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DC supply for inverter Answered


I need to figure out the component values for a "simple" high current power supply... and whether the component costs would even be feasible.

I plan on buying a 3Kw output 12 DC inverter for use in Spain. I have several US devices that I don't want to run off a step-down transformer as running them at 50Hz instead of 60Hz will cause heating and eventual failure (I experimented with my Technics power amp, it got very hot and started to smell, then the output went way down... it's now on the "one of these days I'll open this up and take a look" pile).

So, what I need to design is a 220 to 12 volt/300 amp unregulated supply.
Ideas, anyone?



8 years ago

I think a VFD (variable frequency drive) is what you need. http://en.wikipedia.org/wiki/Variable-frequency_drive

Ha! That looks like it might be more in line for my icebox. Now I do have a couple of options. The discussion of voltage/frequency on the Wiki page was quite helpful too. Thanks to both of you. Now I can delete the inverter from my Amazon shopping basket.

I think your antique sewing machine would be OK, it's an old motor and possibly without a transformer, and probably with speed(power)-control?
The blender won't have a transformer, and should be OK too (short runs).

A transformer will take more current at 50Hz because the impedance is less at lower frequency. Unlike motors the transformer is permanently powered for longer periods, and doesn't have off-time to cool down.
I'm thinking some kind of choke for the devices with the transformer problem might be a better fix - to reduce current flow.
Or use a step-down which takes you below 120V.

If you're still stuck, I'd suggest you ask a different question of the form "How can I get my US devices to run on Spanish-juice (with V and Hz). My stepdown to 120V is no good as they're running ~20% over what they should due to the Hz difference" - let people see this again from a different perspective.


Thanks. I wasn't thinking about the sewing machine and blender having short duty cycles, but of course they would. Neither motor has a transformer, of course. They both use rheostat type speed controls, I'm sure. So they would just always run some 17% slower than in the US.

The appliance that would be more problematic is the icebox. I thought briefly about trying to find out if a 240 volt motor would be available for it, but then decided that would do too much violence to the poor thing. It's been moved, stored and otherwise manhandled for many years (it's a 1950's GE that we've had since new) but it always has powered up and put modern fridges to shame (I know people who have changed fridges 2 or more times since I've known them). Running that motor almost 20% slow would drastically shorten whatever life it has left, I'm afraid.

Talking about the transformered gear, wouldn't a stepdown transformer at 100 volts, say, cause the unit's transformer to heat up more, since not only has the resonant frequency changed but now the transformer also can't draw the voltage it needs and would try to pull even more current?

A current-limiting choke sounds like a good idea for the other gear (electronic organ). I'll have to look into that and dust off my calculating skills (or lack thereof).


At 100V it would take less current, think of the voltage "pushing" rather than the transformer "pulling".


300A is a lot, like heavy-duty welding current. You don't need to do it this way, you've just decided this would be best.
Which specific devices are you taking, and do any of them have an input switch on the power supply that flips them to 220V? (it may be inside the case).
What do you think about selling your stuff and replacing it with 220V gear?


I agree, 300 A is a lot. The inverter specs state 280A @ peak power draw (5k). It's a Trip-Lite fixed installation automotive-type inverter. The devices don't have 220 volt option, if they did I would use that, obviously. It's not a question of supplying voltage. I have a stepdown transformer to supply 120 to my heart's content. The issue is equipment that doesn't like 50Hz. Running 60Hz gear on 50Hz causes the transformer to run hot and consequently can drastically shorten the life of the gear. What I want to (ideally) run are antique appliances (McIntosh tube amplifiers, icebox, blender, sewing machine) and an electronic organ. So no. Selling my appliances is not an option.