how to read diode with single colour band?

 

Easy - the end with the coloured band is the cathode.
That’s it.

 

How do we know the value?

 

Look at the schematic, or bill of materials.
D10 is the same as D19.

lol, I know we are not much help.
I think a good replacement is 1N4148.

 

Without the circuit diagram you can’t be certain that it isn‘t a zener.

 

A lot of surface mount components can't contain sufficient identification. So during manufacturing it's taken on faith that the parts supplied by the parts supplier ARE what they're supposed to be. We then take it on faith that the technician who loaded the pick & place machine has done the job correctly. That's where quality control comes in - they either monitor or verify the parts being placed into the cartridges are correct. The cartridges are bar code labeled and the machine can read which cartridge it's picking from. If the tech put two cartridges swapped the machine will know it. Provided that the engineer who wrote the software did their job correctly.

When we receive a circuit board we often don't get the schematic or the BOM (Bill Of Materials). The silkscreen on the PCB is there for the test technicians at the manufacturing facility to quickly identify components for troubleshooting. If a batch of boards all have the same issue it can be identified by lot and date code back to who set up the pick & place machine.

Do we know what that diode is? Well, if you're talking about D10 then it's whatever the BOM says D10 should be. Or if it's D19 - or whatever designation on the PCB says - the BOM is where we would glean the part number, and thus, reveal the specs for that component. Just a simple glass tube with no identification on it whatsoever - it's not possible to say what it is. We can guess, but that's all it is, a shot in the dark. But if we had the schematic, even if it doesn't tell us what the part is - the folks here can take a pretty good educated guess at what it is. But in the end, without the BOM - it's just a guess.

 

Modern components are not meant to be read/identified by people unless they are reading a roll label when being placed on a robot that installs them on a board.
If the manufacturer prefers to save 3 cents that way, now you know why.

 

If the manufacturer prefers to save 3 cents

The part does not cost 3 cents. I pay 2 cents US price. Digikey.com or 0.9 cents China lcsc.com.
Who can read " 1N4148W-T4 " on the part?

 

Some small parts that are too small to have their identification printed on them may have a code something like 3D. From that you can't tell anything. You need to know the manufacturer first. THEN you can look up their code and 3D will tell you what it is. It may be a 2N2222B transistor or it could be an N channel MOSFET (or something). But you can't print all that on the part.

There's another thread - "The Right to Repair". It discusses some of the issues with manufacturers not wanting the average person fixing their devices. They'd rather it end up in a land fill and you buying a new one FROM THEM! However, in this case, your component is too small to have any meaningful designation on it. Hence, the need for the BOM. For a while I was able to find BOM's but haven't seen one of those in quite a few years.

 

How do we know the value?

what value you want to know? length? diameter? weight?

if you cannot get product marking either from the component itself or from schematics or from BOM, there really isn't much to go on. in that case you can desolder it, and test it... determine its characteristics that you are interested in (reverse voltage, leakage current, forward drop, max current...). with that in hand you can look for an equivalent...

the other option is to see how it is used and what function it does in the circuit.

 

you can desolder it, and test it.

Works great IF the component is good. However, if it's good - why replace it? Why desolder it? If it's bad - you're not going to glean any useful information other than "the smoke leaked out".

 

i am not here to judge - TS simply asked how to identify component, not mentioning particular scenario. so i mention a way to identify it - nothing more.

of course one must ask himself how important or valuable this information is...

if this component is part of something vital, rare or obscure it may be worthwhile to spend time and investigate, and plan accordingly - before failure occurs. doing any analysis after failure is much more limiting and costly if possible at all.

if there is a failure, chances are that same component is used many times on the same board, specially if it is discrete component as image shows. so there may be good chance of locating not damaged one.