Hi everyone,

I am trying to design a circuit which will convert AC input to constant DC output.
I want uniform output voltage of 5V DC and 0.5 A of current. I have an input source as 12V AC f= 1Hz.

I know roughly that I need bridge rectifier to convert AC to DC, then use smoother to smooth DC pulses and a chopper to get rid of voltage ripple.

Diodes for the rectifier will be 1N4001.The problem is I dont know what type and what value to use for Resistors and Capacitors in the circuit (for smoother and chopper). So I was wondering if there is anyone with experience or knowledge in assigning values for the capacitors and resistors for my desired output from the whole circuit. I will attach schematic diagram (just with the rectifier and smoother/filter) which may be helpful for understanding the problem.

Hope i have got enough information to understand the problem if not pls feel free to ask.

Thank you.

RB

 

Hello,

12 Volts at 1 hz (what a low frequency).
You will need a large buffer capacitor for it.
(you did see the PDF I have shown to frisbee4all).
See this page of bill bowden on stabelizer circuits with a LM317.
http://ourworld.compuserve.com/homepages/Bill_Bowden/page12.htm

Greetings,
Bertus

 

Might be easier to use it to pulse charge a battery.

 

Hi Bertus,

Thank you for response. The reason for low frequency is because the voltage is coming out of a generator which is rotated using human motiont (walking). You are right, you do need a massive capacitor, when I did rough calculation, I found the capacitor should be of 0.1855 F (185,000 uF), not sure if that magnitude cap is available or not.

LM317 circuit design looks complicated to me, i was thinking to kee it simple with just a smoother and a buck/boost chopper.

I have a word file attached to this post where i have done my calculation if you may wish to have look at it

Thank you

 

Might be easier to use it to pulse charge a battery.

But shouldn't input voltage for a battery charger to be constant for it to work?

 

LM317 regulator design is really not very complicated; however it is certainly not the most efficient regulator available.

There is a minimum 1.7v drop between the input and the output. For a 5v output, the input must be at 6.7v or higher. Were I working the treadmill, I would not want that kind of energy wasted.

 

Is the input 12V RMS or 12V Peak-Peak?

If it is on a generator setup that you are building, could you use a smaller generator through a 30:1 to 60:1 gear train? That would provide about the same load, the generator would spin much faster, leaving you with high voltage/low current with out major dead times. This could then be put into a buck switching regulator to get 5V @ 500mA.

As is:
Could maybe use low drop diodes in the rectifier section, followed by a mid sized Cap/Inductor/Cap pi filter to feed a buck/boost switching supply at 5V. It would be very "iffy" for the period that the voltage goes under 5V (if it is 12V Peak-Peak, it probably won't work), the filter would supply the regulator.

As mentioned above, using a linear regulator is a nasty waste of power, so some sort of "SMPS on a Chip", just add inductors and caps, that would efficiently work if you had the large cap(s) after rectifier.

You can get 0.5-2 Farad/16V Capacitors used in mobile audio systems, but they are very spendy. The 2 Farad are about the size/weight of 2 Fosters beer cans stacked on top of one another. Probably not what you are looking for if it is a portable application, but you'd have smooth output.

Might also look at the "Ultra Capacitors", several of those in series/parallel would do it, but expense would again go way up.

 

How about using a switcher? The output capacitor problem (size) would go away.

 

Thankyou for responses everyone,

LM317 might give me a problem with voltage drop. I am already loosing 1.4 V from the diodes in the rectifier.

I think I should explain whole problem so it gives a good perspective in what I am looking for.

I have a rack and pinion set up which goes on the thigh. Up and down movement of the rack while walking rotates the DC brushless generator in one direction while moving up and another direction while moving down, effectively giving AC current over a cycle of walking (f = 1 Hz, 0.5s to move rack up and 0.5s to move rack down, rotatin the generator). With a specific generator (Maxon Motor) it was found that it would produce 12 V (peak to peak).

Now i want to use this voltage to charge a battery using a charging unit whose requirement is of uniform 5 VDC and 0.5A.

Problem: Voltage from the generator will be 12V AC (peak to peak) (f=1 Hz) has to converted to 5V DC 0.5A so it can be used to charge a battery(or mobile phone with incorporation of USB port) while walking, running.

Rack is driving the generator shaft directly without any gearing at the moment. Diodes i am thinking to use is 1N4001 which has 0.7 forward voltage drop.

[Device is similar to what this guy made http://news.bbc.co.uk/2/hi/technology/7226968.stm ]

Thank you.

 

Hello,

When you use scottky diodes to rectify, you will have less losses
(0.3 - 0.5 Volts per diode, depending on current).
For the stabelizer you could use the LM2940, this works from 1.25 Volt difference between input and output.
(see datasheet).

Greetings,
Bertus

 

Good choice with the Maxon motor!

From your diagram, it looks like a 2 phase motor. If you were to get a 3 phase, this would be MUCH easier, as there would only be a about 30 degrees of total rotation where the output wouldn't be capable of staying above ~3V.

Do you have a waveform of the output? Now that I understand the application, I am guessing the output would be closer to a sloppy square wave instead of a perfect sine wave.

A "slow rise/fall time" square wave would be much more efficient and easier to rectify than a true sinewave.