Blown Mosfets in battery controller

I am writing in regards to a 12V Battery Controller kit which was described in the January & February 1996 issues of "Electronics Australia". After assembly, I followed the instructions in the test procedure. Applying 12V was fine; applying 13.5V was fine and the appropriate LEDs lit up.

However, when grounding the AUX terminal, Mosfet Q1 burnt out. The leg going to the AUX terminal has blown off completely. I tested that there was no continuity between the terminals MAIN, AUX and ground, and the Mosfets and ground. There appears to be no flash marks from the mica washer to ground.

I would appreciate some information on what could have caused the problem and how I can go about fixing it. We are travelling around Australia and I need to get the kit going soon. (S. K., via email).

It seems likely that during the test where you connect AUX to GND, you did not have AUXMON connected as well. In other words, AUX and AUX MON should have been both connected to GND. This would tell the monitoring part of the circuit that the AUX battery voltage was low and would throttle back the Mosfets and allow the test to be safely made.

If you did not have AUX MON connected to ground, you would probably blow both Mosfets. We would be surprised if they are both not blown.

Sound-activated switch for lamp control

I am doing a Year 12 major assessment task and I have decided to do a sound-activated lamp (clap and it turns on; clap again and it turns off). It needs to be 12V.

Have you ever produced a kit for this or would you know where I could get a circuit diagram? (T. B., Port Macquarie, NSW).

While we have not produced a sound-activated switch as such, the Dog Silencer project featured in the April 2004 issue includes a circuit which could be adapted to your application.

6V DC-DC converter for vintage car Could you please tell me if SILICON CHIP has done any projects on DC-to-DC up-converters (namely 6V to 12V)? I don’t seem to be able to find anything available commercially. I wish to run some 12V items in a vintage car that has a 6V system. (W. H., via email). The DC-to-DC Converter from the June 2003 issue of SILICON CHIP will do the job. It provides 2A output. To operate at 6V, we recommend replacing Mosfet Q1 with a logic FET. Jaycar sell these – Cat. ZT 2271.

More LEDs for USB lamp

I’m from Argentina and I really LOVE your web site. I want to build the "Itsy-Bitsy USB Lamp" from the March 2002 issue but I want to use six or eight LEDs instead of one.

Is this possible? Should I connect them in series or parallel? Should I change the resistor? (M. B., Buenos Aires, Argentina).

Since the USB only supplies 5V, you cannot put the LEDs in series. You could have two or more in parallel but they would each need their own 47Ω limiting resistor.

SCRs in a big battery charger

I have a number of fairly big SCRs (200A) already bolted to heatsinks. Is it reasonable to use these as rectifier diodes in a charger for my 48V 650Ah lead-acid fork-lift battery? Presumably I could simply turn them hard-on by hard-wiring their gates.

Clearly, a 3-phase bridge would give much better ripple voltage. What benefit would accrue if I went that way? (Don’t all real men have 3-phase in their garages?)

By the way, I presume with a battery charger that the current only flows while the instantaneous rectified voltage exceeds the battery voltage. Is that right? Would it not then give a pretty odd-looking current waveform?

Can you suggest an existing circuit for a trickle charger that could be tweaked (just a little) to do my job or do I need to go from first principles?

Also, with a single phase, centre-tapped transformer of say 1A rating, if I use just two diodes as a rectifier, what current can I get in terms of DC? It appears to me that it would be a little more than a 1A (1.4A?), since while each half of the secondary can take 1A (if I’m lucky), it will only be driving the load every half cycle. But with I²R copper heating, the heating will not allow me to use twice the current for each side of the transformer. Your thoughts? (D. W., via email).

We would not hard-wire the gates to the anodes; instead, connect them via a 220Ω 0.5W resistor.

Assuming you had a 3-phase transformer, a 3-phase bridge would give lower ripple but otherwise there is no benefit.

In any rectifier, current only flows when the diodes are forward-biased and the voltage is greater than that at the load; eg, the battery. The resulting current is a series of short pulses at 100Hz, for a 50Hz single phase bridge rectifier.

We published a 12V trickle charger in October 1998 and this was modified for 24V operation in the February 1999 issue. In principle, the same circuit could be adapted to 48V operation.

In any rectifier circuit, the power output can be no more than the transformer can supply and since the rectifier current is in the form of high-current pulses, copper losses in the transformer and power losses in the rectifiers can be significant. If your transformer has a rating of say 30VA, you can’t expect any more than about 28VA from the whole circuit without overloading the transformer.

Cordless connector for guitar I am a guitar freak and need an easy solution to connect my guitar to an amplifier, as I find it difficult to perform with a long cord hanging around my guitar. Can you please suggest a circuit for a cordless solution? (A. G., Delhi, India). You could use an FM transmitter and FM receiver such as a standard FM tuner. Have a look at the FM Transmitter for Musicians in the November 1998 issue. This can be used to transmit guitar signals.

Optocoupler for PC-controlled switch

I am having trouble finding a part for the PC-Controlled Mains Switch (SILICON CHIP, September 2001). I can’t find or track down an SFH601-3 optocoupler, as Dick Smith Electronics doesn’t stock this part any more.

The article states "DO NOT SUB-STITUTE" and I have tried Farnell, RS and various other electronic suppliers with no success. Can you make a suggestion? (M. T., via email).

The CNY17-3 optocoupler is also suitable. It is available from Farnell Electronics, Cat. 359-8380.

Which amplifier to build

I am in a quandary about which amplifier to build. I want something with quite high power and there are two amplifiers to choose from: the 500-watt monster described in August/September/October 1997 or the more recent Studio 350 in the January & February 2004 issues.

I do have problem trying to obtain a suitable 800VA transformer but apart from that, all the parts seem to be available. Can you throw some light on my problem? (R. B., Berala, NSW).

As you have probably realised, the decision on which one to build is not clear cut. On the one hand, the 500-watter described in 1997 is a complete amplifier with power supply, fan-cooled heatsink, relay protection, thermal cutout, etc. It is basically intended to be a real work-horse, whether in PA, music or domestic use.

The Studio 350 has been presented only as a PC board module and you would need to do a lot to end up with a complete amplifier in a case, although you could use the fan control and relay protection from the 500-watter. And while there is nominally a large difference between the power outputs of the two amplifiers, in decibel terms, it comes down to a difference of about 1dB which is hardly worth worrying about.

Where the Studio 350 amplifier is a clear winner is in terms of residual noise and distortion, although the margin would be less if the two were adjusted to have the same closed-loop gain. By way of explanation, the Studio 350 has a voltage gain of 23 (27dB) while the 500W amplifier has gain of 33 (30dB).

What would we do? For home listening, where you want the best possible distortion figures, we would choose the Studio 350 but there would be quite a lot of work to turn it into a complete stereo power amplifier. For a general workhorse, we would pick the 500-watter.

If you want an 800VA transformer, contact Harbuch Electronics. They advertise each month in SILICON CHIP – see page 87 in this issue.

Crossover wanted for subwoofer

I made a 50+50W amplifier several years ago which drives two Philips speaker enclosures, with tweeter, midrange and 12-inch woofers. A while ago, I decided I did not have quite enough bass and because the speakers are five metres apart (the room is a combined lounge/dining room 5.2 x 10m), there is a "hole in the middle", as I have heard it called.

About a year ago, I had a subwoofer box built to suit a 12-inch (300mm) Polycone Woofer (Cat. CW2130) which I bought from Jaycar. I also purchased a 4-inch (100mm) midrange speaker with a fully enclosed back and this will also go in the enclosure.

I have built a 50W amplifier from a kit and intend to use it to drive both woofer and midrange speakers. I don’t need a huge bass output and I don’t have "golden ears". I intend to drive this "centre amplifier" from the speaker output in each stereo amplifier, through isolating resistors of a few kilohms, of course.

My query is, how do I calculate the series choke for the woofer and the series capacitor for the midrange to give a reasonably balanced output from both speakers? (N. W., via email).

There are a number of problems you need to address. First, you need to know the efficiencies of the two drivers. It is probable that you will need a series resistor to attenuate the level of the midrange to match the proposed woofer.

Second, simply placing an inductor in series with the woofer may not give a rapid enough rolloff below your chosen crossover frequency. Third, what is your proposed crossover frequency? It should normally be well below any cone resonance of the midrange.

Fourth, and related to the third question, a simple capacitor crossover may not be adequate for the midrange. You might need a second order filter (ie, a capacitor and inductor) and this needs to take into account any attenuating resistor. That said, it is not possible for us to give definitive answers to your questions. You may like to obtain a book on speaker design before you proceed further.

Ultrasonic cleaning fluids wanted

I have finally given up trying to find the necessary components and information to build an ultrasonic cleaner and have bought one. However, I am still in the dark as regards the correct cleaning fluids to use on various items such as silver-plate, brass, copper, glass and plastics. I would be grateful if you could tell me where I might get this information.

I have been unsuccessful on the internet and no information came with unit. (E. P., via email).

For most cleaning, normal detergent in water (ie, for washing dishes) is fine. Don’t use dishwasher detergent – it is too caustic. And don’t use an ultrasonic cleaner to clean "paste" jewellery – there is a danger that the "paste" will dissolve.

There is quite a lot of information on the net. Just search for "ultrasonic cleaning fluids".

Query on speaker earth return I have been a purchaser of EA/ETI/AEM/SC magazines and kits since the Playmaster 136 onwards and have built and repaired so many (both professionally and for friends) that I can not remember them all! My query is regarding the SC480 amplifier module. The PC board layout as described is excellent but for the best damping factor why doesn’t the speaker common return directly to the power supply reservoir capacitors instead of to the PC board (voltage drops, etc)? (D. B., via email). Connecting the speaker return to the main capacitor centre point would give a worse result since the feedback is taken from across the output and centre earth point on the board. In any case, the damping factor of the SC480 is more than 140. We doubt whether any EA or ETI design could better this.

White noise a hazard for tweeters

The Loudspeaker Level Meter in the April 2004 issue is a great idea but I have a niggling worry about your suggestion for using inter-station noise as a source. Isn’t it nearly white noise?

If it is, there’s a possibility that if you turn it up too loud for too long, you could endanger your tweeters. I could be completely wrong about this – I really can’t remember how much HF energy is in inter-station noise. (G. B., via email).

Inter-station noise is white noise and the signal from a typical FM tuner is quite high. However, the level meter has quite enough gain to allow you to do the tests at modest levels. Even a few watts of white noise is very loud and we doubt whether many people would run their systems at such a level as to put their tweeters in danger. It’s a good point though.

Infrared remote controls explained

Could you please answer a question for me? In a remote control for a CD player like a Pioneer DEH-P3500 or a TV remote control, which has the emitter diode? The unit or the remote? Which has the detector diode? The unit or remote? I would greatly appreciate an answer! (S. A., Hot Springs, USA).

All infrared remote controls contain an infrared light emitting diode while the unit being controlled (CD player, etc) has the detector diode to pick up the bursts of infrared. However "learning" remotes also have a detector so that they can receive and learn the codes put out by other remote controls.

Using a CRO to monitor the mains

I seem to recall you published a circuit to allow a CRO to safely monitor the 240VAC mains without doing in either the vertical amplifier or the human but I can’t find a reference.

I wanted to analyse the waveform produced by a cheap portable generator and using a 9V AC plugpack for isolation seemed to mangle the sinewave into a triangular wave. Did it exist or am I dreaming? (D. H., via email).

It did exist – the Differential Input Buffer for Oscilloscopes, published in April 1992.

Weak Video From A/V Transmitter

I built the Audio/Video Transmitter featured in the July 1999 issue. It transmits audio perfectly but video has "snow" on screen.

I checked component polarity, PC board placement, etc and all is OK. The soldering is very neat and good quality. I am transmitting around eight metres in a plasterboard and timber frame home. The antenna on the transmitter is a telescopic whip (990mm) and on the receiver is a telescopic "Rabbit Ears" (700mm).

Can you please help with this problem? Or can the circuit be improved to be more reliable and give better quality? (M. V., via email).

One of the upc1688G amplifiers is probably not working or is reversed in its positioning on the PC board. Check the orientation and soldering of each device. Alternatively, one of the coupling capacitors between these amplifiers could be shorted or is the incorrect value.

Logging Motorbike Fuel Mixtures

I recently purchased a Fuel Mixture Meter kit, although I haven’t begun assembly yet. I would like to record the A/F ratio relative to RPM and throttle position and have also purchased a data logger kit which has both digital (16) & analog (8) inputs. I also have a strain gauge and amplifier to measure loads on a motorbike dyno I’ve designed and built in the back shed.

With the fuel mixture kit, I thought I would fit some DIP switches so I can choose to switch resistors R1, R2 and R3 on and off. This I can manage but what I would really like would be a digital output to the data logger of the air-fuel ratio and it is beyond my abilities to work out a method.

I assume I could pick up the raw digital signal at pin 1 of IC2a but this won’t give me a very user-friendly figure to log. Can you recommend a way to export a processed digital output signal of the A/F ratio to my data logger? (B. G., via email).

The Fuel Mixture Meter is not designed to connect to a data logger. This is because it does not produce an output in serial form, as it was designed just to drive a display.

At best, you can log the voltage from the oxygen sensor but make sure the loading on the sensor is 1MΩ or more. You can convert from the raw oxygen sensor voltage to an air-fuel ratio by comparing the voltage against a graph of the sensor’s output curve.