How to convert High voltage Dc to AC?

Discussion in 'General Electronics Chat' started by fabieville, Sep 11, 2010.

May 25, 2009
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I have a solar panel that is rated for high DC voltage but low current. Here are the specs:
Power (W) 80 Watts
Open Circuit Voltage (V) 100.00 Voc
Short Circuit Current (A) 1.62 Isc
Maximum Power Voltage (V) 71.00 Vmp
Maximum Power Current (A) 1.27 Imp

As you can see the open circuit voltage is 100V and the rated voltage is 71V and the rated current is 1.27.
I need a circuit or some explanations as to how to convert the high dc voltage to AC first, then use a step down transformer to step down the AC and increase the current and then use a rectifier to convert it back to dc giving me a lower dc voltage at about 16V or little higher but with lot more current than the 1.27 to charge a 12V battery.
Basically what i need is how to convert the solar panel by running it through a circuit so that the voltage is step down while the current is step up.

2. Kermit2 AAC Fanatic!

Feb 5, 2010
3,852
968
I have 25 solar panels of the same type. I presume these are Amorphous silicon panels? My panels are 60Watt. rated 100 volt open circuit and 67volt under load at .97 Amp. I use them to charge a 48 volt battery bank. I have an Outback brand charge controller, but before I purchased that I charged the battery bank through a power isolation diode DIRECT from the solar cells.

Mean Well makes a 48 volt pure sinewave inverter, but it costs about \$900. Also Samlex is another brand that has 48 volt units.

Using those panels to charge 12 volt batteries will be very wasteful and presents a power conversion nightmare. If you can, go with a higher voltage battery string. At least 24 volt. 24 volt inverters are common and therefore much cheaper than 48 volt inverters.

3. thingmaker3 Retired Moderator

May 16, 2005
5,072
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A dc to dc converter is called for here, as the output from the panel will be variable and unpredictable. Most solar charge controllers make use of a dc to dc converter.

4. Kermit2 AAC Fanatic!

Feb 5, 2010
3,852
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is this just ONE solar panel you have?

Even changing it down to a proper level for charging would mean days and days of charging for a battery large enough to be of use for running any kind of power equipment. If its just LED lighting or something else that is very low power drain, then this could be a good way to go, but if you plan on running a 12 volt AC power inverter from the battery, the size of battery you will need will NEVER be charged up properly from just one solar panel.

May 25, 2009
26
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yes i only have one panel. Is it possible to step down the 100VDC to about 20VDC using a dc-dc converter and the current would increase at the same time? Just like how a step down transformer works when stepping down AC voltage.

May 25, 2009
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How can i convert high voltage dc for eg. 100VDC to AC of the same or close to the same value?

7. gootee Senior Member

Apr 24, 2007
447
50
Yes, it is possible. You essentially want to be able to step down 71 V DC @ about 1.3A max to 13.6 V DC @ about 6 A max (6A assumes 90% efficiency in the conversion).

You will probably have to design, or find a schematic for, an appropriate step-down DC-to-DC converter, also known as an SMPS (switch-mode power supply), in order to have the conversion be efficient-enough to get 6A out of it at 13.6 V. And "your mileage may vary".

You can check at Linear.com and National.com , to see if any of their SMPS ICs can handle the high input voltage. If so, they both have free software that could design the relatively-simple circuit for you. And it should only require a cheap off-the-shelf inductor, rather than a custom transformer, which is a big plus.

8. gootee Senior Member

Apr 24, 2007
447
50
I don't know if it could be efficient-enough to convert it to AC and then step it down with a transformer and then rectify and smooth it back toi DC. But, if it is, it's easy to convert DC to an "AC" square wave, using a simple transistor square-wave oscillator circuit that oscillates at 50-60 Hz. (Or maybe use 400 Hz so you can use a smaller and more-efficient surplus aircraft transformer, or wind your own.)

9. marshallf3 Well-Known Member

Jul 26, 2010
2,358
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Transformers aren't the most efficient devices in the world and they get even worse if you try to feed them a square wave.

I'd find or custom design a buck converter.

10. timrobbins Active Member

Aug 29, 2009
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If you are ok with the output being non-isolated from the input (which should be fine in this application) - then search on buck converter mppt, as that is what is needed - they started using that design more than 30 years ago. However, given the posters likely lack of electronics experience then I suggest strongly that they contemplate a commercial mppt.

May 25, 2009
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with all dc-dc converter step down circuit u get an increase in output current while the voltage is decrease at the output?

12. timrobbins Active Member

Aug 29, 2009
318
16
The dc-dc converts power with high efficiency. In simple terms:
Vin x Iin = Vout x Iout

In your case you will be unlikely to get more than 80W = Vin x Iin, so use the above equation to indicate the output voltage and current levels you may achieve.

13. marshallf3 Well-Known Member

Jul 26, 2010
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Depends on what type of converter and how it's built. A good one can achieve close to 95% efficiency, a simple resistor can come close to 0%.

May 25, 2009
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i am not saying that i need to get more than 80w. I am just asking that if u make a dc-dc step down converter, is it always the case that the amps u had at the input u will always achieve more amps at the output after the circuit have step down the input voltage?

15. marshallf3 Well-Known Member

Jul 26, 2010
2,358
201
No, because it depends on how the voltage is stepped down.

16. timrobbins Active Member

Aug 29, 2009
318
16
Marshall - I believe he is just referring to dc/dc switchmode converter generic operation, where you would obtain a higher output current than input current, due to constant power characteristic.

17. Kermit2 AAC Fanatic!

Feb 5, 2010
3,852
968
True. Basic, step down the voltage by a factor of three and the current will step up by about the same amount.

100V 1A to 10V 10A. In a perfect world of course.

18. marshallf3 Well-Known Member

Jul 26, 2010
2,358
201
Start at the first, it's a bit more complicated of a question.

A good DC to DC inverter can achieve up to 95% power conversion efficiency but for what he's wanting to do it would have to be custom designed.

19. Kermit2 AAC Fanatic!

Feb 5, 2010
3,852
968
Do you already have the 12 volt battery you wish to use in this scheme?

I may have an idea for you, similar to the way I charged my 48 volt string direct from panels identical to yours.

20. amit sharma New Member

Sep 14, 2010
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you can use thyristor to invert DC.
as we know thyristor works as switch. thus by using this property of thyristor you can easily invert DC to AC. the circuit diagram for this can be found from a number of sites. further you can use the same thyristor for rectification.