Hi
I have two step up booster 1200W 20A 80V DC-DC and two solar panels 36v dc 7a, can i connect that two boosters in series to double voltage?.
This are step up boosters that i have two pcs.
I have two examples of connecting.
Will this work?.
Thanks 
Depends on whether the output is isolated from the input. If the output is not isolated then in your first example you are shorting out the output.
The second example is OK, but I would be concerned about the panels not being able to supply the same current as each other.
Thank you for help.
I am trying to use 220v ac grass weeder with universal motor, with only one dc booster it works but not powerful enough and it's almost useless and that is why i want to try to combine two dc boosters to double output voltage.
Edit:
I have tried and second example and it have worked, i got double voltage on output but when i try to run grass weeder one step up booster looks like stop running and voltage drops like only one dc step up is working. Step up booster are heating to much, i will try to put on + wire schottky diode so power don't go back direction to step up booster and heating up.
It's more complicated to do than I thought
You are forgetting that if you double the voltage then, for the same input power, the available output current is halved.
Or, looking at it a different way around, by doubling the voltage to the motor it is going to draw twice the current (I know, I know, but I'm keeping this simple), so twice the current, twice the voltage, 4 times the power being taken from the solar panel, which probably can't supply it.
I am trying to use 220VAC grass weeder
I only wish I could translate the excellent cartoon in my head into an actual cartoon on paper, unfortunately (or maybe that should be fortunately) I never progressed beyond stick men and rectangular houses with 4 windows and 1 door in the middle.
Anyway, in my head I am imagining you with your grass weeder and 2 giant solar panels on your back in hot sunshine sweating madly and not really clearing any weeds.
I wanted to say weed cutter 220v ac :). Yes i understand, this was my idea to make solar powered weed cutter by connecting weed cutter 220v ac to dc step up booster powered by one solar panel, but it seemed to be to low voltage and current was enough. I have tested earlier and this weed cutter draw 0.650a when run full speed at 220v ac and my solar panel is 275w 36v about 6-7a so it should be enough amperage. When i connect step up booster to solar panel output and boost power i got about 90v dc and i didn't measured amps (I'm not sure if I can measure amps with a multimeter at the output of step up booster 
).
I'm not that familiar with electronics but i imagined this wouldn't be hard to do.
Hi,
Welcome to the forum.
Please read the post at the start of any forum , entitled "How to use this Forum".
OR
http://forum.arduino.cc/index.php/topic,148850.0.html.
Trying to power a load directly off solar panels is a waste of time as you have found.
The output from your panels is not going to provide an even energy supply and the panel output voltage changes with load.
To improve your project you need a rechargeable battery to store your solar energy to help provide an even supply to the converters.
Your panels will need a charge controller to charge the batteries.
What is the specs of your panels?
What are the specs of your motor?
Do you have a DMM?
Tom..
I have tested earlier and this weed cutter draw 0.650a when run full speed at 220v
If you mean while not cutting weeds that power consumption will be lower than when the tool is under load doing its job, possibly a lot lower.
my solar panel is 275w 36v about 6-7a
If you mean that is what it says on the data sheet / packaging / marketing bxxxxxxx then you need to understand that means it will provide that voltage or that current in strong sunshine, possibly only at midday with the panel carefully aligned to the sun. Also note voltage OR current, not both at the same time. You now have a hint of the ways in which many of the claims for solar power and other forms of renewable energy are considerably exaggerated.
A little exercise if you are interested:
Find out what the available energy is from sunshine per square meter at the earth's surface. Find out how efficient your panel is. Do the maths for the size of panel you have and thus calculate the maximum possible energy output from the panel. Don't forget that the available energy at the earth's surface depends on latitude and the time of day.
Please let us know the results.
If you connect the outputs in series you must use parallel diodes to prevent back-driving - should one
converter cut-out or be unpowered you don't want the other reverse-powering it, that will instantly destroy it in
all likelyhood. A parallel diode routes back-drive current around that converter, rather than allowing reverse
voltage to build up.
Also you are then using voltages that can kill, remember, and DC is more dangerous than AC...
High power high voltage DC is difficult to switch, a specialized switch or contactor may be needed to avoid
arcing over inside the switch.
