Meter Checks

If you mean to use a standard DMM then the checks you will be able to make will be pretty basic, such as resistance of windings, diode forward drop etc...

For passive components such as inductors and capacitors you will need an LCR meter.

For the active devices such as SCRs and especially ICs you will need specialised equipment.

I really think you should first of all identify what it is you want to 'check' with each of these components, then come back with those details and ask again...

John.

Yes, especially how to check an IC is like saying how do you check a car. There are a lot of different systems in there..

well i am thank ful for your reply!

if a TV (or any circuit board) main board gets faulty and if there are certain components eg. capacitors, transistors, diodes, etc installed on it.

if i want to locate which particular part has gone faulty and where is that located..

then what shall i be doing then?.... start checking with a digital multimeter(i have that only) each component? its continuity or what?... powered on or off?

whats the thumb rule for checking or knowing that a particular electronic componet malfunctions so that replacing that could be made easy!!

Thanks gain

waiting for the reply

this guest is me NAUSHAD

THANKS

The "thumb rule" is very very simple.

"know what you are doing"

'Cause if you don't, there is no meter in the world than can help you...

Wangito.

Diagonosing such sophisticated Cct Bds requires full theoratical skill & there you will learn how to repair & trouble-shoot electronic Ccts & systems.

Checks with DVM & Analogue meters differ for continuity tests [eg + & - terminals in DVM has on-terminals same polarity as on terminals markings while in analogue meter it is opposite to terminal markings]

Large capacitors will show short-Cct as they take much longer time to charge & confusion occures as short-Cct.

So it is of little use to check thousands of components on-board except losing time.

Now it is the IC era & most of the equipment need thorough theoratical knowledge & Practical skill which needs time labour.

If you wish to test everything except the ICs then I suggest you purchase a component tester. These can test resistors, inductors, capacitors, diodes, most types of transistors and small batteries. They cost about the same as a good DMM and can be a great help. The only problem is that you will almost certainly need to lift most components out of the circuit to test them and this sort of defeats the purpose in most cases.

If you have a circuit diagram then all you need are a DMM and an oscilloscope....

If you are an engineer you will be aware of the hazards of working on a live circuit, so I wont repeat them.

With the circuit diagram you should also be able to tell roughly what voltages and waveforms should be where and so you can trace a signal and the dc voltages around until you find the fault.

John.

What is your extent of training in electronics?

Don't discount the all important visual check. If a component looks bad, it probably is. Many problems occur in electronics due to overheating, shorting, etc. which often will show up externally on the component. If it looks blackened, or has a brown haze on it, looks melted, or has an obvious broken lead, you may have just found your problem. Leaking fluid or other substances can be a bad sign also. Cracks, bulges, just about any irregularity could be a bad sign as well.

It may or may not be your only problem, but a damaged component ought to be replaced anyway, even if it has not failed, because it may likely be the next one to go!

Also, look for areas of poor soldering, bridges, gaps, cold solder joints. The problem might have originated in manufacture or assembly, but only show up later in use after being subject to heat, cold, or vibration. Connectors that are used often are also sources of trouble. The trouble may be in the mating contacts but often are also found in the attachment of the connector to the circuit. Sometimes a bad joint or connection will work until it oxidizes some time later. These things can be easily checked with a DMM as well. The most frustrating problems are the ones that occur sporadically, i.e. now it works, now it doesn't, now it works again!!!!!!! Check for resistance, not just continuity.

This is all good advice, but it only touches the tip of the iceburg.

Depending on the complexity of the circuitry, there are too many variables involved to accurately be able to diagnose most circuit malfunctions with only a DMM. I am a professional electronics technician and have been so for 30+ years, and I must say that I would be lost without : oscilloscope, power supplies and meters along with other specialized equipment that I need (some of which I had to specifically design

myself.) As to troubleshooting with Power Applied ; Extremely Dangerous even if it is low voltage. A 5 volt high current supply when contacted with metal such as a ring on your finger can literally melt the skin off if not worse. High voltages can kill.

Use the Electricians rule: "Aways work on high voltage with one hand in your pocket"

(not literally, but keep in mind) you do not want to be across the voltage with your hands-it will pass directly through your heart and stop it.

What type of circuitry are you working on? Again I ask what is your extent of education in electronics or are you just curious and wanting to learn electronics?

Either/or--Please Be Careful!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

PS- I just re-read this thread and did you say that you are working on a TV circuit and wondering if you should APPLY POWER??

If you are still alive--GO TELL YOUR PARENTS WHAT YOU ARE DOING AND DON'T TOUCH ANYTHING EVEN IF UNPLUGGED--CRT's CAN RETAIN LETHAL VOLTAGES!!

Very true...... I used to work at Marconi designing oscilloscopes and spectrum analysers etc...

It is amazing how the capacitance of a glass tube (a CRT) can hold a charge enough to knock you off your feet and daze you for a while.....

Then even when you discharge the thing with a screwdriver or whatever its also remarkable just how much charge can recover from a 'discharged' capacitor - certainly enough to make you wish you had left the short in place a while longer!!!

Seriosly though, if you're not an engineer as I have assumed you should be reading these boards, then you shouldn't be messing around with any live circuits as Steve-o said.

John.

dear participants!

i am extremely thankful to all of you for your replies/suggestions!

many asked about my qualifications.. well ! i am a fresh graduate electrical engineer, but i am intrested in electronics.

my precise question to all is that how a good transistor, diode, capacitor, inductor etc reads if i put DMM probes on it.. and what would be the erratic readings in case of each component that can prove helpful in giving an inkling that this has gone faulty... is there any general rule for each component?

what i have seen some TV technicians that they put their DMM probes on different components on PCB and after few minutes here and there checking they come up with a particular componets(diode, transistor, capacitor etc) and say instantly that this is faulty... i just wish to ask what and how they check and whats the majic wand in their hands?

they dont have ESR, OSCILOSCOPE etc.. just a MULTIMETER and thats it.....

Thanks again for the replies

Naushad, So glad to see you're still alive!!

As for TV technicians and their use of just a DMM to fault find... I think that you will find they will only be fault finding to the board level....

From what they will have been told, by an experienced engineer hopefully, is to look at the TV and what it does or doesn't do, that will almost certainly take them to a local area of the TV.... Then they will be able to check a few rudimentary voltages etc... to either prove it is the board at fault, or to find the connector that's dirty or loose fitting....

Fault fixed!! either by cleaning and repositioning a connector or two or by replacing the board with a good one.... The faulty board being taken back for an experienced operative to either repair or scrap.

John.

I was stunned one day, literally as well as figuratively, with the charge a CRT picked up just sitting there. I had discharged it the night before, came back to it the next day and ZAP "SΔ♦T" it got me. I havn't gone into the theory of how a charge builds up but I assume that it would have to do with things like the fluctuations in the magnetic field of the earth as things move about. Maybe that would be a good question for a CR-4 challenge?

masu - If you think of a capacitor as just two plates held apart with an electric charge between them there would be an attraction of the two plates towards each other... Some movement will take place...

If you now remove the charge quickly by discharging the capacitor the plates will try to revert to their privious position, but some flexing will result in the plates taking time to stabilise, so if the short is removed to early the plates will still be moving and a charge will build up until they stabilise....

As a teenager I remember experimenting with a 100uF capacitor and being surprised at how, after discharging it, the voltage built up slowly again....

As for the effects of earth's gravity I'm not sure.... I'm sure some one will correct me if I'm wrong, but I wouldn't have thought there would be an appreciable effect from this.

John.

Ok, good you're an electrical engineer. In answer to your questions:

Hopefully your DMM has a diode check on it; turn it to the diode scale and the readings (out of circuit) should be:

Diode forward drop: 0.5 to 0.7. A few high current diodes measure 0.3 .Diode reverse is infinity .If the meter reads 0.0 you have a shorted diode. If infinite both ways (leads reversed) that is an open diode.

Capacitor ( on resistance scale) red lead to + and black lead to - short out the cap 1st (again out of circuit) small cap readings start at 0.0 and rapidly charge to infinity. Fluke meters infinity as ".OL". Large caps charge slowly fo from 0.0 to infinity.

Infinity means open circuit. OK. A transistor (not a FET)-(turn back to the diode scale) will read 0.5 to 0.7 on your meter from base to collector and base to emitter , you will have to reverse the + and - leads depending on if its a npn or pnp transistor.

Have a nice day!

I was talking about induction as the earths MAGNETIC field is distorted by objects move through it as well as the mains supply of 50/60Hz distorting it. I wasn't thinking of the gravitational field. There is probably a lot of reasons that a capacitor will charge up by itself.

I'm sure there are engineers out there (and probably here where I work) that could explain it-- probably something to do with electron depletion of the capacitor plates and molecular inverse reactions or something like that..

________________________________________________

How do you know that you're not a figment of your imagination?

Ooops apologies masu, I obviously got my magnetics mixed up with my gravity fields....

I will blame the keyboard and my typo's!!

John.

I believe the Earth's magnetic field is too weak to have an effect in charging a capacitor. You would really have to have a large and/or fast motion of a conductor cutting directly across the magnetic flux lines to have an e.m.f. that would build a charge in a capacitor. Most likely, what you experienced was the residual charge after only a partial discharge.

On the other hand, there are some ways a charge might build up again. Faulty wiring or shorts could lead to stray voltages in the circuit. Not all circuits are cut when power is shut off on most TV's and many other appliances. Here is one you may not have thought of. If the TV remains connected to an antenna system, satellite dish or directional antenna, static charges can build up just from the air friction of the wind blowing over the antenna, especially if the feedline (shield/braid portion of coaxial cable) or the chassis is not well grounded. Indirect lightning effects can also charge the air and so sharge the antenna system as well. There are many products available to bleed off static charges from feedlines and cables.

Well I'm here at my electronics bench at work so I tried it..

47uF 400V cap. Charged to 21.89VDC Dicsonnected PS and shorted cap for 5 sec--meter connected to cap read 0.00V. Left it sit while I went to the bathroom, came back and wa-laa...0.392volts Repeated procedure except diconnected meter and it recharged to almost 0.5 volt. I dunno..Ive got a flourescent light sitting over my bench but...

I think it must be aliens.

I don't think aliens would be interested in leaving slightly charged up capacitors around...!! ;-)

Its a basic characteristic of capacitors design .... the dielectric loss or absorbtion is the key to this phenomena...

As I said in a previous post the force between the capacitor plates togewther with the sudden shock of being discharged causes the dielectric to adsorb some energy from the plates moving..... this dielectric force takes some time to dissipate and if the short is removed too quickly the force is still present to re-charge the capacitor....

The dielectric absorbtion of a capacitor is extremely important when designing integrating A/D convertors for instance as the charge must be accurately known and varied to get the maximum precision out of the A/D.

This is why for an integrating A/D its best to use polyprop., capacitors and not polyester PETP etc...

Electrolytics would certainly be useless in those applications... :-)