V8 Troopie posted:

I would have thought a large capacitor should do the trick, cheaper than your suggestions too.
Klaus

Mike Harding posted:

It'll charge to the peak voltage, not the RMS.

V8 Troopie posted:

Exactly, the peak voltage a battery will be charged at if left hooked on indefinitely.

BTW, you can't have "RMS" voltages from a full wave rectified DC output, average yes, RMS no.
RMS is the DC equivalent voltage when the output voltage waveform HAS a negative (AC) component which a rectified voltage will not have. A 100Hz pulsed DC voltage is still DC, no AC is present. So, no need for a RMS meter.
Klaus

Mike Harding posted:

RMS (Root Mean Square) is a mathematical (essentially statistical) concept not an electrical one. It is possible to derive an RMS value for any quantity which changes or varies. ie One could calculate the RMS value of a set of different sized baked bean cans on a supermarket shelf and arrive at an RMS baked bean can – let’s see you measure _that_ with an oscilloscope :)

Do not confuse the common use of RMS where it refers to the household AC mains, there is no requirement for any value in the set to have a negative value in order for an RMS value to be calculated however RMS _will_ handle negative values (which an averaging function won’t - not very well in the case of electricity anyway).

Any signal which changes is an AC signal there is no need for it to have a negative component – indeed, how do we measure “negative”? Such a measurement requires we set a zero baseline and we may choose to do that with any value – so if I have a 12V system and decide 6V is my zero then a value of +2V will be 8V and –2V will be 4V and my system can supply maximum voltages of +6V and –6V (if you see what I mean :) I suspect you are assuming negative voltages are referenced to earth (ie the planet) but that is usually not the case (although the mains supply in Oz is) and especially not with a free standing generator – what is your reference point for positive and negative with a gen? If you measure between earth and the gen’s active output you will not obtain any reading.

In regard to the battery charging; what you say is partly correct. The battery will charge to the peak voltage *eventually*. The issue occurs because the charging source (the 100Hz) is not there all the time, ie. it’s not DC, so there are gaps of time between the peaks. Consider a much worse case: suppose we have a charging source of 16V but it is a pulsed waveform of 5% duty cycle ie. we get 16V for 5 seconds and 0V for 95 seconds repeated ad infinitum. If you connect a capacitor across the charging voltage and measure the voltage you will read 16V (after a few pulses) but any calculations you make in regard to the time required to charge the battery will be out by a factor of 20 (or maybe 19? :) In reality with a full wave rectified 100Hz source it probably won’t be too far out but you would need to analyse it to be sure – these things can be _very_ deceptive.

The following links provide useful information in regard to RMS, the Wiki one is pretty mathematical but the other looks at the electrical application rather more.

en.wikipedia.org/wiki/Root_mean_square

www.ece.unb.ca/tervo/ee2791/vrms.htm

iirc when I looked at the DC output of my GMC gen on my scope it had a peak, unloaded, value of about 22V dropping to around 18V under load – use caution when charging batteries from these outlets, don’t leave them on charge too long - I don’t know that Honda et al are a lot better, DC wise, are they? I did an analysis of both the AC and DC outputs of my gen and posted it to this site a couple of years ago, if you can find it!

Mike Harding

mike_harding@fastmail.fm

inspired posted:

Ummmmm, not being an electronics engineer....Are you saying that my digital voltmeter cannot correctly read a DC signal from my modified sine-wave generator?

--Inspired

Mike Harding posted:

What's a modified sine wave generator? Do you mean an inverter or are you talking about the GMC gen DC output??

It depends: but it is unlikely a meter you paid less than, about, $250 for will have a true RMS facility therefore it will only read DC which does not have an AC component (battery type voltage) or a sine wave accurately - how inaccurate it is depends upon a number of factors and I cannot estimate that without knowing much more about your system.

Measurement instruments are subject to all sorts of issues which may, indeed do, affect their accuracy - never assume that simply because a measurement instrument produces a reading it is correct, especially with digital multimeters!

Mike Harding

Jimbo posted:

That Grandson of your's must be mighty fit to be still riding dirt bikes at 50.

Member - 'Lucy' posted:

Que'

He is 5 you wacker and rides a Honda 50 & 70 like there is no tomorrow.

Now where have you been old son. Very quiet of late I must say.

Jimbo posted:

"He is 5"

That must be your Great Grandson LOL.

I'll call you tomorrow if I have a better day.

Mike Harding posted:

My little GMC 550W is still running well. Must be more than two years old now, it does everything I require - mainly battery charging via a 240V 3 stage charger - and for $98 I, most definitely, got much more than I paid for.

Horses for course, people.

Mike Harding

Mike Harding posted:

A 600Wish microwave probably has a current draw around 4A or 5A - off the top of my head - so I doubt a 550W/750W gen would have a prayer?

Mike Harding posted:

Ah! Just re-read your post.
Microwaves don't "reduce" power as such - they cycle the magnetron on a relative time duty cycle which is proportional to the requested power. So 50% power gets you (say) 10 seconds at full power followed by 10 seconds at zero power. Your gen would still need to provide the 4A or 5A?

Mike Harding

Robin posted:

Thanks, yes they cycle now Mike , should have remember that as mine cycle.

Used to own a Sharp 500w some 20 years ago , and it actually powered down
although limited, but there was a lot more inside it than todays models.

But these days you don't seem to be able to buy one(cheapie) less than 700w.

Never looked into them but I suppose one would have to upset the internal
magentron driving voltage some how.

I got a kilowatt 110v transformer somewhere so might try to run one of it
and see if it fires up.

Robin Miller

Mike Harding posted:

>A more expensive gen is a target for thieves!

Good point!!! Explains why I found that envelope tucked under the handle of my gen recently with a $10 note and a message inside saying "You need this more than I do" :)

Robert posted:

But not everyone has had problems - mine works fine.