AC/DC Volt/Current meter for generator?

[mark_dingbaum]

Hi,

I've got a generator I'm adding an electonic digital (LCD) hourmeter to. Really nice little device from Enco for about $29.

I got thinking that it would really be nice to have a voltage and current meter for both the AC (115V/13A) and DC (12V/10A) outputs, maybe two displays (voltage/current) and a switch for AC or DC depending upon the current use. Upon checking into the available meters, most are $70+ per guage and I'd need four. Price goes down somewhat when I look at automotive DC volt/ammeter guages.

I sure wish someone sold an electronic digital AC/DC voltage/current meter at a very good price similar to the hourmeter I was able to buy.

Anyone know of such a device? How about any kits that could be built? At this point, I'm considering butchering a DMM and using switches to control input and the DMM to control 'flavor'.

Seems like a such a natural device for a generator that I'm suprised there aren't cheap volt/current meters available (or installed already) at least for the AC portion (I realize my DC output is not normal on a generator).

I guess as long as I am putting together my 'wish list', there should be a frequency meter also.

Anyone ever checked a generator for consistent Hz/Voltage idling and under load? Consistent voltage and frequency when responding to load? Are guages unnecessary?

Enough for now...your thoughts?
thanks,
mark

[timekiller]

I'm not up to date on meter capacities, however have you considered a digital VOM meter? I have a Fluke meter that is rated for 10 amp, and will do AC and DC volts. Just a suggestion [img]/ubb/images/graemlins/cool.gif"%20alt="[/img]

[gmac]

A homier $6 deal will get you a DMM that you can wire up for AC and DC volts. $75 or so will get you a ratshack DMM that will add frequency ( be careful here- frequency goes bananas when welding because the meter will measure the ARC frequency including many of the espikes - this is NOT the generator frequency).


adding current is a lot trickier because the measurement of current is through a series device - you would need a current transformer feeding a fixed resistor attached to the DMM to do this, with some type of electrical damping to filter out the arc noise. you could probably wind it yourself if you can find a suitable toriod core, then route the welder feed wire through it inside the machines. I would NOT start out using the current input on the DMM for this unless you have a few to burn up in the experimentation process, but it is feasible if you have some design background and experimenting/ measuring instruments.

[J_Tiers]

For the AC, most cheapies use a rectifier and averaging filter then convert average to RMS.

You can do the same, and scale the resulting DC output to fit one single display, such as a 0-20 volt range etc. You pick the scaling so that your DC and AC can be accommodated on one scale and get close to full scale readings both ways. You may need to do some mental decimal point moving, as 120 V might show up as 12V in this setup.

Current is a pain, because you have power dissipated in a series resistor. If you do a current transformer, it only works on AC. An AC/ DC version needs a hall-effect sensor, and supporting circuits.

I would be in favor of a nice "needle meter" got surplus for DC current. Use a regular resistor shunt for high currents, they are available in 50 mv and 100mv types. This is standard for most higher current "needle" type meters, above maybe 10 or 20 amps.

[Fredp]

Cheep meeters: HF sells little yellow volt ohm meeters for $5.00. They are not a bad meters for the price. If you have room to mount them you could wire in two or three. One set for DC, one set for AC, one set for ??, with a on off switch on each or them. $5 per meter $2 per switch. Or you could take them apart and use the guts out of them.

[Cliff]

Let this admittedly absent-minded and poorly educated individual brainstorm somewhat -- isn't there a way to wrap a few turns around a conductor and the resultant voltage across that coil would be a function of the voltage AND the current of the conductor, so that if you knew the voltage, you'd be able somehow to calculate the current?

But then, this could be WAAAAY off base.

[Wally]

Hello Mark,
Attached is an image of the meters on my 4KW Onan. The volt meter is Radio Shacks Micronta name. The vibrating reed Freq meter is government surplus, I think it came from the Surplus Center. I've had both for at least 25 years. The frequency does vary a bit with load, load changes and hunts some at no load maybe a couple of HZ. My work is a unit operator at a coal fired power station. The MAPP grid stays within + or - .03 HZ by our instruments and averages dead nuts 60HZ.

[mark_dingbaum]

Thanks for the input guys.

Seems like a market opportunity for the right person, but the market is maybe a lot more limited than I realize.

I appreciate the info, not sure yet if I am ambitious enough to start with a digital multi-meter and make one. I know, or have access to people who know, how to do this by using a DMM as a starting point. Might be a neat project.

What, in your opinion, would be most important in monitoring a generator:
AC Voltage?
AC Current?
AC Frequency?
DC Voltage?
DC Current?
The DC part of my generator is a battery charger and having a voltate/ammeter would give me a rough guess that 'soups done'.

[Chip_Sweeper]

Gen Owner:

I have used several of these for various applications.
They run in the $13 range, but require some electrical
know-how.



You can get them from ...

http://www.mpja.com/viewallpict.asp?dept=52&main=51

Chip

[Wally]

Mark,
I looked up the setting specs in Onan's book. Voltage min load 132 max 108. Freq max load 57 min 63. On the generator I have they can't be adjusted independantly so a volt meter would suffice. The voltmeter would probably do for load indictation also, when it dropped to minimum you are about at the end of the power. I could see where a battery volt meter would be good for the charge system.

[Doug_C]

Hello Mark,

The challenge is keeping this cheap.

If you can find these used and in the right configuration they will work as you expect. Probably not as small as the hour meter you mention, but it has many more features as scaleable engineering units and programmable decimal places.

Intelligent Panel Meters

It will take luck and resourcefulness to come up with these as used.


If you do some research on circuits to measure each value you require, with the same spec on the meters full swing. All you would need is a muti-position selector switch as in any multi-meter to change to the display desired to the proper circuit.

Most digital meters are not meant for extreme weather. Heat and cold, and UV rays seem to deteriorate them over a few years unprotected.



DC

[dkinzer]

I have a 12.5KW diesel genset that drops about 15% in RPM (thus frequency) from no load to full load with a fixed throttle position. To rectify this, I built a small controller using the Basic Stamp 2 to monitor the frequency and adjust the throttle using a linear actuator. With this in place, it holds the output to within 0.5Hz from no load to full load.

While I was at it, I fabricated some simple current transformers so that I could monitor and display the current draw on each leg. They are terribly non-linear but I was able to come up with a piece-wise linear approximation of the transfer curve and programmed the Basic Stamp controller to use that information to transform its input to the current reading. It tracks within 5% or so of a clamp-on current meter's reading over the range.

As a side note, while I was working on this project I needed to come up with a load for testing the generator - one that was somewhat variable and could handle up to 12KW. I ended up using 4 of the large heating elements for a kitchen range (about $5 each, used) each controlled by a separate switch. With this setup, I can vary the load from zero to 12KW in 3KW steps.

The Basic Stamp is an interesting little controller with substantial capabilities. A tutorial can be found here.

Don Kinzer
Portland, OR