Cheap DMM Alert

[zombiwoof]

I often read posts by guys who don't have a Digital Multimeter (DMM), and I think it is essential in pedal building and modding.  For the guys that don't think they can afford one, I recently saw this one on sale for $5!:

http://www.parts-express.com/pe/showdetl.cfm?Partnumber=390-500

It reads AC and DC voltages, DC current, Resistance, and has transistor and diode test.  The only thing it doesn't have that mine does is a capacitance test, but it has most of the features that you need to do pedals.  At that price, I don't think anyone can say they can't afford one!.

I can't believe how cheap these meters are getting.

Al

[trjones1]

My dad gave me one of those cheapo DMMs when I was 12 and told me I should always have one because they come in useful.  I thought he was crazy and put it in the closet until I started building pedals almost 15 years later.  Then I realized he was right.

[defaced]

I use the elcheapo Harbor Freight 3 dollar DMMs for B+ measurements on tube amps.  Even a Fluke can't hold up to user error when you're measuring that much voltage. 

http://www.harborfreight.com/7-function-digital-multimeter-90899.html

[zombiwoof]

I use the elcheapo Harbor Freight 3 dollar DMMs for B+ measurements on tube amps.  Even a Fluke can't hold up to user error when you're measuring that much voltage. 

http://www.harborfreight.com/7-function-digital-multimeter-90899.html

That one looks to be almost the same meter, except it's red and the on/off switch is on the front (I don't see it on the black one).  Probably made by the same company, and even cheaper!.  Specs seem to be about the same.

Al

[blooze_man]

Here's one that can measure transistors

http://www.multimeterwarehouse.com/m810bf.htm

[PRR]

eBay always has heaps of $4 DMMs.

I've owned dozens of multimeters. 5 or 6 true VTVMs, so many passive meters, even a Heath 2.5 digit Nixie with the one-UJT ADC.

I like my $200 Fluke.

But I've examined the Harbor Freight $5 DVM and it appears to be more meter than most pedal or amp builders will ever really need. With today's technology, a cheap DVM can be reliable and accurate, just like $2 watches tell time (better than the Accutron!) and $2 calculators can figure taxes.

[Taylor]

Indeed, the 3 to 5 dollar DMMs are definitely good enough for the vast majority of DMM use, and everybody who builds pedals should have one. Mine hasn't crapped  out yet.

I recently grabbed one of these:

http://www.dealextreme.com/details.dx/sku.48194

What it has over the absolute cheapest ones is capacitance measuring, a bigger screen, and a continuity mode that beeps, which makes testing continuity quicker. It's only 12 bones.

[Hides-His-Eyes]

I swear there are only like 4 kinds all made in the same factory. My dad has one like that taylor; branded completely differently of course...

And I have one of those yellow "cheap but still with HFE measure" ones, and I kinda like it, but it doesn't seem to think I'll ever be able to change the battery; maybe it's secretly charging every time I measure 9v

[amptramp]

The resistance measurement on the one in the first post only goes up to 2 megohms.  Some people use higher input resistors and some LFO's from tremolos. phasers and chorus units use higher values than that.  I have one DVM that goes to 20 megohms and another that goes to 30 megohms that can do everything I want because the largest standard resistor value is 22 megohms.  Be sure the DMM will do all the measurements you want or you will just have to get another one later.

[LucifersTrip]

...and the winner goes to:

http://cgi.ebay.com/New-LCD-Digital-Multimeter-DT830B-not-include-battery-/220628109061?pt=LH_DefaultDomain_0&hash=item335e75c705

.99 + $2.02 shipping.

A friend just got his in the mail last week and it's working fine...so far.

[ayayay!]

This one at HF is only $20 AND it has a cap tester built in.  Only goes up to 20uF, but I have one and love it.  

http://www.harborfreight.com/ac-dc-digital-multimeter-37772.html

[Taylor]

The resistance measurement on the one in the first post only goes up to 2 megohms.  Some people use higher input resistors and some LFO's from tremolos. phasers and chorus units use higher values than that.  I have one DVM that goes to 20 megohms and another that goes to 30 megohms that can do everything I want because the largest standard resistor value is 22 megohms.  Be sure the DMM will do all the measurements you want or you will just have to get another one later.

True, but I have to say that I have never used a resistor larger than 10m in all the circuits I've ever built. My $12 one goes up to 2 giga-ohms (2000m); needless to say I haven't had a use for that mode.

This one at HF is only $20 AND it has a cap tester built in.  Only goes up to 20uF, but I have one and love it. 

http://www.harborfreight.com/ac-dc-digital-multimeter-37772.html

The $12 one I linked has a cap meter that goes up to 200uf.  But yours looks like it has a frequency counter? That's cool.

[dedguy]

If you get HF flyers once in a while they have a coupon for the meter, and you can get it for FREE!!!!!!!!!  I have one of those and a couple of LED flashlights.  If it's free its for me 

[frank_p]

I have only cheap DMMs, but I realy can't measure Vac over 100 kHZ.  Can a 200$ Fluke (or other brand) will get more precise on measuring Vpp at those relatively high freq. ...  

[PRR]

> I realy can't measure Vac over 100 kHZ.

Many (maybe older) DVMs loose accuracy above 400Hz.

If you routinely measure higher-frequency voltage, you want an ACVM. IIRC the old Boonton barely made 100KHz, but that was 1938. IIRC the H-P transistor ACVM (VERY pretty machine!) was 400KHz.
post-War Boonton: http://www.radiomuseum.org/images/radio/boonton_electronics/sensitive_rf_voltmeter_91h_699986.jpg

You can read larger voltages (several volts) with any DC meter with a diode+cap detector. We used to put an acorn-tube in a film-can with 5-core wire; crystal diodes make this much easier. Silicon gives a half-volt lag, Ge somewhat less, so as your level gets down below a few volts you will have to calibrate a curved-scale (or lookup table). Or use a flat amplifier to bring level up to a few volts. A calibrated (and flat!) attenuator also removes rectifier/meter linearity error.

Meter measurements:
http://gilmore2.chem.northwestern.edu/ubb/showpage.php?fnum=3&tid=6911
Essay on hi-F measurements:
http://www.tech-diy.com/TestEquipment/TrueRMS/TrueRMS.htm

[frank_p]

Thanks Paul, I am Out of topic so I am going to check that out.  The problem Is that I am working on resonant circuits and it's not in audio range.  None of my measuring equipnents give a similar results.  I 'll come back later on what you said. Thanks.
FHP

[frank_p]

We used to put an acorn-tube in a film-can with 5-core wire; crystal diodes make this much easier. Silicon gives a half-volt lag, Ge somewhat less, so as your level gets down below a few volts you will have to calibrate a curved-scale (or lookup table). Or use a flat amplifier to bring level up to a few volts. A calibrated (and flat!) attenuator also removes rectifier/meter linearity error.
Meter measurements:
http://gilmore2.chem.northwestern.edu/ubb/showpage.php?fnum=3&tid=6911
Essay on hi-F measurements:
http://www.tech-diy.com/TestEquipment/TrueRMS/TrueRMS.htm

I am working on LC resonance and I realised that my mesurements were way off with my calculations (DMM and scope).  Maybe I should *downgade* what I am doing at lower frequencies before investing in exotic equipments...
After all, it's math vs measurements that are fighting here.

++acorn-tube in a film-can with 5-core wire; crystal diodes make this much easier. Silicon gives a half-volt lag, Ge somewhat less, so as ++your level gets down below a few volts you will have to calibrate a curved-scale (or lookup table).


I would like to learn abouth that...  I'll really got to search on that...  Enclosed specialised tubes with special wires... Crystals...

I don't have problems with adjusting curves.  But thanks Paul about looking elsewhere for my  relatively simple problems.

What you are saying to me is that any active device implied in measuring ac over 500 hz have to be specialised active devices, or otherwise , rectified and *translated*.  In other words AC mesurements on an ordinary DMM is doomed if it's not in *60Hz...* (I am exagerating here but...)

Thanks Paul
FHP

[zombiwoof]

...and the winner goes to:

http://cgi.ebay.com/New-LCD-Digital-Multimeter-DT830B-not-include-battery-/220628109061?pt=LH_DefaultDomain_0&hash=item335e75c705

.99 + $2.02 shipping.

A friend just got his in the mail last week and it's working fine...so far.

That one also looks a lot like the $5 one I posted about.  For a buck?.  At that price, you buy 5 or 6 of them and treat them like Bic disposable lighters.

Al

[PRR]

> doomed if it's not in *60Hz...* (I am exagerating here but...)

Yes, exaggerated. For good couple-percent accuracy at 50/60Hz you usually need "some" several-dB response to near 400Hz. You are pretty sure to get that much.

The basic DVM is 200mV DC. The basic diode is ~~600mV threshold. To get error down below 10% of full scale, 20mV, we need gain of 30; for 2mV error (1% full scale or 10% at 10% of full scale) we need gain of 300. Taking a basic cheap low-power opamp, we get unity gain at say 300KHz, gain of 30 won't happen above 10KHz; gain of 300 is gone by 1KHz. We still meet an excellent spec for power-line readings, but bad for audio.

The acorn-tube is obsolete, a joke. When most tubes were large Octals, and the large size and internal lead-length gave trouble by 30MHz, someone needed 50MHz-100MHz tubes. Minature plate and grid, two small glass "bells" pressed together with wires straight out the seam. About the size of an acorn. Although made for amplifiers, they can be used as diodes.



Socketing an acorn tube was ugly. The next step was the "miniature": small but with all the legs in the bottom. With a few refinements, minis are useful far past 100MHz.

For minimum lead-length in the RF circuit, you put the diode _AT_ the circuit being tested. With tubes, that is awkward. Now we have crystal diodes: notably Silicon and Germanium.

Radiotron Designer's Handbook, Third Edition
http://www.tubebooks.org/Books/RDH3.pdf
16MB PDF file

Radiotron Designer's Handbook, Fourth Edition
http://www.tubebooks.org/Books/RDH4.pdf
25MB PDF file

RDH 3rd is more suited to your needs. Try pages 250++.

[StephenGiles]

Talking of stuff on the cheap - I recently bought some "George" brand shirts from Asda in the UK for £3 each. They don't shrink when washed and look good with a tie. If they last until Christmas I'll be happy!!