# PCB Heaven

## General Category => Maths, Geometry, Physics and others => Topic started by: D-the-Greek on December 22, 2011, 14:41:40 PM

Title: Voltage Doubler
Post by: D-the-Greek on December 22, 2011, 14:41:40 PM
Ok,

Most people know what voltage doubler is. I have a question. I have a DC bicycle wheel (MagicPie) and i want to make a electic bike. At the moment i am busy with the steering system so i still have long way to go.

The wheel is a 24-36-48 volt so i was wondering if i can use a voltage doubler (or tripler) from 24 to 48 (or 12 to 36) volt in order to feed the wheel. Will it be ok or is that dangerous for the electronics in the wheel ?

http://www.goldenmotor.com/

Title: Re: Voltage Doubler
Post by: kam on December 22, 2011, 15:08:00 PM
the schematic you posted is actually a voltage multiplier. The best is to use an inverter or other DC-DC converter topology because it can deliver enough current for the motor. The voltage multiplier with capacitors cannot provide enough current because the capacitors do not store enough energy.
Title: Re: Voltage Doubler
Post by: D-the-Greek on December 22, 2011, 15:53:34 PM
Well there are 2 plans on doing this.

1) an alternator will be used(with voltage doubler) to charge the battery and the battery will supply the electric motor.
2) a dc dynamo will charge the battery and the battery will supply the electric motor.

I still have time to deside what will i choose to go on. The efficiency/price of the circuit will be put under serious consideration.
Title: Re: Voltage Doubler
Post by: D-the-Greek on December 22, 2011, 15:57:24 PM
The resson for doing this is weight... 1 12 volt 10A battery weighs less than 3 or 4 battery's...
Title: Re: Voltage Doubler
Post by: kam on December 23, 2011, 08:37:04 AM
if weight is the only parameter, then i think that you should go for 2 batteries instead. One 12v 12Ah lead acid (that i see the specs right now) weights about 3.9Kg, while one 12V 7Ah weights 2.7. So you may wanna use 2x 7Ah 12V in series and get 24V 7Ah for  5.4Kg, 600 grams more than the 12Ah 12V. I think his is a fair trade, plus you save the extra cost for the circuit. Not to mention that you will be needing a powerful transformer for your dc-dc converter which means extra weight.  Plus, the efficiency of 2 batteries in series is much better than using a DC-DC converter.

And something more about efficiency. Power loses due to heat are a result of the current and not the voltage:

P=I2xR

Which means that, if you need to draw 48watts for the motor, if the power source is 24V, the current will be I=P/V = 48/24 = 2A
But if the power source is 12, the current will be 48/12 = 4A

So, the heat losses on the wires for 12V battery are much higher than 24V.

Title: Re: Voltage Doubler
Post by: _pike on January 01, 2012, 15:28:14 PM
Dimitri what are the specs for the motor that you will use?
Title: Re: Voltage Doubler
Post by: D-the-Greek on January 12, 2012, 00:50:39 AM

http://www.devi-motion.com/webshop/bldc-hub-motor/p-1/D1000202.html

Title: Re: Voltage Doubler
Post by: kam on January 12, 2012, 07:55:49 AM
wow! that's big... 8)
2 batteries definitely. You may get 1Hp out of it!  8)
Title: Re: Voltage Doubler
Post by: D-the-Greek on January 12, 2012, 23:50:14 PM
:) thank you...i dont remember but there is a Ampere maximum.

( kam...i have 2 of them ) .... im also thinking to use 1 for helping with biking and the other to recharge the battery, that way i can get more kilometers out of it(and more hours using it)
Title: Re: Voltage Doubler
Post by: D-the-Greek on January 12, 2012, 23:52:12 PM
why im not using both for power ? because here in holland more that 250W electric wheels needs to have license and aproeval for road use/safety etc...
Title: Re: Voltage Doubler
Post by: kam on January 13, 2012, 10:33:56 AM
why im not using both for power ? because here in holland more that 250W electric wheels needs to have license and aproeval for road use/safety etc...
ridiculous law.  :( :( :(

anyway, no matter what, a 250watts inverter will be heavy (maybe not as heavy as a battery) and expensive. In my opinion, it is much simpler to use more batteries whenever possible. Not to mention the efficiency... No matter what, the voltage doubler circuit that you proposed is incapable to provide the required wattage. You will need a DC-DC inverter which is not that simple to make.
Title: Re: Voltage Doubler
Post by: D-the-Greek on January 13, 2012, 18:42:39 PM
The law says 250W because the maximum klm/h must not exeed 25 klm/h. If a 2 or more wheels vehicle runs more than 25 klm/h then it is not allowed to use the bikers road and must then use the cars road. And to use the car road you must go faster than 25 klm/h.
Many crazy laws here i know...
Title: Re: Voltage Doubler
Post by: kam on January 13, 2012, 19:16:36 PM
can you "bend" the law? (not break it). can you for example add a transistor that will provide either 250W or a full KWatt with a hidden switch??? This is absolutely what i would do...