Habor Freight giveaway DMM test

[rhb]

My free gift of choice at HF is the DMM.  I've given away quite a few, but I have 8 of the current red versions.  I just tested 8 of them against an LM399 voltage reference.  I don't know the actual voltage, but my HP 3478A says is is 6.9113 V.

Edit:  I used a couple of 5% metal film resistors to make a 68K:1k divider for the 200 mV range.

#90899 #1 6.92 V  +0.14%      99.2 mV +0.02%
#90899 #2 6.93  V +0.29%      99.0 mV  -0.18%
#92020 #3 6.88 V -0.043%      98.8 mV -0.38%
#69096 #4 6.91 V   0.0%          99.2 mV +0.02%
#90899 #5 6.89 V  -0.29%       98.9 mV -0.28%
#98025 #6 6.96 V + 0.79%      99.3 mV +0.13%
#90899 #7 6.91 V   0.0%         98.8 mV -0.38%
#90899 #8 6.88 V -0.43%        98.8 mV  -0.38%
3478A   6.9113 V                       99.174 mV

There is a trimmer for adjusting the voltage, so I adjusted a couple of older ones I use on a regular basis to read 6.91 V.  I'm going to leave the red ones alone and test them again later.  These were sitting in an unheated space.  I let them warm up, but I'll let them sit a warmer space for the next test.

These all have the original batteries and some may be up to 3 years old. Claimed accuracy is 1% + 1 digit.  So all are well within spec.

I suspect that the AC performance is not as good, but I'll leave that test to another day.  I've got a DMMCheck Plus on order, so I'll make more tests when it arrives.

My reason for doing this is my concern that the voltnut aberration latent in many of us leads beginners on tight budgets to spend more on DMMs than they should.  If you're building things using 5% resistors and 10% capacitors you don't need a 0.01% DMM.

Edit: added HF item numbers.

Edit: For the benefit of non-USA readers:

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

[james_s]

Imagine how impressive those specs would have sounded 30 years ago, a DMM that good would have cost at least $100, which was a significant amount of money back then.

[rhb]

I have a  Radio Shack Micronta  DMM I  bought which now has a dead LCD.  IIRC it was about $70. I did have the good fortune to calibrate is against a high end calibrator before the LCD died.  Before that, my good meter was a 50k ohm Micronta VOM.

The biggest deficiency of the free HF DMMs is they are only 1 Meg input impedance, so if you want an accurate measurement you need to take that into account in many cases.

At just short of 65, I've finally loosened the purse strings and am buying things I've wanted for 40 years.  Monday my spectrum analyzer, an HP 8560A should arrive.  But I am acutely conscious of what it is like to not have access to the test gear you need.  My degrees are in English and geology, so getting access to an EE lab was not an option.

As I learned more, I realized that there are other ways of accomplishing tasks without fancy equipment.  But  a lot of people, my father was the worst, tell you all the reasons you can't, rather than how you can.  And more importna, failure is OK as long as you keep trying until you succeed.

[001]

what is "Habor Freight giveaway DMM"? Is it local US meme?

[james_s]

There's a store called Harbor Freight known for selling inexpensive Chinese tools, there's a similar place in the UK called Machine Mart. They often have coupons for a free item with purchase and one of the common items in the coupon is a multimeter that normally sells for something like $7.99. They're pretty handy as a backup meter, I keep one in my car that has come in handy on more than one occasion.

[001]

Is it "classic" ICL7103 based unit?

[james_s]

I've never opened one up but I'd bet money it's a "classic" black blob of epoxy over an unknown die unit. Who can say what the IC actually is?

[IanB]

I've never opened one up but I'd bet money it's a "classic" black blob of epoxy over an unknown die unit. Who can say what the IC actually is?

It's the most minimalist circuit board possible, with of course the obligatory black blob, and as few additional parts as ingenuity can manage. Every new revision has fewer parts than the previous one. One day, even the trim pots will disappear.

Nevertheless, if you adjust the trim pot carefully, the meters are rather stable and usable.

[rhb]

Here it is in all it's glory.  It uses an "IC1". I have them all over the place.  Toolboxes, battery drawer, etc.  I prefer the older yellow P30756 which used the rotary switch to turn it off.  I've had one actually die out of the large number I bought on sale for $2.99 before they started giving them away.  The biggest problem is the test leads breaking.

[james_s]

That is remarkably simple. I find it interesting that they have a pad layout for an actual IC there, I assume some of them must have had that instead of the blob.

[rhb]

Here are two older yellow models.  The first is the rotary switch OFF. The second is identical to the red one on the outside. It appears to be the same mold, just different color plastic.

[rhb]

I was fiddling with a wonky Fluke 8000A last night.  Probably dirty switch contacts, but I'm out of contact cleaner.  The Fluke is a 70's vintage 0.1% DMM. It's really not worth fiddling with except as an amusement.  People seem to ask ridiculous prices for them on eBay for some reason.  But it got me to thinking about the calibration of these things.

The traditional poor man's voltage reference is a new dry cell.  So I measured a few with my 3478A this morning.  The 3478A was last calibrated in 2004, so on an absolute basis the numbers don't mean a lot, but I thought they might be useful.  The trim pot on the HF DMMs will get them to within 0.1% with a little fiddle.  A 4-6 cell battery holder rewired to place the cells in parallel should do well.  I suspect that Conrad Hoffman's mini-metrology lab series mentioned this.  While technology has changed a lot, it's a great set of projects for the novice interested in electronics or anyone on a tight budget.

1.63105
1.63072
1.63071
1.63145
1.63118
1.63075
1.63112
1.63066
1.63113
1.63034
1.63101
1.63173

Mean: 1.630982
StdDev: 0.000396

It appears to me that this would be good enough to calibrate the HF DMMs to 0.1%.   I find that quite remarkable for the price.  They certainly did not give away Fluke 8000As with any purchase.

[rsjsouza]

Imagine how impressive those specs would have sounded 30 years ago, a DMM that good would have cost at least $100, which was a significant amount of money back then.

They were breakthrough - the Fluke 8020A cost US$169 in 1977 dollars (reference here). Well, 40 years ago.

I've never opened one up but I'd bet money it's a "classic" black blob of epoxy over an unknown die unit. Who can say what the IC actually is?

Due to the external circuitry and the availability of some models in PLCC package that fit the pads, it is certainly an ICL7106 which is a slight variation of the Fluke-designed IC (and surreptitiously re-sold by Intersil) for the 8020A above - good story here. 

It's the most minimalist circuit board possible, with of course the obligatory black blob, and as few additional parts as ingenuity can manage. Every new revision has fewer parts than the previous one. One day, even the trim pots will disappear.

They already did. Check: https://www.eevblog.com/forum/reviews/harbor-freight-cen-tech-90899-small-teardown/msg655719/#msg655719

[rhb]

FWIW I got a freshly calibrated DMMCheck Plus unit from Doug Malone today.  Despite the cal sticker saying  2004, my 3478A was spot on 5.0000 Vdc so the HF DMM absolute errors I calculated are correct.  Time to trim them all to 0.1%.  I am quite amazed.

[james_s]

It would be interesting to leave one in the toolbox, keep one bouncing around in the trunk of the car, one on the bench out in the garage, etc and check them all a year later to see how the stability has held up.

[rsjsouza]

To have a baseline, find some Flukes and leave them on the same places. Then you can have a more solid base for comparison!

[Brak]

They already did. Check: https://www.eevblog.com/forum/reviews/harbor-freight-cen-tech-90899-small-teardown/msg655719/#msg655719

Harbor Freight stuff has "Item Numbers".  There are at least 5 different item numbers for these free meters, I assume there are at least 5 different manufactories making them and they buy from whomever offers the lowest price.

I figured out how to add a trim pot to to item number 98025 if anyone's interested.

It worked for me, no guarantee.

[nanofrog]

Is it "classic" ICL7103 based unit?

They're commonly known as DT830/DT830B. Variations can include backlights or offer different means of turning it On/Off (some use a separate switch, and others use the range switch). Case colors vary too (seen them in black, yellow, blue, and red).

Off position on the range switch:


Separate On/Off switch (Cen-Tech is Harbor Freight's branding):


Separate On/Off switch + backlight button in upper left corner:


As per the IC, I think it may actually be a Fortune FS970X.

Regardless, they're usable in low voltage circuits and make great panel meters at little to no cost. 

[001]

Fortune chip is 5000 counts/ But dt830s are 1999

[james_s]

To have a baseline, find some Flukes and leave them on the same places. Then you can have a more solid base for comparison!

We already know the Flukes don't drift. There are many thousands of them in harsh daily service with field techs and industrial electricians.

[nanofrog]

Fortune chip is 5000 counts/ But dt830s are 1999

I noticed that as well.  Keep in mind however, it's possible to truncate digits. For example, some bench meters allow users to do this in order to obtain faster readings when additional resolution isn't necessary. 

FWIW, I found some schematics that indicated the IC was in fact the Fortune FS970X. Saw some others that indicated it was a 7106; assuming this is the Renasas/Intersil part (ICL7106), it's just an LCD driver that happens to be obsolete.

I'm currently not aware of another chip that would allow them to meet the price point these meters go for.  Would be interesting to know of if available though (I'm nosy).

[001]

ICL7106 DIP is still avaliable at local store for about $2

[Fungus]

Fortune chip is 5000 counts/ But dt830s are 1999

I noticed that as well.  Keep in mind however, it's possible to truncate digits. For example, some bench meters allow users to do this in order to obtain faster readings when additional resolution isn't necessary. 

If you google "DT830B schematic" the chip you see is ICL7106.

ICL7106 DIP is still avaliable at local store for about $2

There's multiple manufacturers of that chip so you can bet there's Chinese clones, too.

There's no way they can be paying more than a few cents for the IC in a giveaway meter.

[nanofrog]

Fortune chip is 5000 counts/ But dt830s are 1999

I noticed that as well.  Keep in mind however, it's possible to truncate digits. For example, some bench meters allow users to do this in order to obtain faster readings when additional resolution isn't necessary. 

If you google "DT830B schematic" the chip you see is ICL7106.

ICL7106 DIP is still avaliable at local store for about $2

There's multiple manufacturers of that chip so you can bet there's Chinese clones, too.

There's no way they can be paying more than a few cents for the IC in a giveaway meter.

FWIW, I hadn't seen any of the schematics (DT830B) using the ICL7106, but I didn't check all of them either. 

If there are clones of that chip that only cost a few cents (would expect so, but not aware of any), that would definitely make sense. As you say, these things are effectively giveaways, so have to be super cheap to manufacture (and still make a profit).

[nanofrog]

Fortune chip is 5000 counts/ But dt830s are 1999

I noticed that as well.  Keep in mind however, it's possible to truncate digits. For example, some bench meters allow users to do this in order to obtain faster readings when additional resolution isn't necessary. 

If you google "DT830B schematic" the chip you see is ICL7106.

ICL7106 DIP is still avaliable at local store for about $2

There's multiple manufacturers of that chip so you can bet there's Chinese clones, too.

There's no way they can be paying more than a few cents for the IC in a giveaway meter.

FWIW, I hadn't seen any of the schematics (DT830B) using the ICL7106, but I didn't check all of them either. 

If there are clones of that chip that only cost a few cents (would expect so, but not aware of any), that would definitely make sense. As you say, these things are effectively giveaways, so have to be super cheap to manufacture (and still make a profit).

The Chinese clone of 7106 in QFP is ~30 cents, so without a package, I bet the bare die is about 20~25 cents.

Out of curiosity, who makes it?

I ask as I've a really tough time figuring out Asian IC manufacturers and what they offer. So any clues/resources that would help figure this stuff out would truly be appreciated.