If the supply current varies a lot, say from a few uA to a hundread mA or more, you can get significant glitches in the supply volts as diodes do not provide a uniform forward voltage drop versus current.
Mostly it does not make a differance, but be aware it can.
There are many 5v 1 amp switching supplies used for cell phone charging etc [little black box plugs into the wall socket] - usually they will operate with 90 to 240 Volts AC input with good isolation [dry conditions only]. In my area people throw them out or give them to thrift stores - they seem to work very well for many uses and usually cost less than $2.
These units are also made for use in cars 14VDC to 5VDC for charging things... often with a USB socket just for 5V output purposes... as they are switching types they don't dissipate much power = heat...
Can I add a teeny note of caution if using old/unknown wall/block PSUs.
Recently I had to check an apparently faulty device which used a Ho Phong block PSU.
The output voltage was meant to be 12V and measured OK with a DMM.
The mystery was solved when holding the (labelled) 0V output pin and resting my elbow on an Earth. Ow!
The damned thing was putting out over a 100V to Earth.
I realise that voltages can leak or be induced and only show up with a DMM but this damned thing had become significantly UN-isolated. Enough to give a painful belt.
BEWARE second-hand electrical devices - there is sometimes a reason why stuff is being chucked out!
TEST before use. Or just spend an extra quid on new stuff from a reputable supplier.
@Dippy - hopefully you have some decades left to find faulty stuff Did you test to see if it was a 'hard' leak? I use a low watt incandescent lamp from each power line lead to output before going for the full smoke out... Mostly I've found a capacitor or resistor to line at times. I agree that high volts is always worth caution. I'm not convinced that new stuff is any better than old stuff on the whole - they both deserve caution and for many people 3 AA batteries are the ideal power source [I use them a lot for convenience - rechargeable].
Keep up the good work and stay safe... have fun...
I did load the lines and there was still a significant voltage.
It got kicked around the room and converted (a rugby phrase) into the skip.
My point really is that some so&so may have simply put something that into a car boot sale and some other bargain hunter may have bought it...
This is one reason why many pre-owned-product retailers won't deal in electrical stuff. Better safe than sorry. I realise this kind of fault is rare, but things like this could end in disaster - and only because someone is trying to save money.
Re-use by all means but get someone with experience to check it first.
I know people won't.... after all, with 5 minutes experience we're all experts aren't we
@Dippy - I usually open the unit up to see if it's an accidental design fault or failure of a good design... and look for good parts to salvage and properly recycle the rest.
I've certainly found many defective items used and new... part of our art is to know how to test stuff - non-destructive testing...
I recall there were two books written 'Why Things Stand Up' and 'Why Things Fall Down' roughly. In engineering both these modes are important to consider [some great pictures of things falling apart... ;-) ] -
It only takes a few minutes to learn a lot about electrical shock - unless your hands have been in salty water then it could take a lifetime [it only takes about 2 watts to to prevent breathing I think] - safety should always be paramount in working with dangerous materials - I still have one good eye and always use safety glasses - and I can count in fractions with fingers and part finger on left hand Still, I keep the fire extinguisher handy if I must work with high wattage stuff... and torches [not the UK kind ]
Thanks for suggestions premelec.
Trust me; as I get older I become more cautious , which is how I have become older.
However, here are the variables.
A) It was at work.
B) It was a work's device.
C) The no-brand PSU was 6 months old (which is good for Made in Ch).
D) A replacement cost £4.
E) It was a bonded enclosure which would require a hacksaw.
The equation goes like this.
A+B+C+D+E = BER ==> chuck in bin.
"accidental design fault or failure of a good design"
- it was unknown-to-me-brand made in Ch so take your pick , but I would go for "cheaply designed, cheaply made with cheap components. Throw me away I'm dangerous."
For onlookers; "BER" means beyond economic repair.
BER varies with
- your budget
- your curiosity level
- your skill/experience with the item in question
- possibly the size/content of your toolbox
I wouldn't take my 10 year old gasoline powered lawn mower to a shop for service because almost any problem could rapidly become BER.
On the other hand, I spent 1.5 hours yesterday cleaning the jets in the carburetor and replacing the gas line. The repair shop cost would have been at least $70. I probably used 10 cents worth of carb cleaner and the gas line had been given to me. It now starts on the first pull of the starter rope and runs very well.
Background - I've rebuilt gas engines across he gamut from flathead V-8 to overhead cam 4, so I don't expect a small engine to have a problem that I can't resolve relatively quickly and usualy inexpensively.
Well I don't want to open a massive debate, but I can't agree with that John, sorry.
Assumine we are talking about BER in the commercial arena it is basically cost of repair versus cost of replacement.
Obviously availability/urgency comes into the equation but when we are talking about a £4 PSU versus an hour hacksawing it open and testing and THEN ordering/replacing one or more components THEN soldering in and reassembling , sorry, but it's a no brainer.
Even a lowly technician is on £20 per hour so a PSU replacement equates to 15 minutes. Curiosity is for another day on someone else's cost sheet.
Yes, of course, a hobbyist with plenty of spare time may wish to spend 5 days saving a quid but I can't be bothered to piddle about like that.
Some guys I've seen would drive 50 miles to save a quid. Plain daft... but totally your choice which I respect.
Hi Dippy. we all have our foibles and are rationalizing rather than rational beings as we represent ourselves - I'm sure you [like everyone] make various enjoyable irrational [in the context of economics $$] choices and that's part of how we enjoy life rather than having it all a grind to make maximum $$ - I often enjoy fixing the unfixable - and the reality of finding another is that it takes time to locate go to a store or order as well as the $$ - so I hope you are having fun with your endeavors as well as satisfying your employers. For the variety of amusing and practical or impractical fixes www.ThereIFixedit.com ... Whatever works... Thanks for all your good advice - I give away all the advice I can't use myself - it's only fair
I was differentiating between work and hobby.
In the normal course of work economics the equation (excluding , as I said before, emergencies) the equation is simple.
At home it's different. But I still haven't the patience to spend a week or drive 50 miles to save a quid. With UK petrol prices it concentrates the mind...
In any event, special caution is needed when buying second hand electrical gear.
The economics of your decision is totally yours.