Could a modified sine wave inverter destroy/damage the AC adapter for a laptop?

Question

I recently encountered a scenario where I attempted to use a Energizer EN500 modified sine wave inverter to provide power to a Dell 180 watt AC adapter. The circuit powering the inverter was 12 volt, 15 amps DC. The AC adapter input is 100-240V ~ 2.34 amps, 50-60Hz, and output is 19.5V, 9.23 amps. I wasn't able to find any more specs online, but it's Dell part # 74X5J, and Dell Model # DA180PM111.

12 volts * 15 amps = 180 watts, and I (incorrectly?) assumed that the adapter wouldn't necessarily need a full 180 watts all the time, and that the worst case would be a blown fuse if it attempt to draw more watts than could be provided. As I'm reading the input on the AC adapter, though, I'm realizing that if it truly can draw up to 2.34 amps, at 110 volts that's over 250 watts...?

When I connected the inverter to the AC adapter, the "power light" on the AC adapter came on when I plugged it into the inverter (indicating connection to AC power), and when I then connected the adapter to the laptop, it started flashing on and off. There was a USB phone charger plugged into another DC power outlet, with a smaller "power light" that also flashed on an off simultaneously, in parallel with the light on the AC adapter. Ever since, this AC adapter has not worked to charge the laptop battery, even when plugged into household AC current. I don't know how many watts it's putting out, but it's just enough to power the laptop at severely reduced operating speed, and the battery won't charge at all.

So it appears that this fried my AC adapter. While it's (presumably) too late to do anything to fix the adapter, I would like to understand possible causes. Could it be due to the inverter putting out modified sine waves? From looking it up online, modified sine waves hardly look like sine waves at all:

Everything I've found online suggests that a laptop AC adapter should work fine with modified sine waves. I did check with Dell and they advised that I use a pure sine wave adapter, but a modified sine wave adapter would still work, although I was "looking at a possibility of shortening the lifespan of the ac adapter." Shortened lifespan indeed!

Or is it likely that the failure was due to the AC adapter trying to draw more current than the inverter, on a 12 volt, 15 amp DC circuit, was able to provide? I wouldn't have thought that insufficient power could kill an AC adapter... could it?

Or is it a combination of the fact that the inverter was providing modified sine waves, and was perhaps "pulsing" on and off due to the excess current requirements? In my question about topicality on meta, DrFriedParts suggested that the failure might have been due to the input clamp circuit failing. Would the input clamp circuit be more likely to fail if the AC adapter experienced a rapid number of "on/off" cycles?

Getting some education on this will impact what I do next. In reviewing my vehicle's wiring diagram, I see that one of my three DC power outlets is a 20 amp dedicated circuit. I could get a pure sine wave inverter and connect it to that 20 amp circuit, providing a "theoretical" maximum output of 240 watts; I know that in reality there are losses and I can't expect a full 240 watts out of the inverter. If insufficient power was the culprit this time, I'd hate to fry my replacement AC adapter again in the same way! If the root of the problem, however, was a modified sine wave, then I can fix that with a better inverter.

Answer 1

"Modified Sine" outputs are very bad approximations of AC

This is a capture of the output of an APC 650 recorded by Jesse Kovach, while under load.

Notice the severe over-amplitude events at the extremes (the spikes at top and bottom). In reality they are actually much greater in amplitude, but the oscilloscope in the image was not fast enough to capture it.

Sharp edges in the time domain equate to broad-spectrum noise in the frequency domain. All of this high-frequency content represents additional energy that must be absorbed by protection circuits. If not, it can exceed isolation withstanding limits in the various input stage components and "burn through". If this doesn't burn out the input it will result in a cascading failure where it will cause something to fail on the secondary side from the resulting overvoltage.

...and that's just one failure mode. There are others. Psuedo-sine waves are poor matches to sine-wave inputs. :(

Go DC-DC instead of DC-AC-DC

A much better (and much more efficient!) approach is to go DC-to-DC directly (note: you can't actually go DC-to-DC directly if your input voltage is lower than your output voltage, but the details of this are well contained inside a "DC-DC converter").

Self-contained switch-mode power supplies for Dell laptops that take DC inputs are available in the marketplace. Here's an example:

which I sourced from:

http://www.amazon.com/Adapter-Charger-Dell-Latitude-D630/dp/B002BK7JEC#

Please note that I have no personal experience with this particular product and many cheap DC converters are poorly designed internally. Be careful.

Answer 2

The AC adapter input is 100-240V ~ 2.34 amps

This means at 100V input, at maximum output power the adapter will draw 2.34A or 234W from the mains - generally any AC/DC power supply will draw maximum current at minimum input voltage.

As I'm reading the input on the AC adapter, though, I'm realizing that if it truly can draw up to 2.34 amps, at 110 volts that's over 250 watts...?

180W out @ 234W in means an efficiency of 76.9%, which isn't out of the realm of possibility. Don't forget that the inverter will also have its own efficiency, making the 12V power port draw higher still.

I wouldn't have thought that insufficient power could kill an AC adapter... could it?

A safe power supply, when exposed to severe abnormals, can most certainly fail, and this is perfectly acceptable to the regulatory agencies so long as the failure poses no safety hazard to the end user - no breakdown of the galvanic barrier from the mains to the output, no thermal hazard, no noxious smoke or shrapnel.

The fact that Dell cautioned you on the use of the adapter with a stepped sinewave input is a pretty good indication that the stepped sinewave contributed to the demise of your charger.

If you didn't blow your cigarette lighter port fuse, you likely weren't in an input-starvation mode, so the failure isn't likely due to limited input power - most likely it just didn't "like" the stepped sine wave.

Answer 3

While modified sinewaves will cause a bit more stress on the electronics than proper sinewaves I wouldn't expect them to cause catastrophic failure of an otherwise good PSU.

Switch mode power supplies can also be stresful on power sources. Lights, heaters etc respond to reduced voltage by drawing less current (and getting dimmer/producing less heat). Switched mode power supplies on the other hand respond to reduced voltage by drawing MORE current to maintain power levels.

Being turned on and off is stressful on switched mode power supplies, so is undervoltage on the input. Given your inverter was visibly cycling on and off I expect it was putting a lot more stres on the power supply than a correctly functioning modified sinewave inverter would.

Answer 4

I too used my Dell 96Watt 240V power supply on a modified sine wave inverter. In this case the inverter was a 12VDC to 240VAC type powered from an RV battery. It worked, but the Dell PS ran noticeably hotter than when run off proper mains AC, so I desisted PDQ.

My mod-sine inverter was a cheap and cheerful 300W unit similar to this one...

While modified sinewaves will cause a bit more stress on the electronics than proper sinewaves I wouldn't expect them to cause catastrophic failure

Luckily nothing was fried for me, but I am interested in others' learned comments re the failure mechanism(s) when a switch mode PS cops a modified sine input. (I do not consider my comments below as being in the category of learned).

Given the almost square wave nature of the so-called sine wave cited by DrFriedParts I would have thought

1) repetitive and too high impulse currents into the input diodes of the switch mode, and/or

2) input storage capacitors overheating due to the caps' equivalent series resistance

would be high contenders in the failure mode stakes. The caps would get a severe current drain workout during the no-power phase of the input voltage cycle, when there is no voltage or current around the zero crossings, and a pasting on charging when the step voltage is applied.

Basic theory suggests a capacitive load is unhealthy for a switch mode supply for the reasons above, and an inductive / resistive load, like a motor or a lamp is far preferred.

Sharp edges in the time domain equate to broad-spectrum noise in the frequency domain. All of this high-frequency content represents additional energy that must be absorbed by protection circuits.

I thought the input stage of a switchmode was not much more than a diode bridge and some series limiting device (small-ish resistor or NTC resistor feeding some high voltage large-ish capacitors, paralleled with a small polyester cap with low ESR to filter off the HF noise that DrF-P rightly mentions. After that a chopper circuit (now usually done with HV FETs), a ferrite core tranny for voltage conversion, more rectification and then feedback to control the duty cycle/frequency/output voltage. What more energy absorbing parts are needed?

I am not convinced that an input side failure would be likely to ripple through to the transformer coupled output stages, unless it be a catastrophic capacitor or diode explosion pasting goo all over the insides of the supply.

Josh says:

When I connected the inverter to the AC adapter, the "power light" on the AC adapter came on when I plugged it into the inverter (indicating connection to AC power), and when I then connected the adapter to the laptop, it started flashing on and off.

As to these observations of indicator leds flashing when they should be solidly on...could that be due to the switch mode delivering no power around the zero crossing, by virtue of the input storage caps being discharged?

That surmise could be tested by running the PS on no or light load, when one might expect the caps to still have some charge left. If the PS is having to run at rated capacity then likely the PS is dropping out.

The fact that Dell cautioned you on the use of the adapter with a stepped sinewave input is a pretty good indication that the stepped sinewave contributed to the demise of your charger.

Hear hear!

I fully agree the better/safer/more efficient way to proceed is to run the laptop off a 12Vdc to 19.5Vdc car charger as described.

Before I found a similar ready made car charger I had intended to build such a device from a ~$AUD4.00 device like this 150W adjustable boost module

but the ready made shown by DrFriedParts at $25.00 is more convenient.

BESIDES AND BEFORE ANYONE TRIES SUCH A BUILD, as I learned just before embarking on the project, Dells have a nasty identity chip built into their power supplies whose function is to tell the laptop what kind of PS it is connected to: reference

http://www.laptop-junction.com/toast/content/inside-dell-ac-power-adapter-mystery-revealed

This is what connects to the third (centre pin) connector in a Dell DC jack. Without it apparently the laptop will refuse to take a charge. I can confirm the pin reads open circuit wrt the +ve and -ve terminals, giving the (wrong) impression that it does nothing.

If the car charger does not appeal, and a pure sine inverter is too expensive, something else to try is to put some inductance in series with the inverter output to get rid of the sharp edges of the output waveform, but one would need to keep a look out for resonances, core saturation and other tuned circuit issues.

As I said, these are surmises without the benefit of having done tests, so I would be most interested to hear from anyone who can throw more and better light on the issue.

Answer 5

5+ years on - doesn't appear to have been explicitly covered.

If there are X & Y filter capacitors (between the live wires and to ground respectively)they will be presented with the step changes in voltage of the "modified sinewave" waveform.

This can cause very large currents and damage or destruction.

Long ago I attempted to power a Hewlett Packard laptop supply from a modified sine wave inverter. It caused a very loud and harsh buzzing noise at 100 Hz - twice mains frequency here. I concluded that the supply had no input inductive filtering prior to the X&Y capacitors. Damage or destruction would have been very likely if use had continued.

Answer 6

The Dell laptop power supply almost certainly contains Active Power Factor Correction circuitry. This means that the current waveform drawn by the power supply closely matches the voltage waveform. As if the power supply was a simple load like a heating element.

Advantages are a smaller high voltage storage capacitor can be used inside the power supply, far lower harmonics on the mains input. As power is almost always been fed into the storage capacitor, instead of only at the peaks of the AC waveform.

By using such a power supply on modified sine wave power, you have almost certainly destroyed the active PFC circuit. and the power supply is now behaving as if it no longer has active PFC. As a result, the power supply now requires a much larger storage capacitor, due to only being fed at the peaks of the waveform. This explains why the power supply is somewhat working, but unable to supply it's full rated power.

In theory, you could feed in a high DC voltage. Say 300VDC, and the laptop power supply will then work at full output again. I definitely recommend that you don't actually do this due to safety reasons. As virtually all switches, fuses etc that are used on mains AC power systems. Are unable to function safely on high voltage DC.

Short answer - don't use a new laptop power supply on the same inverter. You will most likely kill it as well. You need a pure sine wave output inverter - If you can't find a suitable DC-DC power supply for your laptop.