Inverter not putting out enough juice

The friendliest place on the web for anyone with an RV or an interest in RVing!
If you have answers, please help by responding to the unanswered posts.
See posts with no answers
ZuniJayne

ZuniJayne

Well-known member
Joined
Mar 5, 2005
Posts
334
Location
near Albuquerque, NM
Greetings, all....

I recently noticed problems with my two small inverters (200 and 150).  When the solar is charging the batteries, the voltage at the 12v plug is 14.6.  However, the output at the inverters are, respectively, 89 and 99 VAC.  The output is the same when the solar is NOT charging.

These inverters USED to put out the proper voltage.  What could be happening?  ???

TIA
 
Did you measure the voltage at the input to the inverters while they're under load?  14.6V sounds like an open circuit voltage, loaded should be ~13V.
 
Are you using the same meter as you did when the voltage reading was proper?  Some of the cheaper meters will not read modified square wave voltage correctly.  The reading you get is about what could be expected from the cheaper meters.  Does systems connected to the invertes work?
 
As I mentioned elsewhere, I have a collection of meters.  One of them shows 95 volts out from my generator (the rest show 120) I need to re-calibrate it (Which may be a job if I can't find it's service manual) course I think it's older than I am (WW II vintage) but it still works.. Almost
 
Low input voltage (DC voltage) is the primary cause of low output on an inverter. Little else could cause this problem.

But be sure you are getting a correct reading of the AC voltage? As Ron says, many meters will not give a proper reading on a modified sine wave.  Some meters sample the voltage rather than computing the RMS voltage value and a sample on a modified sine wave may not give an accurate result.  An analog meter is probably the best choice among the cheap ones, but that's not fool proof either.

Try some other meters - borrow a friend's or pick up another  $3.99 special at Harbor Freight, Radio Shack or Frye's. See how the readings compare. If possible, get a reading with a high quality meter like a Fluke, but few people own them unless they are heavy into electronics. They are expensive.
 
Greetings, again, all.....

I may have "assumed" (yeah, I know) that the inverters were originally putting out the correct voltage.

Today I used a Radio Shack Pocket meter, a Radio Shack autoranging meter, and a $4 Harbor Freight meter on the inverters inside "Old Green."  The pocket meter said the voltage was about 90.  Couldn't get a consistent reading from the inverters inside from either one of the other meters.

SO - I went to my truck, which has a marine starting/deep cycle battery in it.  Base voltage from the battery at the "power point" was 12.65.  The 120v readings on both inverters was the same as in "Old Green" with each meter.

Could this mean that the inverters (cheapie modified sine wave) just aren't reading right or are putting out really poor waves?  Items like my cordless phone and fire dept radio are not charging right.

Yeah, I'd love to have a good Fluke meter.  I'm saving my pennies...... ;)

 
It may just mean your meters are reading Peak to Peak voltages and multiplying by the conversion factor (Square root of two if memory serves)  Modified sine wave does not peak as high as true sine wave.

Radio Shack used to sell a "True RMS" meter, (I have one) it uses a different way to measure and on a modified sine wave will read higher than a P-P meter  The reason for this is simple... Modified sine wave inverters spend a bunch of time at the peak, it's a "Flat top" so they peak out lower.  This messes up some electronics which rely on that peak voltage
 
Jayne,
John is right on. Your meters are simply giving incorrect results because of the shape of the wave, which is essentially a low plateau (called a square wave) rather than a  tall hill that rises and falls at a modest rate.  That's probably also why some of your chargers don't work right - battery chargers in particular are sensitive to wqave form and some of them will actually buzz loudly or overheat when on a square wave power source. 

The best solution is to get a small pure sine wave inverter to charge those things, but even a small one is expensive. Figure $100 or so for 150 watts (which is plenty for phones, laptops, etc.). Or get an inexpensive 12V charger for the phone - usually around $10 at Walmart or Radio Shack.
 
Was discussing this thread with some friends today at lunch (Note, this is a ham radio lunch and of all the people at the table, as a certified electronics technician, I am roughly the least qualified) one of the Electronics Engineers got out his calculator.  If a peak reading voltmeter is showing 95 volts on a low-end modified sine wave, That is the RMS equivlent of around 125 volts on a true sine wave.

Of course without either 1: Seeing the actual waveform (Easy for Me to do, hands on that is) or 2: Using a true RMS meter (likewise) one can not be sure
 
Thanks, all...

I can spring $100 for a small true sine wave inverter.  Apparently, the msw inverters did a basic job, but took their toll on the electronics after 3 years. I'd heard of this happening.

In my Arctic Fox, the ProWatt 1750 did a fine job of running even my Deskjet printer.  However, I am now living in the old big trailer and haven't hooked up a larger inverter yet.  I got a Chicago Electric inverter for it, and I am a bit concerned about the quality of the msw vs. the ProWatt.
 
Power supplies for electronic stuff (This includes battey chargers for cell phones, PDA's and laptops) come in basically 3 flavors (There are more but these are the big ones)

1: Transformer operated regulated supplies.  Most of these will run on either modified or pure sine wave though they may warm up a bit more on one or the other, Monitor for temperture.  My laptop's power brick, for example, is not much bothered by a modified sine wave

2: Straight transformer supplies (Common with cell phones and pda's) may well have a lower output voltage when working off a modified sine wave as they are "peak reading" devices. no regulation.

3: Switching power supplies (This includes some laptop bricks) Totaly messed up by modified sine waves

The key is regulation, and of course how much overhead is built in.. If I designed and built the supply (I do do that) trust me, it won't care  But alas, I only build supplies for my own use.

Now the "Big Radio" in my motor home (Kenwood TS-2000 all-mode/band Ham rig) does not care if the inverter is modified or not...... It runs on 12vdc and does not use the inverter :)
 
Switching power supplies, as used in nearly every computer, run just fine on a modified sine wave.  This includes both towers with builtin PS and notebooks with power bricks.  Most typical wall wart used for charging cell phones and the like also work just fine on MSW, I have found a FEW that caused the powered device, not the PS, to fail when run on MSW power.
 
John is right get the extech 22-816 true rms dmm for 85 bones at radio shack "wonderful " you will not need another
 
Thanks, Ned, gensetbill, and all....

I'll start saving my pennies for that meter.  I like to know what is going on with my electronic equipment
 
Just a thought.... Any of those meters you have ANALOG (you know, a genuine METER, not a digital readout)

I know some of them are digital (You used the word AUTORANGING which is a digital thing)

I once bought a bunch of cheap analog meters at a dollar store.  For what I needed, they were perfect

Some analog meters will read RMS as opposed to peak (some don't)
 
My old Micranta (Radio Shack) pocket-sized analog meter keeps on ticking and generally gives better results than all but my nice  Extech digital VOM.  Don't know if R.S. still sells them or if everything is digital these days.
 
I have both digital and analog meters from RS and both have their place.  The Micronta brand was one of the better RS labels.
 
If you put an inductive load on the inverter, like a transformer-type power supply, both digital and analog meters will read almost the same rms voltage (.707 peak). A resistive load, like a light bulb, will not accomplish this , nor will a switching power supply or other non-inductive load.
 
You must log in or register to reply here.

Latest posts

Members online

Total: 1,006 (members: 28, guests: 978)

Forum statistics

Threads
131,925
Posts
1,387,586
Members
137,674
Latest member
SPK550
Back
Top Bottom