Sunday, January 9, 2011

CrazyEngineers Forum - Electrical & Electronics Engineering

CrazyEngineers Forum - Electrical & Electronics Engineering

Voltage comparator with variable voltage input. How?!

Posted: 08 Jan 2011 12:49 PM PST

Hullo,
I think that the subject I chose is not the most appropriate but I couldn't find a better one.
Anyway, I am designing my first "serious" circuit. I will use it to manage the power from a bicycle dynamo (permanent magnet generator?). I want it to switch from the dynamo to a backup battery when the output of the dynamo is not sufficient to drive a high power led.
First of all, I will use a full wave bridge to get a DC output.
Then, I was thinking of using a relay followed by a comparator with a fixed voltage reference.
To help you help me,
At 8km/h no load the output voltage is 15V.
At 8km/h with load the output voltage is 8V.
At 12km/h no load the output voltage is 17V.
At 12km/h with load the output voltage is 10V.

Let's say I set the threshold level of the comparator to 10V.
Under 10V, the battery is employed.

The problem:

The bicycle is still (0km/h)
[The comparator sees 0V from the dynamo and switches to the battery.
No problem here.]

The bicycle accelerates to e.g 8km/h and maintains this speed
[The dynamo has no load and its voltage is 15V, the comparator+relay switches to the dynamo. Now the dynamo is under load and its voltage drops to 8V, the comparator+relay now switches to the battery. The dynamo has no load and its voltage rises to 15V, the comparator+relay switches to the dynamo.
That is the problem I am facing. If my logic is correct, this oscillation will keep happening as long as my speed is below 12km/h.]

The bicycle accelerates to e.g 15km/h and maintains this speed
[The dynamo's output is above the threshold level, the comparator+relay switches to the dynamo. The dynamo's output WITH load is still above the threshold level.
No problem here.]

The bicycle decelerates to 8km/h and maintains this speed
[Like before, the same problem with the oscillation occurs.]

The bicycle decelerates to 0km/h and stays still
[The comparator sees 0V from the dynamo and switches to the battery.
No problem here.]

In conclusion, the problem is when the bicycle's speed is 0km/h<U<12km/h.
What can I do to overcome this?
I hope that my question is clear because english is not my native language.
If something is not clear or wrong please point it out.
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