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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 or send an email to silicon@siliconchip.com.au

Recharging Apple USB devices

I can’t quite fathom how a USB port can provide the biasing conditions for a variety of devices, assuming that these iPODs can be charged from a normal PC USB port. I have an Apple Shuffle and a Dick Smith MP3 player. How do I know what biasing is required? (E. J., Otago, Tas).

• The reason that biasing is required for iPods to charge is that when the iPod is plugged into a computer, it communicates with the computer and checks that it can draw 500mA first. According to the USB standard, unconfigured devices are not allowed to draw more than 100mA.

Since there is no computer or microcontroller in the USB charger for Cars & Bikes (SILICON CHIP, May 2010) an iPod can not communicate with it and so can not request the full 500mA. The biasing resistors tell the device that it is connected to a charger and it’s allowed to draw more power without having to talk to a computer.

This is a common problem and unfortunately each manufacturer has addressed it in a different way (and in the case of Apple, a different solution for different devices).

The USB3 standard provides a way to make third-party chargers work with any USB device but unfortunately few devices have adopted it yet.

You may find that the Dick Smith MP3 player will charge without any biasing. Some devices skip the configuration process and just draw power from the port.

We don’t know what bias voltages an iPod Shuffle needs but if it doesn’t charge with D+ and D- unconnected then try shorting those lines together (with a solder bridge). That is the new USB3 method for detecting a charger.

If that doesn’t work, try the scheme in Fig.8 on page 72 of the May 2010 issue. One of these two approaches should do the trick. At least one report on the internet suggests that this scheme (or one very similar to it) works with 2nd generation iPod shuffles. However we have not tried it so we can’t guarantee that it will work.

Soft-starter wanted for switchmode supplies

Have you produced an article or project for “soft-start” switching 230VAC to transformers and computers? I have a problem occasionally when switching on the power to the power board that supplies the above equipment. It doesn’t happen every time but sometimes I see a spark coming from the power point switch at switch on. The total wattage of the devices connected to the power board is only about 450W, so they should be within the 10A rated capacity of the power point switch.

My feeling is that there is an inductive and capacitive load from the conventional wirewound type trans­former/s that power the modem etc that’s causing this. The power point has been replaced with a new one and I am still experiencing this problem which will eventually burn out the switch contacts again.

What is needed is a “soft start zero current” device to place in front of the power board and after the power point that will eliminate the sudden rush of current at switch on. If such a device has not been produced as a project it could be considered for a future project for your magazine as I am unable to buy anything commercially from electrical suppliers that will give a “soft start” at switch on. Any suggestions that you could offer would be appreciated in solving my dilemma.(B. S., Warners, Bay, NSW).

• Your dilemma is very common. In fact, we think that virtually every household has a similar situation in which the in-rush current to electronic appliances can not only burn out the switch contacts in typical power points but depending on the number of appliances being switched, can also lead to nuisance tripping of circuit breakers in switchboards.

In fact, we have a similar scenario in the SILICON CHIP offices where some desks have a typical desktop computer driving two monitors and other ancillary equipment, all with switchmode power supplies. Just as in your situation, the total rated power of the appliances is probably quite modest at maybe less than 400W but the initial inrush current over the first few cycles of the 50Hz 230VAC waveform can be very large, perhaps 30A or more.

One way to reduce the problem is to avoid switching all the appliances on simultaneously at the power point. Switch on the computer first, followed by the first video monitor and so on.

But we realise that few people will want to go to that much trouble. In fact, we have just such a soft-start module in the pipeline and it should be published within the next few months.

Increasing door strike time in RFID module

I built the RFID Security Module (SILICON CHIP, June 2004) which was designed by Peter Smith. How can I change the time the door strike output stays on for (ie, when a valid tag is presented) from a few seconds to a few minutes? (J. H., Bankstown, NSW).

• The door strike on-period can be extended by changing transistor Q3 to an IRF540N N-channel Mosfet. The Mosfet is installed with the same orientation as the transistor, with the Mosfet’s gate in place of the transistor’s base and the drain and source in place of the collector and emitter.

The 1kΩ resistor at pin 9 of IC1 should be lifted and a 1N4148 diode placed in series with this 1kΩ resistor, with the anode at the pin 9 end (the anode end is the end opposite the stripe [K] end of the diode). A 10MΩ resistor should then be connected between the Mosfet’s gate and source.

In addition, a 220µF capacitor must

be connected between the gate and source (negative to the source).

In this way when the door strike is to be activated, the high output at pin 9 of IC1 would charge the 220µF capacitor via the diode and 1kΩ resistor. This would switch on the Mosfet to power the door strike. When the output at pin 9 of IC1 goes low, the diode is reverse biased and the capacitor discharges via the 10MΩ resistor. When the voltage across the capacitor drops, the Mosfet switches off.

The extra time would be 220 seconds or about three minutes. Change the capacitor value to set the time.

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