12V Input current required for Inverters

of course not

calculate from output load wattage assuming around 90% efficient will get you an approx answer. It'll be more than that offload though.

real watts or chinglish microwatts?

NT

snipped-for-privacy@tubes.com wrote on 10/14/2017 6:19 PM:

You are probably looking at a surge current to charge caps somewhere. If they aren't directly on the input, the unit won't draw much current until it sees enough voltage to power up. Then it starts drawing a surge to charge the caps and pop goes the fuse. I know appliance fuses come in slow blow and fast blow, but I don't think I've ever seen a fast blow automotive fuse. They have a lot less thermal mass to heat up. All the automotive fuses I've seen had fairly fat links.

Do you believe everything you read? 16,000 watts is 21 horsepower. Really? 21 horsepower in an inverter? Is it as big as a suitcase and have a big fan to keep it cool? From 12 volts that would be 1,333 amps. Yeah, right!

--

Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998

May as well just connect the batteries in series to get 120 or 240v. If you want ac, just swap the leads over really fast :)

NT

400 W requires 33.33 A at 100% efficiency. So, it could not possibly be a 400 W inverter. But, maybe 400 W peak, for a very short peak. 16 KW? That will need 1333 A at 100% efficiency. No WAY would anyone ever build a 16 KW inverter that ran off 12 V DC. More like 120 V DC input.

You don't NEED a web site! Watts / V = current! VERY simple calculation, (when ignoring efficiency.)

Jon

That's not difficult if you have a 10,000 Ahr battery.

It says on it, 400 Watts continuous, 800 Watts Peak. Your calculation matches the formula you posted. In that case, if it needs 33.33A to operate at full load, that built in 25A fuse wont handle it.

But I connected it to my car with NO load. Why did it blow the 15A fuse in my car? Obviously it's not dead shorted. My multimeter shows around

500ohms on the 12V input, (which climbs over time).

Since I posted this message, I connected it directly to a car battery using jumper cables. It threw a large spark (with power switch turned on), as I touched the cable to the battery. Then it appeared to work. I did not attach any 120VAC device to it yet, because the weather is too rainy to mess with it outdoors. But the LED indicator showed it to be working.

-------------------------

** Likely an "inrush" surge with the first cycle of drive to the output transformer. You said it was "old" so I bet it's heavy too, using an iron transformer to produce 120V AC output.

When 12V DC is applied, the drive circuit applies it directly to the low voltage primary causing the core to briefly until steady AC drive conditions arrive.

.... Phil

Invertors aren't 100% efficient, so 37A draw is more likely at 400w out. A 25A fuse will pass that for a fair while.

NT

It probably blew the fuse because the inrush current to charge the caps was too high for the 15 amp fuse. There are ways to limit the inrush current. I know that the inverter drives for my machine tool spindles limit this current somehow. The manuals for each machine mention this feature. There is a lag in the machines between powering up and when drives signal the control that they are ready. Or in the case of the inverter based welder when the internal contactor for the welding output is enabled. There isn't a big spike in current draw when it powers up or when welding commences. It ramps up the welding current once the arc is established. Eric

I sort of thought the same thing. But when I connected it directly to a battery, using auto jumper cables, as soon as I touched the POS cable to the battery, I got heavy sparking, so I pulled the cable off the battery. That's when I noticed the inverter's switch was in the OFF position. With the battery still disconnected, I flipped the switch on the ON position, and the LEDs on the front of it, lit up for a few seconds. Thus, some capacitor became charged up, and it's stored charge, caused the LED to light.

At that point, I thought that this inverter is just too big for plugging it into a cig lighter. (My old inverter which is no longer working, was rated at 300W peak, and continuous power is 125W. That never blew any fuses).

So, I visited a friend the next day, and was telling him about this, when he grabbed his inverter and plugged it into my car's cig lighter socket. It worked perfectly. His inverter is rated at 700W continuous , or 1400W peak. So, if his 700W cont. works fine, my 400W cont. should not cause any problems.

The seller of that thing told me to just throw it in the trash, if it blows fuses. Of course I wont throw it in trash, and now I am more curious why it blows fuses and why it throws large sparks when it's turned OFF.

The bad thing is that they dont put schematics online for them. At least I cant find one. It's a Cobra CPI 480 model.

With all due respect - does the phrase "danger to yourself and others" have any meaning. As described:

a) This thing blows a 25A fuse on contact. Even instant fuses will handle a n overload for some period of time which is a function of load and duration . So in instant failure = Dead Short.

b) Sure, an LED taking a micro-current might light from residual current st ored somewhere along the current path. But that does not suggest that the s ystem is functioning properly.

c) Who knows what part of the system is being switched. It is *VERY* unlike ly that the actual switch you are using is handling either the 25A input at 12V or the 400-700-watt output, but rather some relay that is capable of h andling the current.

It is very likely that what you have is toast.

verter.jpg is a very basic schematic for a very low output device - but th e principles are the same. You can see multiple opportunities for a short.

Cutting to the chase - either you have a mechanical problem with some part shorting to the case. Or you have a component failure that will require rep lacement. With your well-proven skills, it is unlikely that you will be abl e to diagnose the failed part correctly, or if you find *A* bad part, you w ill not be able to discern whether it is the primary cause or a secondary c ause. In any case, given the cost of a NEW inverter of sufficient capacity to manage your load(s) and not either drain your battery or melt it down - get a new inverter. And, design the proper connectors to use it safely. I h ave seen installations with hard-wired inverters installed with the proper switches. No overheating, no stray wires, no fires. You might consider that option. Get it done by a professional, however.

Peter Wieck Melrose Park, PA

What other kind is there ? /something like stacked Dickson converters ? Or have they found a way to go SMPS with it ?

No danger was involved. It was laying on the lawn outside, I connected the jumper cables to my farm tractor battery and made sure to tap the jumper cable to the voltage, not just clamp it on. After the initial sparks, it attempted to operate.

I since tore it apart, and found the main 12V POS wire was crammed between the board and the case, and it's insulation was seriously crushed between the case and the board. But the case is plastic so I cant see how it would short to that. Anyhow, I taped the wire up, moved it, and turned the inverter on. For the first 5 minutes it worked fine, with no load on the output. I then connected a small 120v fan to it, and the fan worked fine. However, when I unplugged the fan from the inverter, the inverter red light began flashing and the buzzer began beeping, and it no longer produced any AC output voltage. Even after disconnecting it from the 12V battery and letting it sit for 24 hours, it still just flashes the red light and beeps.

I have given up trying to repair it, and bought a new inverter. Because it has SMD devices inside, I wont even try to repair it at this point. It's now in my junk box, intended only to be used for parts. The case will come in handy for some project, it's small internal fan may be used, along with the switch and a few other parts. The circuit board will likely just go in the garbage, because who knows what parts are fried on it. But for now, I didn't feel like ripping it apart again, so the whole thing went in my scrap box.

I have read several websites that say this is a common problem with this model, and it's not highly rated. Cobra has been known to produce quality CB radios, but apparently this inverter is has a lot of problems. I wasted enough time on it. Now it's parts ONLY.

Famous last words, "I know what I'm doing."

And ours.

--
Jeff-1.0 
wa6fwi 
http://www.foxsmercantile.com

He couldn't help himself. You were volunteer labor.

--

Rick C 

Viewed the eclipse at Wintercrest Farms, 
on the centerline of totality since 1998

Imagine like a Class D audio power amp type topology that can put out a

120VAC 60Hz sine wave.

** The most common is a full bridge, mosfet output stage producing a "modified square wave" output.

More expensive units use a similar stage running is a class D to produce a good sine wave.

In both cases, the 12VDC input is stepped up to 170V ( or 340V ) DC to power the output stage.

.... Phil