Herve asked in Science & MathematicsEngineering · 9 years ago

# I want to use a transistor as a switch. The unit is protected by 8 Amp fuse @120VAC. The Load in the unit?

I want to use a transistor as a switch. The unit is protected by 8 Amp fuse @120VAC. The Load in the unit is turned on using a SPST switch @ 120VAC. I have the circuit working with one 12VDC @ 130mA fan. The circuits i need are 12VDC. I have two power transformers input w/120VAC the output 12.6vac @300mA rectified to 12.6VDC out for the two circuits power source . With the supply voltage of the circuits @ 12.6VDC i want to turn on a 12VDC @ 130mA fan and a DPDT coil relay 12VDC @ 75mA. If the fan fails and produces an open in the primary fan circuit i want to use a 75mA coil DPDT relay to drop off closing the normally open circuit and closing the relay on a backup circuit that contains a 12vdc 130ma fan and also in this circuit a 12VDC @ 20mA LED assembly to turn on indicating the primary fan has failed and the secondary fan is running. These components were selected for their physical size allowing them to fit in the units confined housing compartment.

I have 1-1/2 years of college in electronics from 1983. Just eneough to get me in quiet a fix if you know what I mean!

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• 9 years ago

I have trouble understanding this, perhaps you can explain it better? My reading is that you have two 12 volt fans, and if one fails, you want the other one to start up. Is that correct?

But how are you going to sense the fain failure. You seem to be saying you have a 12 volt relay coil is series with the fan, but that won't work, as both the fan and the relay will have less than 12 volts, they will divide the 12 volts between them.

And why a transistor switch if you have a relay?

You can sense the current into the fan via a series resistor, and an opamp or maybe a transistor can sense that and turn on the other fan. Not too difficult, email me.

But fan's don't fail in that mode, although I have little experience with fans. They tend to have a shorted winding and start to smoke, or perhaps get clogged with dust and have a restricted air flow and increased current.

You might want to consider sensing temperature and using that as the warning light.

High reliability fans and switchover are old technology. Did you research what has already been done in that field.

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• 9 years ago

Buy a contactor and reduce the labor intensity considerably. What your looking for is a complex solution where a much simpler one exist.