boost circuit

Hi,

I have a question concerning a boost circuit Vs a dc motor. here is a link to a boost circuit:

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In the step-up circuit, when Q1 is on, L1 is grounded. I feel that this circuit resembles a lot like an electric motor and its coil. The analogy is when Q1 is on, it would be like when the brushes makes contact with the coils. The duty cycles with the time the brushes has contact with the coils. I tried implementing this circuit to a dc motor pole and I was not getting any boosted voltage.The circuit was the same as shown in the link, but I did not add a load resistance, just a big capacitor. Does somebody knows why I am not getting a boosted voltage? There was not filter or smoothing capacitor on the poles of the motor. I used a small electric motor from a remote control car.

lili

Reply to
lilipot
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Why do you want to use a motor?

A motor is not a simple inductor. True it has an inductive component but if the motor turns, it has back emf which would negate any boost effect.

Secondly a motor is a low frequency device. It has high winding capacitance and a core that is not suitable for high frequencies. What frequency were you trying to run your boost circuit?

If you lock the rotor up so it can't turn and run the frequency no higher than a few hundred hertz, you should see some boost effect in the posted circuit. Of course the motor inductance will likely be too low for effective low frequency operation.

In other word, a motor is not an effective inductor for a boost circuit. Why do you want to use a motor?

Reply to
Bob Eld

These things work by storing energy in the coil when the pass transistor is on, and then dumping that energy into the output when the transistor is off. The amount of boost depends on how quickly you can turn off the transistor, since

V = L * I'

Where I' is the rate of change of current with respect to time.

So, the faster you can change the current across the inductor, the more voltage you will get (for a given inductor).

To get a boost effect, the V in the equation above must be greater than the output voltage plus the forward voltage of the diode.

So, to get 10V, for example, given a 1mH inductor, you would need to switch the current from 1A to 0 in 0.1ms.

So, the question is, how are you turning off Q1 above? Also, how much inductance does the motor coil have?

BTW, you would probably have more luck if you bought an inductor to play around with. They are pretty cheap.

Regards, Bob Monsen

Reply to
Bob Monsen

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