# How to reduce vampire power?

1. Nov 10, 2012

### oso0690

Hey guys, I've been building a circuit for a wireless cell phone charger. It's nearly completed but I don't know how to stop the circuit from drawing any power from its source when the phone is not 'connected'. It actually draws more power when it's not. How do I go about creating a circuit to fix this?

EDIT: Not vampire power. I need to know how to reduce no load power.

Last edited: Nov 10, 2012
2. Nov 10, 2012

### yungman

A few pieces of garlic!!!!:rofl:

Seriously, we don't know anything about your circuit!!!! you need to post your circuit so we can look at. Short of this, a few pieces of garlic might work just as well!!!!

3. Nov 10, 2012

### Averagesupernova

You want the charger to sense when the phone is placed on it correct?

4. Nov 10, 2012

### Mike_In_Plano

It seems to me that a timer would suit your needs. Spend most of your time in "stand by" with your excitation off, but the timer running. Then, upon wake up of the timer, actuate your excitation source and look for signs of back EMF from your load. If your BEMF detector spies a phone, then disable the timer.
Even if you come up only once a second to check, the hesitation will be unnoticeable to anyone not familiar with it.

5. Nov 11, 2012

### oso0690

Exactly.

Garlic solution is hilarious lol.

@Mike In Plano - I'll see how a timer works out. Even 1s checks would work much better than no checks at all!

Oh, here's a circuit schematic attached.

*BTW, does the simulator account for effective resistance in a coil?

#### Attached Files:

• ###### Circuit Schematic.png
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6. Nov 11, 2012

### skeptic2

What is your frequency of oscillation and how much is your charging current?

Last edited: Nov 11, 2012
7. Nov 11, 2012

### yungman

You are running open loop, with a set pulse width. You are pumping and pumping with a square wave with close to 50% duty cycle regardless of the load!!! The transformer or the transistor must be getting hot when nothing plug into it. You need a PWM with feedback so when you don't have a load, the pulse width becomes so low you don't draw power!!!

Look into Linear Technology site, they have small, cheap DC to DC converter IC that you can build a circuit from. They might not be 100% oscillation free, but for charging cell phone, don't worry about it.

BTW, You missed one very important thing, you need to reduce the current charging to protect the battery when it is full. I am not familiar with battery charging, someone here with experience can give you advice on this.

Last edited: Nov 11, 2012
8. Nov 13, 2012

### oso0690

Thanks for the reply, I looked into reducing the current charging when the battery is full. My team had assumed that the battery would not accept anymore current when it is full. I found some answers that seem to make sense here:
http://physics.stackexchange.com/qu...harged-battery-when-you-continue-to-charge-it

9. Nov 13, 2012

### oso0690

Frequency is 170 kHz and I'm charging at 100 - 200 mA