Cheapo DMM

They are great for what they are: kick around, who cares if it drops out of the tree and busts, throw one in the tool box or the glove box, meters. I bought some on sale for $2.99 (no typo, *2*.99). On plain DC circuits they are fine, but don't trust them with noisy DC. I run ~50 ns ~ +35 V pulses at about

1Khz into a car battery (used to start my generator). "Real" meters see 13.8 V - the Harbor Freight sees ~ 19 volts. Haven't noticed a problem on AC, ohms or current. They have a Hfe position, which I haven't tried.

And it's a cheap source for a DPM.

Ed

Since they're COB, there's not much you can do to mod them. You can take it out of the case and put it in a panel, or something like that. Maybe use the LCD for another project. Oh, and if you need a pair of test leads and a spare battery for your Fluke, you can't beat it. ;-)

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I bought some of the earlier ones for $2.99 during a harbor freight sale. They are pretty rugged and work well for most of my needs. Would be good for classes or group projects (boy scouts, robot clubs, etc) where each person could have their own meter. The thing I miss is a beep function when checking continuity. They have another version with the beep for a few dollars more. I've see them sold on the net as high as $18.99. Google for 830b meter.

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What're you doin' up in a tree, Ed??

I've noticed that the HP 3478A bench DMM that I recently bought is acting weird when I switch on a compact fluorescent desk lamp that sits next to it. The DC range goes higher, especially at low DC voltages. It's hard to tell, because it changes with where the test leads are, etc. Some of it might even be coming from the lamp, back thru the power strip, and into the DMM. All I know is that when I turn the lamp off, it settles down and gives accurate readings. I may have to go back to a regular blub. But I sure do like the desk lamp. It has two 13W "PL" CFL tubes and puts out a lot of light. Only $25 at OSH.

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Some seller on Ebay sells them for really cheap, and then nicks you for ten or fifteen bucks shipping. I've read some Ebay disclaimer that says they get complaints about sellers that overcharge for shipping. But they basically say buyer beware, take your biz elsewhere, they won't do anything about it. One thing I noticed is that these cheap meters all have AC ranges that go no lower than 200VAC. I wonder if that's because they want to cut corners and save money on switches, etc, or if it's because too many people put the meter on 120 or 240VAC and burn out the lower ranges.

CFL's send a bit of asymetrical, approximately 100KHz RF down the power cord. They can be exciting "friends" to have on a test bench.

The only time I have noticed my HP3478A being affected is on the low AC ranges, if I drape unshielded test leads over the lamp. It adds

50mv AC to the reading.

On even the lowest DC range I notice no effect from draping the test leads over the CFL.

I tend to use shielded test leads with my meters. The best arrangement I have found is a Pomona bananna plug to BNC adapter and a shielded Pomona test lead set with RG58 and a BNC connector on it. I have used the Pomona shielded test leads that go straight from the bananna plug to the coax to the test connectors, but I don't like them because the outer insulation on the RG58 pulls away from the bakelite bananna plug, and reveals the shield.

-Chuck

OBTW, if you have the 50/60Hz switch in the wrong direction, you will see some 60Hz sensitivity on the low DC ranges of your meter. Are you sure when you changed it a while back, that it was really in the 50Hz position?

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position?

Yeah, it was in 50 and I changed it to 60Hz. BTW yesterday I got my

*real* (as in HP p/n) 3478A Service Manual. I bid on it on Ebay and got it for $10 plus shipping. My 7th or 8th manual purchase from Ed Matsuda, but first from him on Ebay. You remember the 'Tucker's tax on shipping' debacle I went thru in April. Well, those Ebay/Paypal SOBs did it again! Their checkout charged tax on shipping! Ed said he didn't know how to change or correct it, so he sent the diff in change in the package. I swear on a stack of bibles! This godforsaken DMM is JINXED!

Try putting it back into the 50Hz position, it is possible that there is something wrong and an earlier tech "fixed" it that way.

As to eBay/Paypal and taxing shipping: logon, and vent your frustrations in their direction. They want to do things correctly, but like everyone, they make a mistake or two along the way. Sales tax rules differ from state to state. Some require tax on services, some don't. They must have CA in the wrong column. If you don't tell them, it might be a really long time before they get it in the right column.

-Chuck

The meters appear to be designed for beginner or casual users that would have little or no use fro the lower ranges like the clamp on AC Ammeters with a voltmeter function. As long as the HVAC guy or electrician can read something close to 24 volts, they are happy with the 200 volt range.

--
Former professional electron wrangler.

Michael A. Terrell
Central Florida

Many people get bit ONCE by the guys who clearly view "shipping and handling" as a profit center, but after that people learn to read the sellers' policies carefully! eBay certainly isn't the only place where this game goes on -- the "New York mail order photo equipment" places were notoroius for years for low-balling the advertised price of a camera and then charging you something ridiculous in shipping.

On the other hand, some places go to the opposite extreme -- Amazon.Com has "free" shipping on most orders over $25, but clearly they have to recover those shipping costs somehow, right?

I think it would be hard to eBay to police this in an automated fashion -- with all the various shipping companies, insurance options, destinations, etc., they'd almost have to employ a bunch of people whose sole job would be to look at each and every auction and decide if the S&H was "reasonable."

I figured it was becasue they wanted to use things like silicon bridge rectifiers and still be able to claim some reasonable percentage accuracy, but I like your idea about people burning out the lower ranges even better.

---Joel

:-) Nothing, for a while. I kinda "donated" a hamstring to the cause at last Sunday's softball game. But a treehouse, night camera, antenna, listening device, whatever zany experiment sometimes belongs in a tree.

Bummer. Maybe the lite belongs up in a tree. :-) What is OSH?

Ed

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for

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Amazon.Com has

recover

For years, I read that Amazon's biz model was to gain market share by cutting profits to the bone, in other words sell at a loss at times. And with gas at $2.40 a gallon and rising, it seems to me that free shipping is asking for trouble. It would make better economic sense to have a shipping surcharge to pay for the rising fuel costs.

fashion --

destinations,

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"reasonable."

I've never sold, but it seems to me that if they required the seller to fill out a form, with certain choices and/or dollar amounts, then it would take care of the automation aspect.

Another consideration is that (at least within Calif), the less the seller charges for the product and the more he charges for shipping, the less the state makes in sales tax. So it would behoove the states to make sure the sellers don't abuse this.

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"PL"

Need a loooong extension cord. You'll see that a lot in two weeks when the latest H.P. book "Half-blood Prince" comes out. Kids will spend all their time reading it in their treehouse. ;-)

I always wondered what would happene of you screwed around and did

or whatever. Tried it, and it didn't seem to make any diff. :-P

Same as

but it just automagically puts the rest in there for you.

Where are the CFL tubes relative to the solid state ballasts?

If they are running the usual 100KHz SS ballasts down in the base, and trucking the wires for the CFL tubes a dozen inches to the lamp head, that is likely to radiate much more trash than the usual CFL replacement for an incandescent bulb.

-Chuck

I haven't pulled one apart yet to find out. BRB...

Well, I pulled the base off one, and found two regular laminated E-I 'transformer' ballasts in the base. Three wires (not twisted) up thru the gooseneck to the tubes. No electronic ballast. But the waveform of the AC is still noisy because the tubes breakdown at some high voltage, so relatively quick risetimes. I saw no caps at all in the base. To help filter some of the noise, I may get a X2 or Y2 rated cap and solder it across the AC line in the base. It's zipcord with a two pin power plug.

Interesting. I have both a CFL (in a luxo lamp), and a regular fluorescent (in a Luxo magnifier) on my bench, and neither bother my HP3456A, or my HP3478A in the DC positions. They do bother both meters in the AC position, but that is expected.

I have to wonder if the 50/60Hz switch is working properly on your meter. What you are describing is exactly why that switch was put there.

-Chuck

Just got a new Harbor Freight flyer in the mail - they are on sale again for $2.99.

Ed

for

First time I'd been there in years. Probably be years before I go again. :-P