High-temperature electrolytics for switchmode supplies

I wish to replace a number of electrolytic capacitors in the power supply of an old fax machine. Beside the value of each capacitor is a temperature figure of 105°C, however all the ones available at retailers are marked with 85°C.

I assume this must be related to either a shift in capaci­tive value with temperature or simply the operating temperature the capacitor is rated to withstand. Could you please clarify this, and if 105°C replacements are necessary, possibly advise where I might be able to source the parts. (T. D., via email).

The inside of fax machines can get quite hot and ventilation is usually very poor so 105°C capacitors are a necessity. If you use 85°C caps they won’t have a long life. We also assume that the power supply is a switchmode type which probably means that the capacitors need to operate at very high frequencies. This means that need to have a low ESR (equivalent series resistance) other­wise their operation will be prejudiced. Low ESR 105°C rated capacitors are readily available from the following: Jaycar Electronics (www.jaycar.com.au) or Altronics in Perth (www.altronics.com.au).

Dolby Decoder Wanted Have you thought of publishing a 5.1 Dolby digital decoder box that allowed uses to take advantage of their existing two channels via a preamp or integrated amp, while offering the extra three channel outputs to a second multi-channel amp? It would be a handy add-on for people who did not want to buy an expensive multi-channel integrated system. With the popularity of digital recording, it would be great to build a DA/AD converter that enabled the home user to record/play via SPDIF, optical and analog inputs and outputs. Some of us have high quality analog preamps that do not have the facilities to connect components via SPDIF/Optical. The unit could be designed around a 1U rack mount box. (J. H., Keswick, SA). We won't be doing another Dolby decoder, for three rea­sons. First, existing Dolby decoder equipment is now cheaper than a do-it-yourself project would be. Second, there is no readily available chipset. Third, standard consumer gear has inbuilt microcontrollers and fancy displays that would be extremely difficult for us to duplicate. And fourth, the Dolby licence fee is now over $30,000 – that puts it well out of our league!

Driver transistors get hot in amplifier

I have built six Plastic Power amplifier modules (April 1996) and find that the pre-driver transistors Q6 and Q8 (BF470 and BF469) get extremely hot under normal running conditions. There is 13.5mA running through them. The text quotes 13mA. The rail voltages are 59.8V due to a slightly higher mains voltage here in Victoria.

The supplied heatsinks seem to be woefully inadequate. Even though it would be difficult to include these transistors on the main heatsink, they deserve much more serious attention in the heatsink department than they have received so far. This seems to be a common problem with most of the kit amps that I have built.

Is this normal or is there is anything else that I could do (other than the obvious increase in heatsinking and possible addition of fan cooling) to reduce the temperature without af­fecting performance. The amplifiers work beautifully and I am very happy with their performance; just concerned about heat. (E. A., via email).

These transistors do get very warm as they are dissipating about 700- 750mW. However, they are rated for 1.8W with a mounting base temperature of 114°C. This assumes a certain amount of heatsinking via the board.

You can certainly use bigger flag heatsinks but paradoxically, because of the better transfer to the bigger heatsink, they may seem even hotter. We would expect the transistors to be running at no more than about 60°C (ie, uncomfortable to hold). This is hot but should not be a problem. We have not had any reports of failures.

Door minder circuit wanted

Have you ever had a door minder circuit in one of the magazines? I heard that you had but I can’t seem to find it on the website. (R. P., via email).

We have published a number of Door Minder projects over the years. The first was in February 1988 and used an electret to sense the pressure change when opening the door. The second was in July 1995 and it used the same method. We also did light beam relay projects in December 1991 and July 1993.

We can provide back issues (or photostat copies where back issues are not available) for $7.70 each, including postage.

High power light dimmer causes flicker

I have built the High Power Light Dimmer featured in the August 1994 issue and it seems to be working fine except for the following strange happening. When it is first switched on (ie, cold) it starts at full brilliance and then after about half a minute, it dims down to the level of the potentio­meter setting with an intermediate level of flicker.

I have thoroughly checked and double checked and cannot find any reason for this strange behaviour. I regret having to trouble you with this one because as a rule every project I construct usually works flawlessly the first time which speaks volumes for the quality of SILICON CHIP’S projects and the at­tendant attention to detail. (R. M., via email).

The flickering and slow action upon powering up could be due to insufficient drive to the Triac or Triac driver. Try reducing the value of resistance feeding pin 1 of IC3. Use 560Ω or 470Ω instead. Similarly, with the Triac drive, reduce the 390Ω and 470Ω resistors from pin 6 of IC3 to the A2 terminal of TR1 to 270Ω and 330Ω respectively.

Railpower minimum speed problem

I have just built the Railpower from the October and Novem­ber 1999 issues of SILICON CHIP. It works as expected but I have a problem setting the minimum output voltage. When I rotate VR2 fully anticlockwise, the voltage on the track jumps up to 15V and you can’t get the voltage down. Even using the speed (-).

After a lot of investigation I found that if you turn VR2 clockwise a quarter of a turn and use the speed (-) the voltage will start to drop but I can’t get the minimum speed setting without the meter scale off (0V on the track and 40 on the meter scale). Can you suggest what is wrong? (S. S., via email).

We think you are winding the minimum speed trimpot (VR2) too far anticlockwise. Another way to adjust VR2 is to firstly set it at about mid setting. Then with the speed (-) button pressed, slowly adjust VR2 until the loco is just about to move. This procedure is as described on page 88 of the November 1999 issue.

Theremin has loose IC sockets

I just completed building the Theremin from the August 2000 issue of SILICON CHIP and was wondering if you could help me with a little troubleshooting advice. The kit is fully assembled and I have checked that all components are in the right place. I am using a Jaycar 12V power pack but get no sound. One thing I am concerned about is that the ICs don’t seem to clip together very well and are loose. Should these be soldered to the board? (L. C., via email).

The loose ICs should be corrected by either soldering them in place or using better sockets. If you have a power supply capable of 9-12VDC, try the Theremin operation with this or use a 9V battery. This will check whether the plugpack you are using is working.

Other tests would include checking the various power sup­plies around the circuit with respect to ground. Check for about 6V at pin 6 of IC3 and pin 8 of IC2. IC1 does not have a power supply pin (as it is a collection of transistors) but the various pins should have voltages on them. The drains of Q1-Q3 should be at about 5V.

Matching a Crane ignition to the HEI I just recently put together your High Energy Ignition pro­ject (June 1998). I also tried to install the Hall effect sensor to the dizzy to replace the points but was unable obtain the "chopper plate" as Bosch does not do them anymore. So I purchased a complete Crane Fireball Ignition system. It was simple to install and works very well. The only problem is that when I connected the output of this ignition model (which uses an optical sensor in the dizzy) to the HEI kit it does not work. The Crane unit says that it is compatible with HEIs and gives a ‘points type’ output. This output actually switches negative and the HEI kit needs a positive input. What can be done to fix this? (P. W., via email). The Crane system presumably requires some sort of power to be applied to a LED within the optical sensor via a resistor from the 12V supply. Have you connected this up correctly and does the opto sensor output also go high and low as the distributor ro­tates when connected to the HEI? If the coil fires correctly but the timing is out because it gives a low signal at the trigger point instead of a high signal, you could investigate moving the sensor within the dis­tributor so it does give a high-going signal at the correct firing point. We published diagrams for connection of optical pickups to the HEI in the "Circuit Notebook" pages of August 1998 and October 2000.

Soft-start circuit for a router

I have for a long time been dissatisfied with the jerk which occurs when I switch on my 1500W router. Recently, it became necessary to replace the switch which went open-circuit and I looked upon it as a chance to install a soft-start device to eliminate the jerk.

Obviously, the soft-start unit has to be electrically after the switch which means it has to be installed inside the router where there are serious physical space limitations. The nameplate current is 6.7A which is only marginally more than the 5A rating of your "Heavy Duty Drill Speed Controller" published in the September 1992 issue of SILICON CHIP.

However, the only available high current Triac I could obtain was the BTA41 specified therein. But then I came up against a blank wall insofar as I could not obtain any thermal resistance information on this Triac. I am wondering whether you were able to obtain this information when the your drill speed controller was designed as your heatsinking seems quite moderate.

I must add that the location within the router will mean that the Triac will be fan-cooled by quite a fair air blast. I have passed 6.6A of DC through the Triac and measured a 1.17V drop across it. This amounts to 7.72W dissipated and it rapidly becomes too hot to touch.

The available heatsink space is a black-anodised plate 51 x 81mm. I have not yet mounted the Triac on the plate but I will undertake further thermal testing when I have done that. Your comments would be appreciated. (R. B., via email).

You have just established the basic rule of thumb that dissipation in Triacs is a little over 1W per amp of current. You need a bigger heatsink. The Drill Speed Controller of Septem­ber 1992 is not suitable because it will not let your router run at full speed (about 80% at no load) and nor will it give a soft start.

You need our 10A Speed Control published in November 1997. This will let power tools run at full speed (as well as being variable over a wide range) and will also give a current-limited start which stops the kick from routers, circular saws etc. We can supply the November 1997 issue for $7.70 including postage.

Curing thumps in subwoofer amplifier I have recently built the Plastic Power amplifier module from the April 1996 issue and an active crossover for subwoofers and have coupled them together to make quite a powerful mono subwoofer system. The problem I am ex­periencing is speaker "turn-on thumps". I noticed Jaycar stocked a kit called a "Universal Loudspeaker Protector" (Cat. KC-5220) which eliminates turn-on thumps and so I bought the kit. I was wondering if you could tell me which wiring diagram I should be following in the instruction guide so as to achieve the result I’m after. I am not sure how to go about hooking up the unit to a mono mains amplifier such as mine. (L. B., via email). The loudspeaker protector you have was described in the April 1997 issue of Silicon Chip and should be assembled as per Fig.5 on page 58 of the same issue. Its power requirements can be obtained from the power amplifier positive supply rail. Connect the protector PC board power connections to the GND and positive supply of the power amplifier. Note that the supply resistor RY should be 220 5W. Since your amplifier is a mono unit, you need only use one side of the relay. Connect the amplifier output to the Amplifier 1 output connection on the PC board. The GND goes to amplifier GND and the Speaker 1 output on the PC board is for the speaker connection.

Excessive hash from the sine/square generator

I recently bought and built the Sine/Square Generator de­scribed in the February 2000 issue. I followed the layouts and instructions to the letter but the final result was pretty well unusable for my requirements of testing aspects of my home-built stereo system.

There was appalling audible hash and at some frequency set­tings really messy waveforms were produced. Inspection with a CRO showed that the output from the TL071 was dirtier than the input! This was traced to the -5V rail which had nearly 1V p-p hash on it. This was cured with a 47μF capacitor across the 0.1μF which is clearly inadequate on its own. The +5V rail has a 100μF by­pass; why not the -5V rail?

This mod basically turned the unit into a usable one, although further reduction of very high frequency hash requires some shielded cabling – which again should really be included in the kit. (M. S., via email).

The oscilloscope waveforms from Fig.3 to Fig.8 in the Febru­ary 2000 issue show that the output from our prototype is rela­tively clean from hash. Perchance you have a low-spec 79L05 regulator.

Having said that, we agree that there could be im­provements made by using shielded cable for the level and range controls and from the output to prevent pickup from the switching circuitry. This would make the circuit more suited to critical audio applications.

Combiner needed for two UHF antennas

I have recently moved to an area where television is all in the UHF region. I need two antennas pointing in different direc­tions. Is there a preferred method of joining the output from these two? Commercial joiners seem to be UHF + VHF not UHF + UHF. (T. S., via email).

You need a splitter/combiner. Normally used as a splitter, if you use it the other way, you can combine two signals into one, instead of splitting one signal into two. Get it? When you go to purchase your splitter, ensure that it can also be used as a combiner, eg, Altronics Cat L-1310 2-way.

Notes and Errata 12/24 Hour Giant Clock, March 2001: The 10μF capacitor on the overlay adjacent to ZD1 should be a 100μF as shown on the cir­cuit. Also the LDR should be a Jaycar RD-3480 not RD-3485. The description for easy daylight saving setting is incor­rect. Changing to daylight saving requires the hour switch to be pressed once to set it to the next hour. Returning to standard time requires the hour switch to be pressed until the previous hour is selected.

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.