# Circuit for converting 120v - 30a AC to 96v DC at 30 amps

#### systemroot

Joined Jan 3, 2021
10
I plan on making a gasoline powered ebike... The gas generator is 3600w and makes 30 amps at 120v AC.
I plan on using a 3kv variac to convert that to 90 V AC. Then bridge rectified to DC using a 200v, 100 amp full bridge rectifier... Then using 2 (63 volts, 100,000uf ) capacitors in series, for a total filter of 50,000uf as they are in series and rating of 126volts... I heard that you got to multiply the 90 volts by the square root of 2 rms, so thats around 130 volts, which is the upper limit of the ebike controller... I just want a smooth DC voltage as long as if its between 90v and 130v (despite title saying 96v).
The problem is i need a pure DC source and my calculations on ripple for C = I / (2 x f x Vpp) , says C = 30 amps / (2 x 60hz x 0.1v ripple) = 2.5 Farads... That is a a big difference from the 50,000 uf... Any help / suggestions here. Thanks

#### sghioto

Joined Dec 31, 2017
5,388
I remember an old rule of thumb for a full wave bridge was 2000uf per amp needed. So 50/30 = 1667uf per amp, probably close enough.

#### systemroot

Joined Jan 3, 2021
10
I remember an old rule of thumb for a full wave bridge was 2000uf per amp needed. So 50/30 = 1667uf per amp, probably close enough.
how come with the capacitor formula it says different.... it says 10 volt ripple!!!

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#### sghioto

Joined Dec 31, 2017
5,388
The frequency is 120hz on a full wave rectifier.

#### BobTPH

Joined Jun 5, 2013
8,945
I plan on making a gasoline powered ebike...
Why? It is bound to be heavier, bulkier, and less efficient than a gas powered bike or a battery powered E-bike. What advantage do you see for your design that outweighs the disadvantages.

Bob

#### systemroot

Joined Jan 3, 2021
10
The frequency is 120hz on a full wave rectifier.
that would make it 5 volts ripple

#### systemroot

Joined Jan 3, 2021
10
Why? It is bound to be heavier, bulkier, and less efficient than a gas powered bike or a battery powered E-bike. What advantage do you see for your design that outweighs the disadvantages.

Bob
Laws... Ebikes have range problems... A ebike running off a gen is a ebike... A bike running of a gas engine is a motorcycle and requires license / registration / etc etc... Yet an ebike with a generator trailer is an ebike and follows ebike laws and has unlimited range... The unlimited range is a key thing as most ebikes at 40mph only go like 40 mile range tops which is not much... Also i looked at a online calculator for ebike wattage and speed and the extra weight only subtracts like 1 mph off of top speed... The generator is 100lbs and i weigh 130, so it would be the same as a heavy set 230lb guy riding my bike.

#### sghioto

Joined Dec 31, 2017
5,388
5 volts at max current. Less than 4% at 130vdc.

#### BobTPH

Joined Jun 5, 2013
8,945
I must say, I wasn’t expecting that!

As for the top speed, it will likely be far less on an uphill. Hope you don’t live around here, Worcester, MA, which, like Rome before it, is built on seven hills.

Bob

#### systemroot

Joined Jan 3, 2021
10
5 volts at max current. Less than 4% at 130vdc.
true but a switchmode supply would have what ripple ?? i need a clean waveform for my ebike controller

#### sghioto

Joined Dec 31, 2017
5,388

#### systemroot

Joined Jan 3, 2021
10
i feel like 100,000uf is the highest i can go. I am thinking of using switchmode supplies from aliexpress off the generator... Do you think switchmode supplies will run off a gen...

#### sghioto

Joined Dec 31, 2017
5,388
How clean is the output from the generator?⁸

#### systemroot

Joined Jan 3, 2021
10
How clean is the output from the generator?⁸
its a cheap 3500w non inverter generator...

#### sghioto

Joined Dec 31, 2017
5,388
Is the output a sinewave?

#### systemroot

Joined Jan 3, 2021
10
Is the output a sinewave?
i dont have a scope sorry... i am 18 and budgets are low. A youtuber had a scope and said it was ok, and im willing to risk it. My question is if i put a capacitor in parrallel to the mains AC, will that filter the sinewave for the switchmode supply... i maybe totally wrong here lol, i head that switching supplies have input filter capacitors, so i figure if i have some ones added parralel to mains, it will help out the supply...

#### sghioto

Joined Dec 31, 2017
5,388
You shouldn't need any additional filtering if it's a sinewave.
Can you post a link to the generator or the data sheet?

#### sghioto

Joined Dec 31, 2017
5,388
Looks OK. They mention operating a computer which normally has a switch mode power supply.
Did notice you would need to wire the variac to both 120 volt outlets.