|john fletcher 1||22/04/2018 17:49:48|
|385 forum posts|
My son bought some LED strip lights for his workshop, thinking they would last a 'life time' only to find several failed within a few months. After searching around he now says some LED (cheaper one) are made to a lower standard than others and those are the ones which fail. Could that be possible ? Having used lots of rectifying diodes in the past with few if any rejects I find that incredible. Could manufactures used inferior material I thought the cost was in the making not the material. John
1890 forum posts
76 forum posts
I had the same experience with the cheaper ones that failed in a short time, month or two, however the more expensive ones cost 70% more have been working for more than two years now and still bright as day one, so perhaps these el cheapo's must be made of inferior "semi conductor" components? I also can not see that the cost of manufacturing could be influenced by a better quality of materials unless they have frequent sales in mind. I must add that in the mid 1960's (yes i'm and old timer) i fitted a new 100 watt incadescent type lamp in my garage and believe or not it still works to this day i don't know if it's sheer luck or outstanding quality of that time. btw it's made by Westinghouse.
|Neil Wyatt||22/04/2018 19:28:54|
13224 forum posts
Yes. Avoid cheap ones unless you have an expensive current-controlled power source.
I have found this with LED bulbs, cheap ebay ones last months, branded ones (even DIY shed and supermarket own brands) - I haven't had one fail yet.
3878 forum posts
Make sure they are well attached to their heatsink. The low quality ones probably are selected as having higher resistance or some other feature owing to being at the edge of the wafer or close to some defect in it.
|2681 forum posts|
I think the LEDs are OK, it's the rest of the lamp that's too cheap.
Bazyle mentions the need to firmly fix LEDs to their heatsinks; yes indeed but one economy is to make the heatsink on the small side in the first place. Hot LEDs don't last well.
In the cheap lamp I took apart the big problem was the power supply. Of the simplest type possible it didn't provide a constant regulated voltage to the LEDs. This shortcoming was made worse because the lamp was designed for 220V and the poor LEDs got even more unregulated volts off my 240V mains. The life of a LED is greatly reduced by overrunning it. Also the circuit didn't have any obvious suppression making it vulnerable to splkes. These cheap basic lamps might last reasonably well running off 220V mains - I don't know.
Another problem with my lamp was that the LEDs were wired in series. As LEDs often fail short circuit, if one fails a greater strain is applied to the survivors, and their lives are shortened too.
I think the more expensive lamps come with a better power supply that ensures the LEDs run within specification while protecting them from from spikes. Possibly better heatsinks keep the LEDs cooler as well.
Edited By SillyOldDuffer on 22/04/2018 21:59:22
|Mike Poole||22/04/2018 22:24:35|
1268 forum posts
The U.K. Pushes its luck by fiddling the tolerance on mains voltage. I have encountered problems with equipment made for 220v not working in the uk. Part of our factory was sitting at 252v so was well out of spec for even the fiddled limits on line voltage. It is time we moved towards 220v as our standard voltage for real and not fiddling the figures to include 240v. It might take a few seconds longer to boil your 240v kettle but as old spec equipment works its way out of the system things should settle down. Of course after Brexit we could go back to 240v but that would just cause more pain and expense and would be unnecessary and pointless, having got this far we may as well align with the rest of Europe and benefit from a large bloc standard.
364 forum posts
Recently purchased an LED security light for the outside of my workshop, fitted it and it works a treat, however at night I noticed that the leds were glowing faintly when the light wasn’t activated. One of my shooting buddies manages a specialist lighting company so I asked him why the light was glowing when off and his explanation was that it would be down to the fact that the components used in its makeup were probably out of spec which leads to minute current leakage allowing the leds to glow dimly. His comment was that expensive units were constructed from components that were certified within spec and these consequently lasted much longer before failing. It seems that in the price paid for led units you get what you pay for, cheap price usually equates to short life.
|john swift 1||22/04/2018 23:01:34|
297 forum posts
since the LED's require a lower current
either the capacitance of the switch cable
or the snubber capacitor that bridges the triac in a solid state relay as sometimes used in LED security lights with a built in passive IR detector , (although I believe most have gone back to using relays) can provide enough current to cause the LED's to give a dim light when switched off or more annoying flash on and off
old switches that arc a bit before they close can cause the fuse in LED lamps to fail because
of the use of a capacitor as a wattless dropper in the cheaper LED lamps
at 50 or 60 Hz main frequency the impedance of the capacitor will limit the LED current to a safe value
but with the high frequency currents produced when the switch arcs
the impedance of the capacitor will be lower and more current will pass through the fuse and LED's
blowing the fuse
Edited By john swift 1 on 22/04/2018 23:16:24
|Sam Stones||23/04/2018 00:53:08|
493 forum posts
Hi John (F)
Big Clive has done a large number of interesting LED light strip-downs. Worth a look if you are prepared to search his website.
One thing he seems to emphasise is the need for adequate heat-'sinkage' and heat-transfer 'paste'.
3139 forum posts
Just junked a 1pound LED candle bulb after about 6 months
It was flickering and far too hot for 5watts
Probably the most used bulb in the house
|Neil Wyatt||23/04/2018 09:27:16|
13224 forum posts
Energy companies oppose this as a resistive load like a heater, for example, uses nearly 20% more juice at 240V as at 220V.
|not done it yet||23/04/2018 10:12:31|
|1731 forum posts|
9% more at 240V than 220V. There would be more leccy used to boil a kettle at 220V because it would take longer and thus lose more energy during that extra time
AC voltage is given as the DC equivalent voltage ie the root mean square of the peak emf. Ie peak voltage divided by root two.
I would expect that the UK generating plants were historically designed for maximum efficiency at 240V and have continued due to the old units in place. Transmission losses would also rise as a consequence of voltage reduction, so increased current, for the same power. Large industrial sized motors might also be affected....
Lots of technical reasons, but not just to sell more energy I think.
Where I used to work, they were still using about 25MW of power at the plant. They have replaced much of this with newer (than the original ~3MW in the mid 1920 - 1940s), but the most modern machinery, put in place about 30 years ago would not tolerate being uprated, for current, by 10% and would be less efficient at reduced power.
We were not permitted to run 3000hp motors at more than 105% of the rating plate under any circumstances. Older motors were more tolerant of overload as the designs were not so constrained as later computerised ones, which are more efficient but less tolerant of any supply changes.
2704 forum posts
I bought 12 "Luceco" LED bulbs (5.5W E14 candle) from Maplin(!) in Feb last year and they have all popped apart from perhaps one or two. If properly designed, the only failures should be due to manufacturing errors like poor solder joints (usually down to the solder mask / paste operation). But when I ripped one apart it seemed that one of the LEDs had actually let the smoke out, so either their LED suppliers are truly crap or the circuit is not fit for purpose. I now only buy Philips etc when on offer.
My record was a purchase of 3 bulbs from one of the pound shops that barely lasted a fortnight between them. We were living in a rented house and due to move out. I had to go out and buy another set of slightly better ones - good money after bad.
The lifetime of filament bulbs is inversely proportional to the 5th or 6th power of the voltage applied (I forget which but pretty certain it's the 6th), so most of the decent quality drivers for halogen bulbs regulate the voltage. If you simply use a linear (mains) transformer, they will be very sensitive to variations in supply voltage.
2704 forum posts
Splitting split hairs. If "juice" means power, then Neil is right. But few heaters run without a thermostat so what difference does it make? The kettle will end up taking the same energy to boil as good as dammit. The amount of excess water you put in the kettle will have a much bigger effect. And the heater will take the same average power to keep the room warm.
I'm going lay some tiles and then machine something now.
|2681 forum posts|
Slight misunderstanding here. It's not about losses in the home, the supplier worries about power loss in his distribution network, not in your kettle. Dropping the volts costs the supplier money.
The issue lies in the relationship between Watts and Ohms Law.
Assume your street draws 100kW of power, and the average resistance of the network is 10 ohms.
100kW at 220V is 454Amps, so the 10ohm resistance of the network means it will lose 4540W as waste heat.
100kW at 240V is 417Amps and the 10 ohm resistance of the network will lose 4170W as waste heat.
In this example, saving 400W (9%) may not seem much, but there are about 40,000,000 households in the UK. You could reduce the losses by installing fatter electric cables but copper is expensive; it's cheaper to up the volts.
Changing the subject slightly, does anyone know how various countries standardised on the various pressures and frequencies they've adopted?
In the UK, where a mish-mash of early local systems were replaced with a grid, I get the impression that they went for the maximum voltage you could put into a home without killing everybody who touched it! At the time 250V. 50Hz was chosen, I think, because it was the best that could be done with the largish number of early alternators available (magnetically rather than mechanically).
In the US I suspect the Edison/Westinghouse marketing fiasco had everybody so terrified of AC that they wimped out big time and standardised on the much safer 110V. By then it was also possible to generate power efficiently at 60Hz, and, as they didn't have a lot of elderly infrastructure to accommodate, they went for that, and saved dosh because 60Hz transformers can be made more cheaply.
I theorise 220V 2-phase became popular because110V has quite a few disadvantages other than being wasteful. 220V single phase has come to dominate in the world because local distribution networks cope with old 110V consumers while offering a good compromise for everybody else; not too dangerous, not too expensive, and not too wasteful.
|not done it yet||23/04/2018 14:17:43|
|1731 forum posts|
Thanks Dave. I did mention transmission losses, but I don’t like to quote figures. I^2R losses are very important to the power suppliers. And yes, Ohms Law applies to resistive loads, so 9% it is.
|Mike Poole||23/04/2018 15:47:51|
1268 forum posts
Changing the voltage to 220V is only going to affect the last few yards from your local transformer to your house. I suspect that the cable sizes for the house to sub would not be an issue most of the time but if the local transformers cannot be tapped for 220v then replacement costs would be high and maybe the reduction in capacity of the transformers may be relavent in some places where they are close to the limit at 240V. The new installations in the factory I worked in were set at 220V but as the equipment we used was specified for plants worldwide it is best to operate it in its optimum range rather than push it to the high end of its operating spec.
|Phil Whitley||23/04/2018 19:02:26|
|628 forum posts|
"Of course after Brexit we could go back to 240v but that would just cause more pain and expense and would be unnecessary and pointless,"
Supply voltage in the UK has never changed, for the above well explained reasons. It has always been nominally 240V. Engineers were told that due to harmonization with Europe, we all had to "SAY" that the voltage was 230V in Europe and the UK, although it was never changed in the UK. This from the wiki:
Just as it was in 1960, the supply voltage to domestic properties in the UK is still nominally 240 V AC at 50 Hz, although the declared voltage in the UK is now 230V AC +10% to -6%. Historically the domestic voltage was 240 V +/-6% (and 415V 3phase), whilst continental Europe was 220V (380V 3 phase).
In other words just more Eurocrap! We always questioned the idea of us harmonising with them, given the extremely antiquated electrical supply network, and the use of unsafe unshuttered 2 pin sockets still widespread. It would have been more forward looking and a lot safer for them to harmonise with us, but that is not the European way. Thank the lord we are coming out!
|Mike Poole||23/04/2018 20:29:32|
1268 forum posts
The whole thing is a load of nonsense, nobody actually did anything apart from fiddle the tolerances. It would make much more sense to actually work toward true harmonisation and then tighten the tolerances so they are actually useful. Although power supplies for electronic equipment can be designed to accept a wide input range much equipment is going to be near the limit of its comfort zone if it has to cover the wide range now specified. It has been discussed before on here before how operating with lower voltage can give much improved component life. By agreeing to the present fudge we probably suffer most as we are buying 230V equipment and running on 240V. I very much doubt that manufacturers build something for 240 and let everyone else under run it.
Please login to post a reply.
Want the latest issue of Model Engineer or Model Engineers' Workshop? Use our magazine locator links to find your nearest stockist!
Love Model Engineering? Sign up to our emails for the latest news and special offers!
You can contact us by phone, mail or email about the magazines including becoming a contributor, submitting reader's letters or making queries about articles. You can also get in touch about this website, advertising or other general issues.
Click THIS LINK for full contact details.
For subscription issues please see THIS LINK.