(I would have posted this to flukecommunity.com but for some reason they don't allow me to read certain forums, nor post to any with my userid and password and they don't answer my messages to the administrator! I hope the question is acceptable here, this is a very informative site for me.)

Anyway, I have a CDI 'peak voltage adapter', sometimes called a 'DVA', made by a company called CDI and supposedly compatible with similar adapters made by Mercury for their outboard motors. I gather it contains a circuit with a capacitor and a diode and is meant to let a voltmeter capture peak voltage regardless of waveform.

I also have a fluke 87 and a fluke 88v. I have only done a couple of tests, but I notice that they read similar values when I put one of them in 'peak mode' and the other in 'min/max' retain mode. For example, on a friends RV with INtellitec battery disconnect relays, I get 155V 'MAX' with the CDI DVA on the '87 and 160V with the 88V set to 'peak' (this is with 120VAC shore power connected to what I gather is a fairly primitive Allanson converter that has a coil about five inches high and four inches wide).

I presume the difference between the two readings is just 'noise'. My question is do I really need to bother with a peak voltage adapter with such meters?

Perhaps another question would be when measuring very weak sources, such as ignition 'exiciters' on small motorcycles, can I also dispense with the DVA?

On the one course I took about this, a marine instructor demonstrated the use of a DVA but I got the impression it wasn't needed with such Fluke meters as these, but being just an amateur hobbyist in electronics, I could well be wrong about that or have misunderstood it in the first place.

 

One point I didn't mention is that the fluke 87 is advertised as 'true RMS' which I gather means it measures power as part of the voltage measurement. I presume this means it might understate voltage for a narrow voltage spike that isn't in the form of a sine wave. Is that reasonable?

Whereas the 88V isn't advertised as 'true RMS'. I wonder if this has to do with manufacturing cost or is it in fact preferable for typical automotive diagnostic purposes?

 

A true RMS measurement technically has nothing to do with power. It is simply a number associated with the waveform that quantifies the waveform's amplitude in a specific way (mathematically, it's called a functional). One reason why it is a useful number is that RMS voltage and current measurements can be used to get the RMS power of a signal; the RMS power can be equated to an equivalent DC power, at least in terms of the rate of energy usage.

Usually, measurements are made to help make a decision. Whether or not this peak detector add-on is useful depends on what decision(s) you want to make. Thus, we can't really answer your question about whether you need to bother with a peak detector unless we know what you're trying to accomplish. It sounds like it might have some application with respect to outboard motors, but you're going to have to educate us on why.

For general purpose troubleshooting and characterization, I can't see a strong need for a peak detector. If I need to know something about the peaks of a signal, I'll use an oscilloscope. A peak-reading meter will miss peaks that are narrower than the meter's bandwidth specification, so you may miss what you're interested in (and digital multimeters typically don't have very wide bandwidths; they usually measure over audio frequencies at best). The same can be true of an oscilloscope, but even an inexpensive oscilloscope will probably have a bandwidth two or three orders of magnitude higher than the meter (and, thus, be much more capable). Now, many folks don't have access to a scope, so the peak reading meter is their tool of choice. They just have to be aware of its limitations.

 

Apart from trying to be reasonably competent to use fairly simple meters on an amateur basis, to help diagnose charging and ignition problems on fairly old two-wheelers and the occasional rv. mine and friends', both just a hobby (I refuse to help with friends' new expensive bikes because I don't the money to replace any parts I might ruin). Usually I can handle something really simple like a 1970 Mobylette without any meter, nor a manual (usually hard to find) and nearly always it is hard or impossible to get a wiring diagram (eg., triplee motorhomes in Winnipeg says their wiring documentation either can't be reproduced or can't be understood by customers). In this case I'd never encountered an Intellitec two-way relay with a permanent magnet, usually the ones I come across are simpler. My limited electronics knowledge is a problem but given this hobby of course I must deal with that. Sometimes it is actually an advantage to know when one is stepping into water over one's head.


The reason I got the 'PVA/DVA' in the first place was because I was ignorant about the peak readings of fluke meters. When I got the flukes, it was mainly because they have a good reputation and the backlight feature helps my poor eyes. It was an extravagant purchase for me, but I often need two meters when tuning engines as my powers of sight and sound aren't very reliable. I have been studying a couple of books about ignition exciters, sometimes called pulse generators on gasoline engines and how their wave-forms aren't usually sign waves, rather tall, narrow peaks. At least one, a book I like published by Haynes about motorcycle electrics, author Tony Tranter, if I recall, mentions this. I can't remember at the moment which service manuals mention peak pulse voltages, but I have seen several that gave a voltage spec' for them. I realize that much automotive diagnostics involve testing several components to isolate the one that can't be tested, but I guess being a little obsessive, I would like to be able to pinpoint a component that is weak by testing it. For example, I would dearly love to know how to test a 'capacitor-discharge-ignition' module, which looks like a fairly simple circuit for an electronics pro' if not for me.


Old Honda manuals mention only certain Japanese analog meters for their tests, but after several years trying, I've never been able to find them in North America. My only guess for the need of these meters is that they use batteries with higher voltages and lower impedances than the modern meters, but that's just a wild guess and I'm probably wrong on some detail or other. One big problem area with old two-wheelers is that many bike shops don't want to work on them because parts are hard to get and the typical economic proposition deters diagnosis and favours parts replacement. Even the trade schools here in Canada have downgraded their technical training to reflect this. So even if one of my friends can afford the shop work, it doesn't help. Plus, I have first-hand knowledge of local motorcycle and rv shops not having competent electrical technicians and charging customers for their own mistakes.


That is a long-winded and probably not very useful answer to the question as to 'purpose'. If this ramble is not acceptable here, I'll be happy to chastised and go away!

 

Should have mentioned that I'm hardly the person to 'educate' anybody here about why Mercury sells a DVA for their motors, nor to try and explain why I don't know much about oscilloscopes - I probably couldn't even hook one of those up correctly, let alone analyze the results! Although if I ever see a used one that connects to an old laptop, I might give it a try. It could well be that I'm in over my head once again and should stop pestering the people here until I understand more.

 

Do not trouble yourself about your lack of knowledge or your uncertainties -- the folks on this board will try to help you understand things and give you the benefit of their experience if you ask well-posed questions and show that you're expending a reasonable amount of skull sweat to understand things.

Let me first explain a little about "peak" readings. What happens in the digital multimeter (DMM) is that the voltage being measured is "sampled" by an electronic circuit. This essentially means the instantaneous voltage is measured by the circuit (here, "instantaneous" depends on the electrical details of the measuring circuit) and converted to a digital number. This number is then displayed on the screen. The DMM also has memory where the largest and smallest values are stored -- when, e.g., the latest reading is lower than the smallest value stored, the latest reading is stored in the lowest value memory. When the user wants to see the maximum and minimum readings, these values are displayed on the screen.

Clearly, the ability of the DMM to spot the "real" peak voltage depends on how close together these sampling points are in time and how rapidly the peak occurs. Your add-on hardware may have circuitry to "stretch" or "remember" these peaks long enough so that the DMM can actually display the "true" peak.

For the testing that you seem to want to do, being able to look at the actual waveforms is very desirable and, hence, the recommendation for an oscilloscope. Fortunately, things like engines are electrically pretty slow -- meaning the signals you're likely to need to see don't run to very high frequencies. Let's make an estimate -- this is an example of "order of magnitude thinking". Suppose we have to deal with an 8 cylinder engine and it's running at 10,000 RPM. Also suppose each spark plug fires every revolution (they typically don't, of course). That means there will be 8 plug firings every revolution. The revolution frequency in Hz (Hertz) is 10000/60 = 167 Hz. You'll see 8 ignition pulses during one revolution, so multiply this frequence by 8 to get the actual frequency. You can thus see that the signals you'll be looking at are on the order of 1 kHz (1000 Hz), which is pretty slow as far as oscilloscopes are concerned. To see the detail in these waveforms, a scope with a bandwidth of 1 MHz would likely be suitable for your needs.

You could probably find a used oscilloscope on ebay or even a new one within the range of $250-$500 (I'd specifically recommend staying away from PC/USB scopes, as they are likely not robust enough for this type of service). Suitable older analog scopes can go for significantly less money and there are good ones still available.

Learning to use an oscilloscope is not very difficult -- you just have to have the desire to want to figure things out. Plus, there are lots of books and web pages that can help. One of the best ways to learn is to get an experienced person to show you some of the basic tasks. Since it sounds like you're interested in motorcycles, you might want to advertise locally that you'll trade some mechanical work for instruction on an oscilloscope.

About the old Japanese analog meters -- I would doubt they were/are anything special. The typical analog meters from Triplett, Simpson, etc. would have been as good or better for the purpose. Most modern digital multimeters have significantly better capabilities than the analog meters. However, an analog meter can be useful for some purposes and I still use a few of them. I had a little Triplett 310 that I kept in my toolbox and it was good for working around the car; I gave it to my sister a couple of decades ago since I had replaced it with DMMs. Triplett and Simpson still sell their analog meters and some folks still use them. If you know their limitations and don't abuse them, they'll probably last much longer than any DMM.

 

Thanks for the tips. I took a few electronic courses when I was young but lost interest in becoming an electrical engineer when I found out none of them knew what an electron was. After that I slowly (very slowly) acquired enough wisdom to know that there are other ways to skin a cat.

Now, in my second childhood, the theory aspects interest me again but mostly just to try and keep my brain exersized. Whereas old kids from the 20th-petro-century like to ride old stuff which usually means some repair to make it work. I get a big kick out of meters and maybe I show off too much in front of my own friends when I tell them you can't see electricity, so the meters are your artificial eyes.

One of these days I'm sure I'll find a used o-scope that is sturdy. So far the people I've met who were selling them had lost the manuals and couldn't demonstrate either. I might try to make a smart charger sometime, just for laughs, but I doubt if I will ever attempt to actually make a more sophisticated transistorized circuit board.

The flukes I have, if I recall, can sample every 250 micro-seconds or so, or 4,000 times a second which means that with a few engine turns they have a good chance of capturing the peaks since a very fast idle might only involve 60 turns per second. As you say, engine speeds don't need the fastest o-scope. I've seen a circuit for a PVA/DVA and it basically depended on a capacitor to retain the peak value although I don't know what the capacitance was, which might affect that. But there are also times when the engine isn't in the picture and some circuit board or other, and in the rv case, various transformers, which piques my interest.

I tell my friends I diagnose, they fix, ha, ha. But I usually end up cleaning and soldering too. Some of them are licensed aircraft techs and they have lots of mechanical tricks I never learned. But it is fun for me to try to deal with the more abstract techniques, such as electrical diagnosis.

I'm pretty sure I have one of the magnetic two-way relays sorted out. Another one to go. (They are the kind that stay on but use a swinging magnet to avoid drawing current when on, the board seems to be able to switch polarity to the coil wires so the solenoid is bi-directional.)

Thanks again for the advice. (Sorry if the paragraphs run together, still haven't figured out what I'm doing wrong in that respect.)