Thinkpads Forum

Hello guys,
I have this great 120v ac inverter that turns 12v DC into 120v AC and I wanted to know if any of you know if this is OK for the thinkpad adapter. Technically, this makes a modified sin wave of 120v instead of a True sin wave. It's more like a square wave than a sin wave. But anyway, I tried it out and everything works fine. The only funny thing (and somewhat expected) is that the AC adapter for thinkpad buzzes a little more than normal. I'm not sure if anyone knows the answer to this, but i have used it in the car for maybe an hour or so with no problems. I'm not sure if lenovo/IBM support would know a answer to this, so I've turned to the forums. Yup. Well thats about it...

Thanks...

The inverter should publish somewhere its RMS voltage. That is a measure of effective voltage. So, my Port Inverter claims to put out 120 Volts RMS from a 12 - 16 volt DC input. It is not a sine wave, but since the effective voltage is 120 volts, it works fine with my TP adapter - I use it fairly frequently. ... JD Hurst

Your inverter is probably what's known as a "modified sine wave". If they add one step from positive to negative, then it's a modified sine wave. Better inverters have more steps... good ones (like my Samlex) are expensive but generate a pure sine wave with no more THD than you see on the average AC line.

The buzzing you're hearing is as a result of the switch-mode power supply not liking a square wave. You'll also likely notice the power supply getting hotter, which will lead to its eventual demise. It probably will take a while, but you will kill it quicker.

I'd recommend staying away from cheap inverters (like Vector) and getting something from Trace/Xantrex, Samlex, or other reputable manufacturer. The best solution is a real DC-DC converter from Lund or similar... rather than going 12VDC-->115VAC-->20VDC (with other intermediate steps on the middle), you go 12VDC-->20VDC with much more efficiency and much less heat.

tvsjr wrote:<snip> The best solution is a real DC-DC converter from Lund or similar... rather than going 12VDC-->115VAC-->20VDC (with other intermediate steps on the middle), you go 12VDC-->20VDC with much more efficiency and much less heat.

If the sole use is the ThinkPad, then this is a good idea. Stiil, such an inverter must first change to AC in order to up the voltage and then convert back.

If you have other uses (I use my Cell Phone charger on it at the same time as the ThinkPad) then a DC -> 120 VAC inverter also makes sense. It depends upon end usage to determine what to buy.
... JD Hurst

I'm another one who votes for a DC-DC adapter. You can buy one direct from Lenovo:

ThinkPad 90W AC/DC Combo Adapter

or simply search eBay for the FRU (40Y7630), but make sure it comes with ALL of the cables. I've been able to buy a couple of 72W adapters for my T23s off of eBay for around $20-$25, but that's the exception.

jdhurst wrote:If the sole use is the ThinkPad, then this is a good idea. Stiil, such an inverter must first change to AC in order to up the voltage and then convert back.

Incorrect. A switch-mode DC-to-DC converter works by applying a DC voltage across a transformer intermittently, controlled by a high frequency switch (commonly 100KHz-5MHz depending on design specs). The transformer stores energy magnetically when the switch is on, then releases it through the secondary windings when the switch is off. The high-frequency switch gives the switch-mode supply its name and is also responsible for the RF interference generated by such a supply. An inverter typically involves a DC-to-DC converter, but also adds a full-wave bridge rectifier to convert the AC waveform to DC... but, this is not an inverter.

tvsjr wrote:

jdhurst wrote:If the sole use is the ThinkPad, then this is a good idea. Stiil, such an inverter must first change to AC in order to up the voltage and then convert back.

Incorrect. A switch-mode DC-to-DC converter works by applying a DC voltage across a transformer intermittently, controlled by a high frequency switch (commonly 100KHz-5MHz depending on design specs). <snip>

Whatever the frequency and however generated, it is still AC (or at least that is how I generically describe it). I did not mean 120V AC Sine Wave in my post, merely that there must be an AC component to transform, which is precisely what you described. ... JD Hurst