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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

6V operation for 12V DC-DC converter

Is it possible to adapt the 12V DC-DC Converter (SILICON CHIP, June 2003) to 6V operation. I guess that rules out using the standard Mosfet but could it work with a logic Mosfet or a Darlington? (D. S., Narrabeen, NSW).

• Mosfet Q1 would need to be a logic-level Mosfet and the available output power would be less than half that quoted for the 12V version. So it would be a nominal 15W DC-DC converter instead of 30W. Inductor L1 would need about 45 turns instead of 33 turns. Zener diode ZD1 should be 8.2V and the 1000µF low-ESR capacitors could be rated at 16V instead of 25V.

Heatsink insulators
for the 12V amplifier

With regard to the 12V Compact Stereo Amplifier (SILICON CHIP, May 2010), neither Jaycar nor Altronics have TO-218 insulating washers. Would it be OK to mount the IC direct to the heatsink and insulate the heatsink from the case? (P. C., via email).

• You could certainly do that. The TDA7377 tab is held at PCB ground potential so there will not be much radiation from the heatsink. The PCB ground is different from chassis ground though, which is why they need to be insulated from each other.

Keep in mind that the TO-3P outline is virtually the same size as TO-218 so those washers will work too. Jaycar sell TO-3P insulating washers made from silicone rubber (HP-1174) but they do not conduct heat as well as mica so the amplifier may run into thermal limiting sooner.

Altronics sell mica TO-3P insulator kits (H-7120).

Substitute transistors for Studio 350

I have the Studio 350 Amplifier (SILICON CHIP, January & February 2004) and need to replace the 2SA1084 PNP low-noise transistors. I can’t find them anywhere and was wondering what I can replace them with? (C. B., via email).

• You can use 2SA970s (used in the Ultra-LD power amplifier) if you can’t get 2SA1084s. They should be available from Altronics or www.futurlec.com Failing that, you could use BC556s although their noise figure is not quite as low.

How To Prevent Momentary Loss Of DVD Playback In A Car

I have two headrest DVD players in the car. The kids get in and start up their DVDs, then I get in and start the car. As the DVDs are connected to the accessories circuit, the power drops out and causes a chorus of “Daaaaddd!!!!!” from the rear of the car. the same happens when we are getting petrol.

I want a device that can keep the power flowing for 10 or so seconds while the car starts.

Can someone come up with a solution? I have not measured the current drain at this point; maybe a big capacitor or a small 12V rechargeable battery that recharges while the car is turned on and a time based cut-off to stop it draining.

Looking forward to a creative (low cost solution). (P. K., via email).

• In all likelihood you would need a big capacitor bank, fed by a power diode. Alternatively, you may need to run the DVDs from a separate battery fed by a power diode.

As a first attempt, we would suggest two 10,000µF 25V capacitors in parallel fed by a Schottky diode with a rating of 10A or more.

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