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A hard to resist problem

Discussion in 'General Electronics Chat' started by Justin Verrall, Dec 22, 2015.

  1. Justin Verrall

    Justin Verrall Member

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    I got some 1/2 watt resistors from a shop called jaycars here in New Zealand and then I wanted to get a lot more and since I'm a cheep student I didn't want to spent a whole lot of money at jaycars I jumped on eBay to get more 1/2 watts for cheaper as I'm sure plenty of people do but when I received them today I found that they were 2x bigger then the ones from jaycar so I checked the packaging on both the jaycar resistors and the eBay ones and tho only difference that I could find was that the jaycar ones were labelled 0.5w and the eBay ones were 1/2w... Correct me if I'm wrong but 0.5 and 1/2 is the same thing right? So why are the eBay resistors bigger? I'll post an image of them side by side too show you what I mean.

    [​IMG]
     
  2. zahwi

    zahwi Member

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    The small resistors get hotter and could be made of more temperature stable metal so they can stay within range at higher temperature.
    If the lead is bigger diameter it uses the PCB as heatsink.
     
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  3. Justin Verrall

    Justin Verrall Member

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    Where can I get the smaller ones for cheap and what are the smaller resisters called?
     
  4. dave

    Dave New Member

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  5. zahwi

    zahwi Member

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    Resistors come in many sizes depending on manufacturer. You can get them cheap on ebay. Most ebay dealers specify the wattage only, you could be able to email them and ask for the dimensions. Check the price of RS or Farnell (I assume you have them in NZ) they give the size.
     
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  6. Justin Verrall

    Justin Verrall Member

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    Cool! Thank you very much!
     
  7. spec

    spec Well-Known Member Most Helpful Member

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    Hi Justin,

    Welcome to ETO,

    Resistors come in different types not just power rating. The different types have different power rating for a given size.

    The main types are carbon composition, carbon film, metal deposit, tin oxide and metal film. There are also wirewound and ceramic for high power.

    Looking at your images, I would say that the smaller resistors are good for 250mW and the bigger 500mW. The specifications for resistor power can be measured in differnt ways.

    You can get resistors from many sources, but they are so cheap that it is best to get them from a reliable source.

    I you want to know more, post tommorow. I am going to bed now.
     
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  8. Justin Verrall

    Justin Verrall Member

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    I'm home and I've taken a photo of the 1/4 watt, "Supposed" 1/2 watt and the bigger 1/2 watt and I've taken a photo next to a ruler and it seems that the 1/4 and the small 1/4 are the same size but I did find the pack that the small 1/2 watt were in and it said it was "mini" could that be the reason it's so small? Resistor.png
     
  9. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    There are other characteristics which can affect vendor choice of body size such as Vmax, flame proof coating
    They do come in different sizes. What voltage rating do you need? 250? 300? 500? 1600?
    http://www.digikey.com/product-sear...=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=25
    A homogeneous film of metal alloy is deposited on a high grade ceramic body. After a helical groove has been cut in the resistive layer, tinned connecting leads of electrolytic copper are welded to the end-caps.
     
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  10. spec

    spec Well-Known Member Most Helpful Member

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    Hy,

    Now that you have indicated the resistor size I can assure you that smaller resistors are 125mW and the bigger resistors are 250mW; neither are half watt.

    The reason why the issue ever arose is because of the vendor’s loose and unqualified description. The only way to establish what a part can do is to get the exact part number and check the datasheet. The trouble is that many second-line retailers sell unbranded and unspecified components. Their description are also loose to say the least. The rule is never buy anything until you have established its specification- vendors descriptions are riddled with errors anyway (not so the mainline vendors like RS, Farnell, Mouser etc, but, even they make the odd mistake). To make matters worse there are even errors on data sheets, but rarely.

    The other thing is that the rating of any component depends on the conditions. For example, the rating of a resistor may be 1W, but that may be at a body temperature of 25 deg C. In practical terms, unless the resistor were immersed in a bath of oil at around 10 deg C, this is a meaningless specification. What you need is the resistor dissipation in free air at 25 deg C.

    But, if the resistor is mounted in your equipment and the air in the equipment is 50 deg C, a fairly normal temperature, then a 250mW resistor may become a 125mW resistor. The controlling factor, with all electronic components, is the maximum temperature of the active part: in the case of a resistor, the ceramic insulating tube (body) and the spiral film (resistor) deposited on it. With a transistor the active part is the semiconductor junction, which typically must not exceed 170 deg C or the transistor will be destroyed or permanently damaged.

    As has already been said, there are a number of different resistor types: some information here: http://www.electro-tech-online.com/threads/transistor-equivalent.146091/page-27 Post 530.

    I’m not sure how much you know about electronics; just in case, here is a bit of useful basic theory about resistor power dissipation:

    (1) The power dissipated in a resistor is: the current flowing through the resistor squared, multiplied by the resistor value. So for example, if you had a 10 Ohm resistor with 2 Amps flowing through it, the power dissipation in the resistor would be (2 * 2) * 10= 40W

    (2) The power dissipated in a resistor is also: voltage across the resistor squared divided by the resistor value. So for the same example, if you had a 10 Ohm resistor with 20V across it, the power dissipation in the resistor would be (20 * 20) / 10= 40W

    (3) The power dissipated in a resistor is also: current flowing through a resistor times the voltage across the resistor. So again for the same example, the current flowing through the resistor would be 2A and thus the voltage across the resistor would be 20 Volts. Thus 2 * 20 = 40 Watts again.

    There is a simple way to check if a resistor is OK as far as power dissipation goes: put your finger on it- if you can’t quite keep it there that is around 70 deg C and is a good design maximum surface temperature; anything higher is unwise. This is not the case for certain power resistors which can run at around 100 deg C.

    After all that, I bet you wished you had never asked. I will get my coat now. :happy:

    PS: you can check for yourself what wattage ratings your resistors are by using the formlae above and the finger test!
     
    Last edited: Dec 27, 2015
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  11. Justin Verrall

    Justin Verrall Member

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    Oh wow thank you! I'm about as new to this as you can get so any info/resources that will teach me how to create guitar pedals as well as be able to tweak there sound will be more then appreciated! :)
     
  12. spec

    spec Well-Known Member Most Helpful Member

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    My pleasure Justin,

    You are @ the right place. I have two axes and have had some amps, but only a practice combo now. Have designed and built many weird things. Long-term project is a guitar pickup: individual low turn coils for each string and built in opamps for each of the six channels- automatic hum bucking without loss of hi frequencies. Another long-term project is electronically controlled tuning and tremolo bridge. I had better stop now.. again
     
    Last edited: Dec 27, 2015
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  13. Tony Stewart

    Tony Stewart Well-Known Member Most Helpful Member

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    Although everything spec explained is good, they "could" both be 1/2w with examples of both sizes in my link above.

    Lessons to be learnt:
    eBay is a good place to buy surplus and and/or off spec reject parts, but without a MFG Part Number and datasheet, you never know for sure what you get. eBay seller never seem to give datasheets or a guarantee to all specs. Ok for cheap, bad for certainty. Data sheets are more reliable documents and also part of a purchase order, which is treated like a legal document in professional business using specific MFG name and PN. But you must understand the fine print.

    For Pd=0.5W =V²/R= I²R
    This is often rated at max body temp with free air at 125'C, so derating 30-50% to keep cool is better.

    Notwithstanding, your 220K Res. would need V=√(Pd*R) = √(0.5*220k)=332V which means the body length or material insulation is affected by V rating .
    ergo ... two sizes depending on Vmax.

    If connecting to AC line , remember, √2*Vrms is just the peak max but does not include 3kV transients often used for line rating safety for withstanding voltage ratings.

    Learn to use Digikey, Mouser, Farnell for looking up parts with filter keywords and drop down menu then compare data sheets and understand them.

    Also learn to write yourself design goals with testable values.
    Use the search engine "images" for schematic searches, but treat them as ideas and judge quality of source. Was it built by a rookie or a pro with test results and layout of design?
     
    Last edited: Dec 27, 2015
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  14. spec

    spec Well-Known Member Most Helpful Member

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    Quite true Tony- there is a lot more to component selection than I said, but it is still a good rule-of-thumb (or finger) and will keep you out of hot water and I wanted to keep it simple.

    Yes, you can buy components anywhere if you have the experience to know good from bad, but my view with resistors, because they are so central to a design and because they are so cheap, is to buy from the mainline distributors, and make sure there is a manufacturer name, part number, and data sheet available.

    There have been quite a few stories going around where people have bought a ton of resistors cheaply, from ebay especially, and found that the tolerence is way out. For someone just starting out in electronics this can lead them astray badly- they normally have enough of a task getting the first few circuits going, even with perfect components.

    As an aside a little, story. My first venture into electronics went like this:

    As a teenager I could weild a soldering iron and do the practical stuff- chassis, fit tag strips etc, but apart from knowing there was this Ohms law thing I had no idea.

    I started to collect bits and pieces: strpped dpown radiograms, scap TVs, government surplus... My Dad brought home a box of bits that were being junked at work. In the box were little packets of resistors, all brand new and organised in decade ranges, so I chucked the old used resistors that I had collected and intended to use the nice new ones.

    At the time transistors cost the earth, but about 6 months later my Dad gave me a couple of OC71, or similar, germaium transistors to play with- magic!

    My first projet was a traditional two transistor differentiating multivibrator. I didn't have a scope, but I figured if I made the timing long enough I could see the collector voltage jumping up and down as the multivibrator fliped over. I built a beautiful little circuit, all neatly laid out, taking particular care with the transistors: hold the lead near the can with a pair of pliers while soldering to prevent the heat damaging the germamium pellet

    Then came the day for testing. I had blown a week's pocket money on a 9V battery so I was all set to go. But nothing. So I started investigating with an ex USAF meter that my Dad reluctantly let me borrow. I just could not make head nor tail of the voltage readings- they just did not make sense. After hours of messing about, I came to the conclusion that electronics was far too difficult for me and didn't do any for about 2 years- I still read the mags and had an interest in hi fi; speakers I could deal with, and record decks.

    Quite some time later when I knew a bit more theory, I came across my little multivibrator circuit- the transistors had been given back to my Dad- but I made a few measurements and all of the resistors were open circuit. When my Dad got home from work I told him about the open circuit resistors he had given me. He smiled and said, those resistors would be open circuit- they are capacitors! My career in electronics has followed similar lines :(
     
    Last edited: Dec 30, 2015
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  15. Justin Verrall

    Justin Verrall Member

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    Awesome!! Can you suggest somewhere to start please? I have no clue! I'm currently saving up to get a few more bits of equipment one of them being a laser printer because it seems that's ink jet printers can't be used to print PCB layouts as far as I can tell.
     
  16. spec

    spec Well-Known Member Most Helpful Member

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    Issue: 04 (2016_01_14)

    Hy Justin,

    Sure can advise how to start- it is a common question. Advise you hold the purchase of the laser printer for now.

    I will put together a list of how to start designing and building electronic equipment. It needn't be expensive.

    A rough list (I will tidy up/fill in) of items needed to get into most fields: linear, digital, power. As I understand it, you are focusing on electronic music associated circuits, so not all the list will apply to you:

    Oscilloscope

    An oscilloscope is in it's own category and the decision to buy one very much depends on your objectives. No instrument will teach you more about electronics than an oscilloscope. Just leaning how to set up the trigger, adjust the X gain & the Y time base will give you great experience. It is the closest that most people will ever get to seeing what the electrons are actually doing and when your first circuits don't work they are the quickest way to locate the problem. There is only one reason why an oscilloscope is not in the essential list: cost. But scopes are much cheaper than they used to be but are still relatively expensive.

    There are basically three broad categories of scopes: real-time, storage and digital. (to be expanded)

    (1) Essential
    (1.1a) electronics reference book/epub . The Art of Electronics
    (1.1b) Notebook (A4 size)
    (1.1c) PC
    (1.2) soldering iron suitable for fine work
    (1.3) precision wire cutterrs- I will recommend- not cheap, but not overly expensive
    (1.4) precision fine nose pliers- as for wire cutters
    (1.5) multimeter with good ranges- Fluke model 15A+ or model 17A+ (15A+ with temperature measurement). Ensure protective case and test leads are included. But see more adavanced multimeter below in Luxury section as cost is not much more.
    (1.6) bench power supply adjustable: 0V to 30V, 0A to 10A switch mode
    (1.7) resistor set- type and ranges to be defined- you have some already
    (1.8a) capacitors solid: 100nF ceramic: (for decoupling)
    (1.8b) capacitors solid polyester (pollycarb much better but expensive): 1nf, 10nF, 100nF (1uF luxury item)
    (1.8c) capacitor precision (for filters & effects circuits):
    (1.9) capacitors electrolytic low ripple: 10uF, 47uF, 100uF, 470uF, 1,000uF
    (1.10) capacitor elytrolytic high ripple (for PSU etc) 4,700uF
    (1.11a) BJT small signal: NBJT=BC546, PBJT=BC556
    (1.11b) BJT low power: NBJT= ???, PBJT=???
    (1.12) BJT medium power: NBJT=TIP41C, PBJT=TIP42C
    (1.13) BJT high power: NBJT=TIP35C, PBJT=TIP36C
    (1.14) FET: NFET= PFET=
    (1.15) MOSFET medium power: NMOSFET=, PMOSFET =
    (1.16) MOSFET high power: NMOSFET= FQP30N061, PMOSFET = FQP27P06
    (1.15) opamp jellybean, dual: LM358
    (1.16) opamp high performance: OPA192
    (1.17) opamp audio: TIL072
    (1.18) variable resistor log: 10K log
    (1.19) variable resistor lin: 100R, 1K, 10K, 100K, 1M
    (1.20) strip board
    (1.21) wire
    (1.22) diode small signal: 1N4148
    (1.23) diode medium current: 1N4007 (1KV), or 1N4006 (800V)
    (1.24) diode Zener low power: BZX84 series: 3V3, 4V7, 8V2
    (1.25) voltage regulator: VREG+ = LM317, VREG- = LM337
    (1.26) timer IC: LM555
    (1.27) Soderwick (fluxed braid for removing solder)
    (1.28) flux
    (1.29) IC socket, turn pin:
    (1.30) surface mount to DIL header board:
    (1.31) digital ICs: selection from 4000 CMOS family
    (1.32) bench lamp
    (1.33) hand pliers
    (1.34) LED, red
    (1.35) switch toggle single pole change over
    (1.36) switch push to make single pole.

    (2) Desireable
    (2.1) eye loup x20 magnification
    (2.2) workbench/table
    (2.3) small vice around 3.5 inch wide jaw
    (2.4a) screwdriver set, flat and posidrive
    (2.4b) screwdriver set jewlers, flat and posidrive
    (2.5) small spanner set metric
    (2.6) allen key set metric and imperial
    (2.7) bolts, nuts and washers: 1.5mm to 5mm
    (2.8) junior hacksaw + good make blades
    (2.9) files: large, medium and keyhole
    (2.10) twist drill set ( 1mm to 5mm)
    (2.11) hand drill brace
    (2.12) scalple
    (2.13) scientific hadheld calculator
    (2.14) heatsink compound
    (2.15) heatsink insulator mica: TO-3, TO-220, TO-247
    (2.16) insulating washers: 1mm to 5mm
    (2.17) epoxy glue
    (2.18) super glue
    (2.19) contact adhesive
    (2.20a) fibre-glass matt and resin
    (2.20b) PinkGrip: filler adhesive
    (2.21) mains plug & socket IEC
    (2.22) mains cable 10A
    (2.23) speaker cable (cheap) Not mains flexible mains chord, but mains 20A house wire is good.
    (2.24) phono plugs & sockets, gold plated
    (2.25) binding posts (screw terminals): red, black, green, yellow
    (2.26) switch rotary
    (2.27) battery holder for all battery types owned
    (2.28) metal ruler: 6", 12", 18" (dirt cheap)
    (2.29) set square 6" (get good quality or waste of time)
    (2.30) centre punch
    (2.31) wet and dry emry paper: fine to coarse
    (2.32) light oil
    (2.33) engine oil
    (2.34) grease lithium
    (2.35) grease silicone
    (2.36) Locktite thread locker
    (2.37) varnish clear polyuathane, yacht
    (2.38) chord, lacing
    (2,38) tape, Gaffer
    (2.39) tape, insulating
    (2.40) tape, aluminium
    (2.41) tape, transformer yellow
    (2.42) tape, self amalgamating
    (2.43) sleeving rubber
    (2.44) sleeving, heat shrink
    (2.45) battery, 9V pp3 rechargable
    (2.46) battery, 1.25V AA, rechargeable
    (2.47) battery, 4.7V LIon, CR3
    (2.48) battery 4.7V LIon 8550
    (2.49) battery button, silver oxide 3V: Cr2032
    (2.50) charger, smart for all battery types owned
    (2-51) cleat, cable
    (2.52) moving coil meter movement +- (cheap)
    (2.53) relay, 12V, 20A, 1 pole change over, auto 80 Ohm coil

    (3) Luxury
    (3.1) ECAD software- EAGLE is free
    (3.2) simulation software- LTSPICE is free
    (3.3) signal generator- not expensive
    (3.4) plugs and sockets
    (3.5) resistors precision: +- 0.1%
    (3.6) voltage reference precision +-0.1%: ADR5041
    (3.7) SCR
    (3.8) TRIAC
    (3.9) Current transformer
    (3.10) High-end digital multimeter: Extech EX540 (Digitech QM1574 same)

    (4) Decadent
    (4.1) small pillar drill (not expensive)
    (4.2) metal bender
    (4.3) bench, metalworking
    (4.4) vice large, engineers
    (4.5) metal working lathe, small
    (4.6) welder TIG AC (for welding aluminium)
    (4.7) gas blowtorch
    (4.8) heat gun
    (4.9) hand electric drill, variable speed
    (4.10) tin snips
    (4.11) solder station
    (4.12) microscope optical (expensive) or electro-optic (reasonable cost)
    (4.13) Dremel minature shaper
    (4.14) BJT extremly high power: NBJT= MAG6332, PBJT= MAG9412
    (4.15) MOSFET, high power, high voltage: NMOSFET = BUZ906, PMOSFET = BUZ906
    (4.16) BJT high power HiFi: NBJT = MJL3281A, PBJT = MJL1302A

    NOMENCLATURE

    BJT
    MOSFET
    NBJT
    PBJT
    NMOSFET
    PMOSFET
    SCR
    TRIAC
    ECAD
    EAGLE
    SPICE
    LTSPICE
    TIG
    LIon
    PC
    IEC
    TO-3, TO-220, TO-247
    A4
    LED
    jellybean
    IC
    CMOS
    CMOS 4000 family
    VREG
    Zener
    electrolytic
    turn pin
    PSU
     
    Last edited: Jan 14, 2016
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  17. Justin Verrall

    Justin Verrall Member

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    I honestly thought you were just going to point me in the direction of a good book but this is far more practical and much easier to get my self going. I'm going to save it to a word document in case someone else I meet down the road could use it.

    I'll go through the list check off all the things that I already have and (as long as you are okay with it) link you to the options that I have in New Zealand for different specialty tools (like the multimeters http://www.jaycar.co.nz/search?text=multimeter).

    I'm a bit slow at typing so I'll take some photos and upload them to my photobucket so you can see what I have so far I don't think you will be able to see what the small parts are exsactly but if you ask me what something is I will have a better look and tell you:
    <iframe width="480" height="360" src="http://s1226.photobucket.com/user/Arkanim/embed/slideshow/"></iframe>
     
  18. spec

    spec Well-Known Member Most Helpful Member

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    Hi,

    glad you found the long list of some use and not too daunting.

    There are heaps of books around, but none I have seen yet hit the sweet spot of simplicity and practicality. With a bit of easy-to- learn basic theory, you can do your own designs, some pretty sophistoicated. Like everything in life, it is knowing the fundamentals that is enabling.

    Just a silly example. When I started playing golf I could never get out of the sand in a bunker. I would swing away in clouds of sand and the ball just would just roll forward a few inches, so my swing count went up and up- I felt a right dick and my mates laughed their heads off.

    But then we had a couple of lessons from a golf pro. He explained that you do not try to hit the ball with your club head as you would do on the freeway. No, instead, in sand, you hit down into the sand about an inch in front of the ball. I gave it a try- still a great cloud of sand but the ball popped up out of the bunker, no problem. Now, I have even holed from a bunker. If it hadn't been for that simple piece of advice I would still be thrashing away in the sand with no result. Now- I quite like bunker shots.

    It was the same thing with writing. I used to stuggle like hell getting a report done until, one day, an old hand in the technical publications department at work took my spaggeti proposal and turned it into silken text in about half an hour. Just like the bunker shot, I learned the techniques and now writing of any sort is no probs.

    What I cannot get with though is the artistic side of life: dancing, singing, painting, playing a musical instrument (can't play my guitars), spelling- to me those are a complete mystery.

    The list looks onerous but it is not. I have had an intro to electronics piece in my mind for years now. I plan to cover the whole thing, but in simple terms. The headings I have in mind are:

    Safety
    Theory
    Designing
    Documenting
    Test Equipment
    Electronic Components
    Tools
    Materials
    Board layout
    Mechnical (chassis, case etc)
    Building
    Testing
    Faultfinding
     
    Last edited: Dec 28, 2015
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  19. Justin Verrall

    Justin Verrall Member

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    I guess what I should do then is get a multimeter. Do you think this one would be good ( http://www.jaycar.co.nz/Test-&-Meas.../Cat-III-Multimeter-with-Temperature/p/QM1323 ) to get me started or is it something that I need to spend a bit of money on?
     
  20. Justin Verrall

    Justin Verrall Member

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    Also what about playing the guitar/music are you finding hard? I might be able to help or at lest give you some info that might point you in the right direction.
     
  21. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    Not a bad multimeter.
    It has auto-off which is good.
    Accuracy specs are crummy - not a real problem
    Lacks a bar graph display - Useful at times
    Not TRMS - not a big deal
    Non-standard input Z (7.8 M vs 10 M) - dunno
    They don't list the voltage burden for current

    Biggest missing part is lack of a bar graph display. Best good part is auto-off and auto/manual ranging.
     
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