

 The Ultra-LD will handle nominal 6Ω and 8Ω loads OK but we don’t recommend 4Ω loads because as you point out, loudspeaker loads are highly reactive and they can dip to very low values. We wouldn’t recommend driving electrostatics either.

Avoid stupidity with 240V speed controller

I seek your advice concerning the 10A 240VAC speed controller described in the November 1997 issue of SILICON CHIP. I have blown several BUP213s (usually owing to my own stupidity) and have noticed the aforementioned semiconductor usually fails before or at the same time as the 10A fuse blows.

Consequently, I have wondered whether an IGBT upgrade might be the best option (as my stupidity is probably a permanent, but fortunately intermittent, factor!). Since the price of higher rated IGBTs appears to rise exponentially with their current ratings, would it not be feasible to connect two BUP213s in parallel? Of course, some physical modifications to the layout would be required, but nothing too challenging. Would any other component modification be required? (J. B., Burragate NSW).

 We are intrigued as to what particular form of stupidity causes the BUP213 IGBT to blow. If you are intermittently stalling the power tool in question, we would expect the over-current limiting to take care of the situation. Perhaps you should check the circuit at low voltage, as described in the article.

On the other hand, if you are making some sort of connection to the circuit while it is powered up, there is always the risk of blowing the BUP213 and other parts.

Having said that, the BUP213 can be upgraded to a BUP314 which is a larger TO-218 package and has a current rating of 52A rather than 32A for the BUP213. These are available from Farnell Components; phone 1300 361 005. We don’t recommend parallel connection of IGBTs since they will not share the current equally. In an overload situation this might mean that you blow one and then the other, within milliseconds of each other.

Insufficient signal from reluctor

I have one problem with the Multi-Spark CDI system described in the September 1997 issue. I connected it up to a reluctor from a Chrysler and there is no output from the coil. Then I shorted out the wires for the pickup and got a spark from the coil. I then tried it on a Falcon XF with reluctor and it worked perfectly. The resistance on the Chrysler pickup is about 460Ω and on the XF it is about 1.2kΩ.

I looked at the trigger input for the ignition system featured in the June 1998 issue and noticed there was a 2.2kΩ resistor in it but the equivalent resistor in the September 1997 circuit was 10kΩ. I made the change anyway but still no go. There doesn’t seem to be enough pulse from the Chrysler pickup. Can I modify the circuit so that it will work properly? (M. K., via email).

 The reluctor signal is usually quite substantial at around 30V peak-to-peak and so the circuit should operate. The actual resistance of the reluctor coil does not indicate much since the output is dependent upon the strength of the magnet associated with the reluctor, the number of turns on the coil and the reluctor gap, as well as the rotational speed.

The sensitivity of the reluctor trigger circuit can be increased by reducing the value of the 47kΩ resistor at the cathode of ZD5. This will reduce the level of current holding Q8 on when there is no signal from the reluctor. You could try using a 220kΩ trimpot and adjusting it until the circuit works or try various values from 68kΩ to 220kΩ.

The 2.2kΩ resistor you refer to is used in the collector of the transistor and will not affect the sensitivity of the reluctor circuit. Either value could be used here.

By the way, Chryslers are pretty old now; it is possible that the reluctor is faulty or you are using reversed connections to the circuit. Try swapping the reluctor leads.

Coolant alarm query

I have built the Coolant Level Alarm from the June 1994 issue of SILICON CHIP and I feel that there is a problem with the connection of the "hot" side of the indicator lamp. Fig.1 has the connection between the 33Ω resistor and the anode of diode D2 but Fig.2 has the 12V and the lamp commoned. Which one is right? (D. H., via email).

 Fig.2 is right; Fig.1 is wrong. This was noted in the Errata we published in the February 1995 issue.

Multipurpose fast battery charger

I’ve built the Multi-Purpose Battery Charger described in the February and March 1998 issues and it is not quite up to spec.

I’ve tried it on a couple of 2.3A.h 12V SLA batteries and the charger only stays on FAST for about three seconds before switching to 100% even though the battery voltage does not correspond to the "full" voltage.

At a battery terminal starting voltage of 11.3V, the voltage across TP CELL and TP GND is 0.97V/cell but ramps up quickly during the initial 3-second period to about 1.8V, before stepping back to a steady 1.65V during the 100% stage. The charge terminates some time later but I’m guessing this issue relates to time-out rather than the batteries reaching full charge. Admittedly, they are older batteries so this might well be the problem.

Do you know why the TP CELL voltage for a 12V SLA battery is not equal to the terminal voltage divided by six cells? I read something about scaling it back to a NiCd equivalent. I’ve downloaded a copy of the application notes from the Philips semiconductor site and this reading has prompted a couple of questions:

(1) Given a 27kΩ Rref resistor on pin 20 and an 820pF capacitor on OSC/pin 14, I read from the chart that the oscillator frequency will be about 50kHz. This is consistent with your notes. However, reading across the chart, I get time-out periods of 45/90/180 minutes or three times the values that you arrived at. When I ran the SLA battery described above I had the switch on 30-minute time-out but am sure that it was reasonably longer than this. Am I misinterpreting the chart?

(2) Given Rb of 3.3kΩ, Rsense of 0.05Ω and Rref of 27kΩ, I calculate an Ifast of 3A rather than the 6A quoted in the article. Am I overlooking something here? (P. J., via email).

 Voltage regulation at Vbat (pin 19) is 1.63V for an SLA battery. Since the end-point for a 12V SLA battery is 14.6V, the 0.111 division by the resistors for the 12V position on S5b will give 1.6V for the cut-off voltage.

The fact that the voltage on your SLA batteries reaches 1.8V before dropping to 1.65V suggests that either the divider resistors for the 12V position on S5b are incorrect or the batteries are high impedance and so faulty.

The timeout period is also set by the POD voltage at pin 6. This is actually tied low via a 33kΩ resistor (see overlay diagram) for a divide by 1 selection and so the 15-minute timeout is set to about 22 minutes. The timeout is 226 x tosc x POD x PTD. PTD is set to divide by 1, 2 or 4 by the selection at pin 7 via switch S2.

The charging current does not follow the formula detailed in the data sheets because we are using an unfiltered DC voltage to charge the battery.

600W DC-DC converter questions

A few years back, I bought the 600W DC-DC converter kit from Jaycar in Perth and I didn’t have the time to start on it until recently. Before I start, I have some questions: (1) Do I need to upgrade the car’s alternator to a higher capacity? My present car is a 1600cc Toyota Corolla CSI. I think the output of the alternator is about 70A. (2) Will there be a surge that will destroy any of the car’s electronics if I wired the converter to come on as the ignition is switched on? (3) Is there any requirement for a slow start circuit? (4) Do I need to increase the capacity of my present battery? (A. W., via email).

 You will not need to upgrade your alternator unless you are expecting to drive your amplifiers at full power when all the headlights and accessories are on. You can switch the inverter on via the ignition and this was shown on the circuit.

The car battery should not need changing since the circuit is intended to operate only when the engine is running, so as to keep the battery topped up.

Problem with TENs unit

I recently built one of your TENS units (SILICON CHIP, August 1997) and found it terrific for pain relief for an inoperable spinal injury I suffered about 10 years ago. The problem is it was working fine one day then the next day it just would not work.

I am in my first year of a diploma in electronics course in which I have passed my hand soldering with a 97% pass mark. I have checked all resistors, capacitors and diodes and they are fine, so I was just wondering if there have been any problems with the units after they had gone into production? Alternatively, could you advise me on where to look for any possible faults? (J. C., via email).

 The TENS Unit does not appear to have any particular problems. The problem with your particular unit could be with the transformer (T1) or any other component such as IC1 or IC2, or the Mosfets Q1 and Q2. Or simply, the batteries may be "flat".

We suggest you check the operation of each part of the circuit to isolate the problem. First, check the voltage at the drain of Q1 – you should get a reading of 80V. If this is not present, check the supply to IC1 at pin 6 (6V). If this is OK, the windings in T1 may be shorted or open circuit.

If you do measure 80V, check the supply to IC2 which should be around 15V. You should also check the output voltage at the electrodes – this should give readings up to 40V (as measured with a DC-reading multimeter), depending on the pulse width setting. Lack of output voltage may mean that IC2 is not operating or that Q1 or Q2 are short circuited.

Further checking can be made with an oscilloscope. You should obtain similar displays to those published.

Notes & Errata

RoomGuard Intruder Alarm, April 2000: the 100kΩ resistor at the junction of D1, D2, D4 and pin 9 of IC1 on the circuit on page 31 should be 10kΩ. The wiring diagram on page 32 is correct.

LED Dice, May 2000: LED6 is shown back to front on the PC board component overlay on page 60. Its cathode (the flat side) should go towards the top of the page. The circuit diagram on page 58 is correct.


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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