Problems with 12VDC inverter

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Re: Problems with 12VDC inverter

Post by prawncrackers on Wed Sep 25, 2013 7:13 am

Thank you for your reply Runem,

i have a 2k2 resistor from the TL072 outputs to the gates, are you referring to a 1k resistor from gate to ground? ill try that but ill have to remove the 2k2 resistor, right? OR for initial purposes, ill swap the mosfets with a bjt.

you may be right about "phase crashing". MOSFETs not switching sides cleanly or pulling on each other's drains thats drawing a lot of power/heat.
damn its hard to verify this without a scope. but maybe...

ill connect only one of the secondaries to the inverter and thsi should give me a pulsating output wrt ground, measure idle current and do with the other side.

im keen with the inverter thing because ill be using existing lighting fixtures. true, 12v lamps will be more efficient though.
im just thinking of the practicality where whenever theres power-out ill just cut off the main breaker, plug the inverter to ANY wall socket and it will deliver power to all lighting across the house. 

cheers!

prawncrackers

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Re: Problems with 12VDC inverter

Post by Runem on Tue Sep 24, 2013 5:17 pm

There are people here that most likely can give a better answer, but I'm giving a stab at it anyway. I am guessing your problem has something to do with the gate capacitance of your mosfets. Maybe the TL072 isn't capable of driving the gate low properly so you have a "phase crash" going on in the transformer(?). Maybe a gate resistor (about 1k) on each mosfet will do the trick?
Well, i'm only guessing here...
Anyway, since I'm also plagued with power outages, I have installed a few 12V LED lights in the hallway and kitchen powered by a stack of paralleled SLA batteries, trickle charged just to maintain the cell voltages. A lot more practical in my point of view, and I can have light for at least 40 hours continuous, if needed. The biggest problem is light fixtures, so if you want something that looks OK, but at the same time can take 12v LED bulbs, marine light fixtures is probably the best way to go (I did). Sometimes it is also possible to modify 115/230V bulbs to 12V, if you can take them apart.

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Runem

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Problems with 12VDC inverter

Post by prawncrackers on Tue Sep 24, 2013 4:58 am

Hello,

i built Harry's 12v to 200v square wave inverter for uhmmm emergency uses. [our place is plagued with power outages]
i cant access www.sm0vpo.com right now maybe its down.
anyway this is the one with the astable circuit feeding comparators and driving mosfets.
i used IRF3205 as mosfets, TL072 as comparators.
I used a generic over-the-counter 12V-0-12V 1A transformer as test. i connected it to a cheap 8W CFL and it lights up fine

HOWEVER

i get a rather large current draw from the battery [around 18w in all]
the AC voltage output flies all over the place [very high and low]
the transformer sings halleluiah in 120Hz square wave [i dont have a scope but i know how it sounds]

to test why the output voltage is so cranky, i connected a 68k dummy load across the AC output, put a diode and cap to test one side of the output.
measured 212VDC, good voltage. then reversed the diode/cap jig
measured 340VDC, weird.

i then switched the mosfet connections [i.e. mosfet 1 pulls from the 2nd 12v winding instead of the 1st]
from the setup where i got 212VDC i now get 220VDC, and
from the setup where i got 340VDC i now get 325VDC.
and for a 68K dummy load it draws around 0.8A current from the battery. and sings 120hz.

i have this 12v-330v UPS transformer and i got same results [even higher voltage around 500v i thought id blow up my dummy cap]
and the UPS transformer draws radiculous current... i dont wanna remember it.

whats going on? is this a transformer quirk im not aware of?


also followup question, if i get this project right, will running the transformer at higher frequency [120hz or 240hz] give me better power-to-trafoweight ratio? id only use it with appliances that rectify mains anyway.

thank you for your patience reading this.

prawncrackers

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