Silicon Chip Online - Ask Silicon Chip

Query on adjustable voltage regulators

I was interested in a comment in the February 2007 article on the Remote Volume Control & Preamplifier Module. It states: "We’ve used adjustable regulators in this design because the ADJ terminals can be bypassed to ground to improve ripple rejection".

Don’t these bypass capacitors just restore the ripple rejection back to the same levels that the standard 78XX regulators have? The NSC datasheets seem to indicate that either circuit will have around -80dB ripple rejection at 100Hz.

Is this just a misconception that is being perpetuated (I’ve seen the same thing done in other designs) or is there a real performance reason to prefer adjustable regulators over fixed?

Also, could you further explain "two reverse-connected diodes (D7 & D10) across the output prevent their respective rails from being driven to the opposite polarity (eg, if a regulator fails)"? Did you mean "rectifier"?
(S. J., via email).

You are quite right about the ripple rejection figure. Once the 10μF bypass capacitors are installed, the ripple rating of the adjustable regulators is essentially the same as the fixed output devices.

Our text wrongly implies that we used the LM317 & LM337 adjustable regulators purely for the ability to decouple the ADJ pin. We neglected to mention that the adjustable devices have a tenfold performance benefit (according to National Semiconductor) over the fixed devices in line and load performance. They also feature improved overload protection.

Additionally, we envisioned that these supplies would be useful in a variety of projects, hence the benefit of adjustable outputs.

Regarding the reverse-connected diodes, it is indeed possible for one rail to be driven to the opposite polarity due to a failure in the powered device (not in the regulators, as stated). In this case, the diodes protect the regulators by clamping their outputs to one diode drop below (or above) ground.

Capacitors for valve preamplifier

I am currently replacing the capacitors in the valve preamp for hifi use (SILICON CHIP, February 2004) but I don’t seem to be able to find a 680nF 630V capacitor from either Jaycar or Altronics. Would a 470nF 630V unit be OK to use instead? (R. C., Doncaster, Vic).

It would be OK to use a 470nF 630V capacitor instead of the specified 680nF. The only difference would be a slight reduction in the far-bass response which probably will not be noticeable.

If this does worry you, you could connect a 220nF 630V capacitor in parallel with the 470nF unit, to give a total of 690nF.

Killing that sound blast from TV commercials

Have you ever published a circuit to limit the surge in sound when TV programs go to commercials? If not, do you have any suggestions? (S. K., via email

A number of schemes have been devised over the years to combat the rise in audio level when adverts are on. One commercial system sensed the video "fade to black" at the start and end of commercials to turn off the audio but the TV networks have seen around that. We don’t know of any more effective method than killing the sound with your remote control or using a PVR to do time-shift.

Transformer for 110V iron

Some time ago while in Canada I bought a 110V iron to use for ski waxing. The iron is about 1kΩ and a transformer that would be suitable would be quite expensive. Have you ever published a project which would allow such a high-powered appliance to safely operate from 240VAC? (I. L., via email).

While an electronic circuit such as a zero voltage switching power controller (see our Heat Controller from the July 1998 issue) could reduce the RMS voltage from 240V to 110V to suit your iron, it would not be a safe option as 240VAC would still be applied to the heating element and it may not be designed to take that voltage stress.

The only alternative approach is to use an auto-transformer or a fully isolating step-down transformer. Both will be relatively expensive. They are available from Harbuch Electronics in Hornsby (Sydney). Phone (02) 9476 5854.

Moving coil RIAA preamplifier

You recently published a moving coil variant of your Magnetic Cartridge Preamplifier in the Circuit Notebook pages (March 2007 issue). I would like to build this but what is its gain?

Is there enough gain, say +62dB, to raise the output of my Ortofon cartridge (with an output of 200μV) to line level, say 250μV? (T. R. Woodend, Vic).

The gain for the moving coil RIAA preamplifier shown in Circuit Notebook in March 2007 is +55dB (x 560) at 1kHz. Gain for the second stage is up to x11 or +20.8dB, so a 200μV signal can be boosted to 1.2V with VR1 set at maximum. The minimum setting for VR1 would provide 112mV output for a 200μV input. Therefore the gain should more than enough for your Ortofon cartridge.

Remote switching with the SMS controller

I recently built the SMS Controller which was published in SILICON CHIP in 2004. I also built several Remote Relay Switches, published in May 2006. I intend to use the controller to switch lights as well as other appliances on and off on a property on the Gold Coast.

I have tested the Remote Relay Switch with the SMS controller for short periods of time and it works quite well. I propose to connect three Relay Switches to the controller. The Relay Switches may have to be energised for extended periods of time (several days).

Do you see any problems with excessive loads on the switching integrated circuit? I have soldered it directly to the PC board. Or would you advise me to use the relay switch published in the November 2006 issue instead? (C. N., via email).

The Remote Relay Switch from the May 2006 issue would be a good choice. It uses a relay with a 200-ohm coil, so up to eight units could be driven from the SMS controller outputs without any problems, particularly as you’ve soldered the ULN2803 chip directly to the PC board.

The DC Relay Switch described in the November 2006 issue is not suitable, as it cannot be used to switch 240VAC.

Can Radar Speed Gun measure cricket balls?

My three grandsons, being extremely keen cricketers, want me to build the recent Radar Speed Gun described in the November 2006 issue. Could you proffer an opinion if it could be used by their cricket club trainer to measure their bowling speed? (D. C., Bribie Island, Qld).

The Radar Speed Gun could probably be used to measure the speed of cricket balls but you may need to spray the balls with metallic paint, so they will reflect enough of the microwave radiation to give a reading.

Controlling audio level when burning CDs

I burn LPs onto CD using my hifi record player and using the output signal from the headphone jack. This works well. However I cannot keep the volume the same between copies as my hifi volume control is by pushbutton and not by potentiometer.

Can I use the kit in the March 2007 issue, called the Universal Infrared Remote Volume Control & Preamp, to do the job? I would like to choose my volume and see exactly how high or low the headphone output is. (T. Z., via email).

The Universal Infrared Remote Volume Control & Preamp is not suitable, in part because it does not display its input signal amplitude. There are a number of ways to address this problem. If you’re happy with the quality of the recordings that you’ve made thus far, then it may be possible to retrofit a volume control potentiometer and VU meter to your record player.

An easier method would be to monitor and control the signal level using Windows-based recording software. Additionally, you can make use of the special LP noise filters built into many of these packages.

Check out our feature on transferring LPs to CDs in the September 2006 issue for details.

Power supply for Mighty Midget amplifier

I have built the Mighty Midget amplifier (SILICON CHIP, March 2002) and need some advice. Instead of lugging the family car battery with me, what sort of power supply would I require? (B. W., Toukley, NSW).

Given that the Mighty Midget can deliver up to 70 watts on peaks, the supply would need to deliver about 14V at 10A, which is a very big supply. The alternative is to use the same power supply arrangement as we used in the PortaPAL PA system featured in the February and March 2003 issues. In essence, this used a 12V SLA battery with a float charger.

Electronic bell circuit wanted

I am looking for an electronic bell circuit. The one I have in mind is similar to the types used in school amplifiers for end-of-period, etc. I have hunted the "web" without finding anything remotely suitable. Has SILICON CHIP ever done such a project? (R. J., Kangaroo Flat, Vic).

We published a DingDong doorbell project in the May 1992 issue.

Fuel Mixture Display shows overload

I purchased a Fuel Mixture Display from Jaycar. I have put the kit together and have one problem that I cannot figure out. I cannot set the span. It always goes to OL.

Can you provide some guidance? (B. A., Oregon, Illinois, USA).

Check that you are getting -2.49V across REF1. VR1, VR2 & VR3 are different values and must be placed in the positions shown. VR1 should have a 504 code, VR2, a 254 code and VR3 a 203 code.

The span problem could be because the offset is not set or VR2 is the incorrect value. Make sure the other resistors on the PC board are the correct values, in particular the 180kΩ & 100kΩ values associated with IC2a.

NiMh battery charger

I recently purchased the Cordless Power Charger Control Kit (SILICON CHIP, December 2006) with a mind to adapting it to charge 10 AA 2400mAh NiMH batteries connected in series. This raises a number of questions.

First, what voltage and current ratings would I require on my plugpack to recharge the 10 cells, assuming, say, a 5-hour charge time?

Second, what additional circuitry would I need to ensure that the current going to the cells is limited. I gather from the article that most power tool battery packs contain a current-limiting resistor. This being the case, could I simply introduce a current limiting resistor in series with the 10 AA cells to be charged? If so, how would I go about calculating the required value for this resistor?(L. S., via email).

The charge rate for a 5-hour charge for the 2.4Ah battery would be 670μ A. So you would need a 1A DC plugpack at 15V or 18V.

For 15V you would need a 3.9Ω 5W resistor. For an 18V plugpack use an 8.2Ω 10W resistor. The value may require some adjustment because the charge depends on the actual plugpack voltage and the voltage that the battery reaches during charge.

Porsche Takes Off In Second Gear

I am the happy (almost) owner of a 1985 928 Porsche. This vehicle is fitted with a Mercedes Benz 4-speed auto transmission and it is normal for this transmission to take off from rest in second gear (a strange Teutonic quirk). The only way to make it take off in 1st gear is to mash your foot to the firewall. This closes the kick down switch and powers up a solenoid. Unfortunately, this results in much protestation from the rear tyres.

In the interests of fuel consumption and driveability, I would like it to take off in 1st gear. I have fitted a manual switch in parallel across the kick-down switch but this has become tiresome in traffic. I want to have the kick-down switch automatically close after the car comes to a complete stop and to open after the car has moved away but with the opening point (speed) variable according to the throttle opening.

Is there a kit or some other way I can achieve this? The vehicle is standard with an electronic speedo and fuel injection so picking up a road speed pulse and variable voltage from the throttle position sensor would not be a problem. The auto transmission is a fully hydraulic unit apart from the electric kick down system. (R. A., via email).

You could use the Frequency Switch this month on page 82. Use it to close a relay at the stopped (or almost stopped setting) for the kick down and release the relay at a higher speed, as set by the hysteresis setting.

Hesitation In High-Energy Ignition

I built one of your High Energy Ignition systems, as featured in the December 2005 & January 2006 issues. It has been installed in a 1996 6-cylinder Falcon.

I built the engine management version but rather than take a signal from the distributor output before any conditioning by the vehicle’s management system, I chose to take the signal from the coil primary connection, so that any timing changes made by the ECU will still have the same effect but will also be processed through the High-Energy Ignition.

I fitted the 100Ω pull-up resistor as per the points version of the ignition and the vehicle runs very well. The benefit is immediately noticeable. However, at the initial moment the accelerator pedal is pressed, there is a split-second hesitation – the motor stutters then away it goes. Other than this moment of indecision, the motor runs wonderfully.

I was wondering if the value of the pull-up resistor is causing my problem. The coil’s primary resistance is around 0.5W so the car’s management is not seeing the normal load. Do you think this might be the reason I am seeing the hesitation or could you suggest an alternative approach?

Also, if I were to fit a resistor of around 0.5Ω in place of the 100Ω unit, would the current drain have any impact on the High-Energy Ignition itself? (G. M., Endeavour Hills, Vic).

A 100Ω resistor in place of the coil should be fine. You may need to slightly increase the dwell period so that the coil is ready to fire with sudden demands such as acceleration at low RPM. This should stop the hesitation.

What we think is happening is that at idle the dwell period for the coil is correct, as the engine is running at a relatively constant RPM. But as soon as you put your foot down on the throttle, the engine load changes and the firing point (or timing) is changed.

This change can reduce the dwell period momentarily as the microcontroller readjusts its calculations. As far as the microcontroller is concerned, the engine has had a sudden RPM change.

This loss in dwell can prevent the coil from delivering its spark and so the engine hesitates.

Making The Active Crossover A 2-Way System

I was most interested in the article "An Active 3-Way Crossover For Loudspeaker Systems" which was published in the January 2003 issue of SILICON CHIP. I subsequently purchased the kit from Jaycar and it sat around for a year or two before I began work on the project.

Although I had initially planned for a 3-way system, I’ve since settled on a 2-way loudspeaker system and am therefore keen to know whether it would be easy to modify the kit or bypass part of the circuitry to allow the crossover to be used on a 2-way system.(L. D., Perth, W.A.).

A 2-way crossover can be implemented if you omit the bandpass filter and just use the low-pass and high-pass filters. The crossover frequency would then be set to the same value for both the low and high-pass filters. The low-pass filter is for the woofer and the high-pass filter is for the tweeter.

The unused bandpass filter circuitry includes op amps IC3c, IC3b, IC5d & IC5c in the left channel and op amps IC4c, IC4b, IC6d & IC6cd in the right channel.

We recommend that you install the components for these op amps (use the default value 47nF capacitors and the 10kΩ and 20kOmega; resistors) to prevent them from oscillating.

WARNING!

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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