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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

WIB regulator
gets hot

I recently put together the Web Server In A Box (WIB, SILICON CHIP, November & December 2009, January 2010) and I am very happy with its versatility. I constructed the kit as per the instructions as I always do and tested it according the instructions. Everything worked straight away, which doesn’t always happen.

My question is regarding the LM317; it runs very hot to the touch. I understand they do get hot and that’s the reason for the heatsink. I am just thinking of the small enclosure and the heat the device produces.

The kit is drawing 200mA but the input voltage is more than 9V from both of the two plugpacks I have tried. One was 11V and the other 14V. From what I understood of the LM317 it can accept an input voltage higher than I am using. Any help would be appreciated. (B. S., via email).

• While we did specify an input voltage from 6-9V, that does not necessarily mean that you should use a 9V plugpack. As you have found, a 9V plugpack typically delivers a lot more than 9V and so the regulator will run hot (when using a 9V plugpack). The best approach to reduce the heat generated in the regulator is to use a 6V plugpack. These will typically deliver around 8-9V. The lower the input voltage, the less heat will be generated.

Speed control
for water pump

I wish to remove the engine-driven water pump on my car and replace it with an electric pump. I need a controller for the pump motor and have been looking at the High Power Reversible DC Motor Speed Controller in the August 2010 issue of your magazine.

A question I need to ask is this: can the speed set-point be derived from a temperature-related source? In particular, can I delete VR1 and use two thermistors in series such that a PTC one connected to an NTC one to give a facsimile of a potentiometer with the characteristic that the common or centre tap resistance moves like a pot and is related to temperature?

I’m thinking that this will be easier than using a thermocouple. For the pump I have, the motor speed will need to be 20% when cold (0-20°C) and rise linearly to 100% at 82°C and above.

I’ve not yet looked at any one type of thermistor to use. If you can recommend something I would appreciate that a lot. I have had experience in replacing the pellet in OEM temperature sensors, so packaging a thermistor is no problem here. (M. W., via email).

• The August 2010 controller is not suitable since the circuit relies on the resistance of VR1, the speed potentiometer, remaining at 5kΩ in total. A thermistor and fixed resistor or two thermistors (NTC and PTC) will not remain at 5kΩ over a temperature range. Nor will their total resistance will be linear with temperature.

The High-Current Motor Speed Controller from our June 1997 issue would be more suitable since the speed is voltage controlled. It is sold by Jaycar Electronics (Cat. KC-5225).

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