Author Topic: Converting Ac to Dc current.  (Read 10603 times)

Offline benchmark

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Converting Ac to Dc current.
« on: January 18, 2011, 05:49:14 PM »
Pardon me as my Physics is not at sharp as it used to be.
Probem is tat i ae a small stepper motor i want to use as a generatr run on one f my model steam engines.
The motor as 4 wires leading out of it , 2 pairs each that generate up to 75 vots Ac current each when i test run it.

Now, apart from me aking a diode bridge rectifier to achieve this, do you guys know any place or link to any small ready made cuircut that can serve?

Secodly what would be the best configuration of connecting the 4 wires be to get maximum perfomance.
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Offline BillTodd

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Re: Converting Ac to Dc current.
« Reply #1 on: January 18, 2011, 07:10:36 PM »
Just use two small bridge rectifiers like this: (it's a very common component so you should be able to find one locally)

http://www.rapidonline.com/Electronic-Components/Discrete-Semiconductors/Bridge-Rectifier-Diodes/1.5A-Bridge-rectifiers/29687/kw/bridge+rectifier



connect both AC inputs (marked ~) of one rectifier to the wires of one coil and AC inputs of the other rectifier to the other coil. Then connect the two positives (+) together and the negatives (-) together as the outputs.

The output voltage could potentially be quite high when unloaded (75vac will generate 75 * 1.414 = 106vdc)  ,  so be careful what you connect it to.

Bill



« Last Edit: January 18, 2011, 07:12:23 PM by BillTodd »
Bill

Offline John Swift

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Re: Converting Ac to Dc current.
« Reply #2 on: January 18, 2011, 07:28:02 PM »
Hi Benchmark ,

with the bipolar motor you can connect a bridge rectifier to each winding

then either parallel the DC outputs to give the maximum current out


or if you smooth the DC with a capacitor across each bridge rectifier
wire the two in series to give a higher voltage

if you had a unipolar motor you could have used just two diodes per phase


 John
« Last Edit: January 18, 2011, 07:31:18 PM by John Swift »

Offline benchmark

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Re: Converting Ac to Dc current.
« Reply #3 on: January 19, 2011, 05:01:03 PM »
Chhers guys, i like those small bridge rectifiers, i guess the 1.5A 200V ones will do?

However , following the connection arrangement you suggest 106 Volts DC is  just too much as i will only be lighting up small bulds and LEDS , any other suggestion for a connection that gives about 50 volts?
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Offline picclock

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Re: Converting Ac to Dc current.
« Reply #4 on: January 19, 2011, 05:18:35 PM »
Hi Benchmark

If you connect the windings in series, getting the phase right to give you 150 volts you can connect that to a redundant mobile phone charger (hide it in the base). They will normally run from 100 - 240v AC and they are not fussy about frequency. The output of these is normally 4-5 volts, just right for running some LED's (via a resister  :zap:) and small bulb's from. If you remove the charger case the electronics are quite small. the largest thing about them is the mains connector, just remove that and solder your generator wires to the connections.

Good luck and show us the pictures.

picclock
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Offline benchmark

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Re: Converting Ac to Dc current.
« Reply #5 on: January 19, 2011, 05:29:49 PM »
Hi Benchmark

If you connect the windings in series, getting the phase right to give you 150 volts you can connect that to a redundant mobile phone charger (hide it in the base). They will normally run from 100 - 240v AC and they are not fussy about frequency. The output of these is normally 4-5 volts, just right for running some LED's (via a resister  :zap:) and small bulb's from. If you remove the charger case the electronics are quite small. the largest thing about them is the mains connector, just remove that and solder your generator wires to the connections.

Good luck and show us the pictures.

picclock

Thanks for the response but the problem with this will be that the gen wont make any current when the engine is running slow since the phone charger need minimum of 100 volts to work.

What if i just connected the resistor in series in a Half-wave rectification  with a single diode in a one-phase supply. Like this



how many volts will i get DC? and con someone please explain to me what 'R' stands for in the diagram above? finally what diode can i use for this, the same bridge rectifier diode linked by BillTodd?
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Offline BillTodd

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Re: Converting Ac to Dc current.
« Reply #6 on: January 20, 2011, 07:42:55 AM »
Hi Benchmark

If you connect the windings in series, getting the phase right to give you 150 volts you can connect that to a redundant mobile phone charger (hide it in the base). They will normally run from 100 - 240v AC and they are not fussy about frequency. The output of these is normally 4-5 volts, just right for running some LED's (via a resister  :zap:) and small bulb's from. If you remove the charger case the electronics are quite small. the largest thing about them is the mains connector, just remove that and solder your generator wires to the connections.

Good luck and show us the pictures.

picclock

I like that idea  :)   but the stepper phases are likely to partly cancel if series connected, probably better to rectify first (switch mode PSUs will work happily with a DC input)

Bill

Bill

Offline picclock

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Re: Converting Ac to Dc current.
« Reply #7 on: January 20, 2011, 09:50:56 AM »
@ Bill

My Bad  :hammer: - the phases for a std stepper are 90 degrees out of phase. Not sure it would make much difference as this would still put the rms value @ 110 volts, and most of these chargers will work down to 90v or less depending on load. I would think that is worth a try.

Technically better option as you say is to bridge rectify both windings, with capacitor smoothing, and connect the outputs in series as this will result in the smallest motor load.

@ Benchmark

Half wave rectification results in a dc current flowing through the windings which is likely to demagnetise the magnet in the motor. Much kinder to the motor to use fullwave rectification, as it reduces the current peaks into the load and works the magnetic motor components about the centre point. If it helps think of it as an unbalanced flywheel .. . it will work but the bearings will be shot in no time.

The R in the circuit is the load. Without an input capacitor the average voltage is 66 % of the peak - so 75 volts is 106v peak, or 70volts average. As its half wave rectified you would think 35 volts but things are much more complicated than that. As an example, a 35 ohm resistor will draw 1 amp at 35 volts, dissipation 35 Watts. However doubling the voltage will give 2 amps but 140 Watts.

IMHO try the two windings in series and a phone charger. Its quick to do, and will allow a large current draw at a lower voltage which is what you need for LED's and bling lamps. As a bonus the output is isolated so no nasty shocks.

The frequency of operation will not matter much for a device like this.

Good Luck

picclock
 
 
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Offline BillTodd

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Re: Converting Ac to Dc current.
« Reply #8 on: January 20, 2011, 03:36:53 PM »
Quote
Not sure it would make much difference as this would still put the rms value @ 110 volts, and most of these chargers will work down to 90v or less depending on load. I would think that is worth a try.

I agree , well worth trying especially for a simple application  like this :)

Bill
Bill

Offline benchmark

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Re: Converting Ac to Dc current.
« Reply #9 on: January 23, 2011, 04:07:42 PM »
Well Guys, the rectifying diodes came yesterday and i finally finished the generator and it is performing well above my expectations.

I will let the video speak for itself

Thanks for all the input.



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