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High mains voltage an equipment hazard

A mate of mine commented about the mains being slightly high at 257VAC. We really should look at a project to stabilise the AC or limit potential rising AC voltages.

I remember many years ago a device with two transformers connected back-to-back, ie, 240V : 24 : 24 : 240V and it had some sort of simple ferro-resonant 24V DC circuit which affected the 240VAC output. OK, so it was not rocket science and but it sure took care of the ups and downs. (R. B., via email).

• Ferro-resonant AC voltage regulators are still be best way to go, particularly as they also have the benefit of cleaning up the AC waveform to some extent. We are not sure whether you can still purchase such products but they would be very expensive. We have also seen AC voltage regulators that were based on servo-controlled Variacs. They work extremely well but they have a relatively slow response to sudden changes in the mains voltage. Again, such a product is likely to be very expensive.

Most equipment should be able to handle wide variations in the mains voltage but that is clearly not always the case which is why we have produced the Mains Moderator project featured on page 66 of this issue.

Monitor for lithium-ion battery wanted

I am looking for something to monitor a Li-ion battery and to disconnect the load if the battery falls below 10V. I did find the Universal Voltage Switch project (from SILICON CHIP’s Performance Electronics for Cars, 2004) but it required an additional 12V supply. I need something that operates off the monitored battery only. (M. B., Bedale, UK).

• The Universal Voltage Switch project is not really suitable for disconnecting a battery because it uses a relay to do the switching and this would tend to discharge the battery itself, even without considering the load the battery is connected to.

We have published two battery protectors in SILICON CHIP: first, the Micropower Battery Protector from July 2004 that can be set to switch off power from the battery at a preset voltage. This circuit draws 20µA. Second, the Battery Guardian from May 2002 which performs the same function but with larger batteries and larger loads. Its current drain is 2.5mA.

Ultrasonic Cleaner Makes “Snufferling” Noise

My Ultrasonic Cleaner project (SILICON CHIP, August 2010) does not work. No bubbles. The power LED is on. There is 5V between pins 1 & 8 and 5V on pin 4 which drops to zero when the button is pushed. The timer works and the running LED flickers a bit as it is mainly on.

The gate voltages are 1.3V and the Mosfets are hot. With an ambient temperature of 30°C, Q1 is 70°C and Q2 is 77°C. The tranny frame is 49°C.

My CRO is no good for high voltages, so I looked at the transformer primary between each drain and earth. The frequency in both modes is about 40MHz. The transducer makes a weak “snufferling” noise. Any suggestions, please. (P. L., via email).

• The Mosfets and transformer should not run hot. Check that the transformer is wound correctly and that the zener diodes and diodes are oriented correctly. The cleaner works best from a 12V lead-acid battery so that the transducer can be driven with maximum peak power.

If gate voltages are only reaching 1.3V this would prevent their turning on. Presumably this value is a multimeter DC measurement instead of an oscilloscope measurement. The frequency of operation should be between 20kHz and 40kHz rather than the quoted 40MHz.

Battery eliminator
for vintage radio

Have you ever described a battery eliminator for vintage battery sets? I have sets that need 1.5V and 90V. and another that requires 2V and 135V. At the moment, I am using a series of 9V batteries but for 135V, 15 batteries is a bit over the top. I would prefer a supply that runs off batteries; maybe a gel cell. (P. C., via email).

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