Model Boat Mayhem
Technical, Techniques, Hints, and Tips => The "Black Arts!" ( Electrics & Electronics ) => Topic started by: pnr/p on October 09, 2008, 09:46:25 pm
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I feel this may be a dumb question but.....can someone explain Forward and Reverse voltage as seen in LED specs please
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Is this any use?
http://www.kpsec.freeuk.com/components/led.htm
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I think it's the maximum positive (forward) voltage an LED will take before it "burns" out, the negative (reverse) voltage before it sorts (breaks down) out or starts conducting.
http://www.designledlighting.com/led-polarity
http://www.ledsmagazine.com/features/4/8/1
http://books.google.co.uk/books?id=E_wKgWBu8rUC&pg=PA128&lpg=PA128&dq=Forward+and+Reverse+voltage++LED&source=web&ots=1Ut5wDK7MN&sig=kFgKJVbzCEibnK3lAsmJt2skhlw&hl=en&sa=X&oi=book_result&resnum=6&ct=result
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PNR/p
The key bit about forward and reverse voltage is the last letter D for Diode
A diode conducts in one direction and blocks in the other so forward voltage is connecting it the way it lights up, and reverse is , er, the reverse (no light, no current flow)
No damage is done connecting either way, so "trying it" is, to me, easier than learning which lead is which :-)
Best diagnostic tool for leds is one of the lithium 3V button cells - just hold the leads to the faces of the cell, if it stays dark - try it the other way
Hope this helps -
andrew
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No damage is done connecting either way, so "trying it" is, to me, easier than learning which lead is which :-)
Stone me, Andrew! "Short leg goes to neg". I have it written on my workshop wall, next to Wombat's First Rule "Red to Positive, Black to Negative, cut off anything left"*.
BTW if you're buying any stuff from Component Shop then ask Iain for a copy of his guide to using LEDs - it's excellent and very easy to understand....and FREE!!
FLJ
*Don't try this at home, kids.
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FLJ,
I'm an old mechanical engineer and try not to remember anything!
If I understand it, I don't need to remember it
If it doesn't matter I don't need to remember it
What I have written on my workshop wall are:
Pobody's nerfect!
Engineers are (approximately) perfect
Don't think what you should think till you know what you should know!
But I cant remember why
Short leg
goes to neg
Hey, I love that
andrew
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The "peak inverse voltage", or reverse voltage, is what is will stand up to before dying. It will normally die quietly in this mode, just turning itself into a plastic blob totally indistinguishable from what it was before.
The "forward voltage" is the voltage that you can measure across it when it is in circuit and conducting. The available voltage will normally be greater than this, but there will be something in there to limit current. Sometimes this will be a resistor or other current limiting circuit, sometimes reliance will be placed on the internal resistance to limit current. If an LED tries to pass too much forward current, it will respond by dying, in this case usually with a very bright flash.
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Not on topic of the original post but....
Is it possible to get leds that have a broad beam or visible from the sides rather than a focused beam ie for bouy lights?
Nick
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In short, yes.
The light output from an LED is mostly directed through the domed end, and different beam angles are often available. The beam angles are usually specified on the LED data sheets.
Domed LED's are not really designed to be viewed 'side on', but the ones with the broadest beam angles produce more side scatter which makes them appear brighter than the narrow beam ones when viewed from the side.
Malc
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Not on topic of the original post but....
Is it possible to get leds that have a broad beam or visible from the sides rather than a focused beam ie for bouy lights?
Nick
Yes
Paint them white for internal reflection.
Paint them black on top of this, for reducing light leakage.
The file them flat across the top. You will get a flood light with approx 45 degress of light.
Or buy frosted LEDs of frost them yourself with sand paper. Then you get 360 degrees of light in all planes.
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hi all. Am i going mad. Wright i have been wiring up a loco for dcc and it has LED lights 2 red 2 yellow. I have found that if you look at an led the bit with the cup on it is - and the other bit is + or vic versa. Any way it is all the same for all LED i thought? On this loco it has the red LED cup + and the yellow LED cup -. I have checked this 6 times same every time can this be. I all ways thought that ALL LED ran one way regardless of the colour . I have checked with my stock of LED and they ALL run one way. So is this some sort of new LED Please help as my head is spining %% %% %%
john
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Have a look at:
http://www.kpsec.freeuk.com/components/led.htm (http://www.kpsec.freeuk.com/components/led.htm)
Malc
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hi there that link says what i thought the all LED operate the same way ie + for anode and - for cathode. But The red LED were the other way round ie - for anode and + for cathode. I was wondering are these some sort of new leds.
john
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Answer is simple really.
What you have in effect is 2 leds mounted in parallel inside the same package.
one is reversed so you have in effect
+...............-
-................+
so no matter which way round you connect the supply, it will be right for one of them to work.....the other remains 'off' until the polarity of the supply is reversed.
The one that was off comes on....and the one that was on goes off!
Rex
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Hi all, a couple of things.
1 I have had some blue LEDs for some years and have never got them to glow. Any ideas?
2. Has anyone experience of the new bright white LEDs? And where is best place to obtain them?
Regards to all, Roy
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Hi all, a couple of things.
1 I have had some blue LEDs for some years and have never got them to glow. Any ideas?
2. Has anyone experience of the new bright white LEDs? And where is best place to obtain them?
Regards to all, Roy
Have you tried increasing the voltage?
I think blue/white are 3v, and green/red/yellow/orange are 2v.
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Hi all, a couple of things.
2. Has anyone experience of the new bright white LEDs? And where is best place to obtain them?
Regards to all, Roy
I hate to have to ask, but have you looked in the Trader's Directory?
Malc
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Hi all, a couple of things.
1 I have had some blue LEDs for some years and have never got them to glow. Any ideas?
2. Has anyone experience of the new bright white LEDs? And where is best place to obtain them?
Regards to all, Roy
we have used hundreds of them.... for work as well as for models boats too....
we get them via www.ledlight.co.uk - they are agents i tihnk for a multitude of LED based items
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Hi all, a couple of things.
1 I have had some blue LEDs for some years and have never got them to glow. Any ideas?
2. Has anyone experience of the new bright white LEDs? And where is best place to obtain them?
Regards to all, Roy
ho there . the cheapest way to get white led is go to a shop and buy Christmass lights there very cheep now as we get close to the big day. eg 50 led for a £5 at one shop thats 10p an led can go wrong
john
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Thanks very much to all,
regards Roy
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i always use a firm called goodwill sales on ebay very good stuff at great price never been let down by them and i buy quite a lot.
no ties with them either just happy with them sorry couldn't make a link can't remember how :embarrassed:
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i always use a firm called goodwill sales on ebay very good stuff at great price never been let down by them and i buy quite a lot.
no ties with them either just happy with them sorry couldn't make a link can't remember how :embarrassed:
Do they have a website?
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Evenin' All,
Going back to the question from Nick regarding light scatter, do the rectangular LEDs emit to the side or is it still from the end?
I want to put a couple of 'trough' type deck lights on the current project and can include foil reflectors so the frosting option would work as a backup.
Tony
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From the end.
Malc
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Evenin' All,
Going back to the question from Nick regarding light scatter, do the rectangular LEDs emit to the side or is it still from the end?
I want to put a couple of 'trough' type deck lights on the current project and can include foil reflectors so the frosting option would work as a backup.
Tony
Depends on the type of the LED - some are side emitting, some are top emitting.
Wom
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Thanks both,
Wom, is there any way of identifying which type is which from catalogue descriptions? They all give the view angle and the majority seem to use the word 'radial' which I would have thought described the 'sideways' scatter but .........
Tony
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I used LEDs right across my model, they are all in parallel, in that I have a + & - wire leading up the superstructure and I have 4 pairs of 'searchlight's' (8) off that plus 2 nav lights and 1 mast light. So 11 LEDs.
The rear has 2 lights and another 2 pairs of searchlight (so 4 LEDs)
So that's 15 LEDs off a battery pack of 2 AA batterys.
My query is I didn't know they had a voltage, I knew they blew with 6 v, but 3 v lights them all OK.
The searchlights are yellow as there was no white in the mid 90s. The red & green are a cert for nav lights. [GOW bulbs where too big.]
Seriously, I didn't know there was various voltages, I just used to buy 'LEDs' and I've even had non-techy family members buy them for me when they've seen them at car boots, etc. So am I in danger of blowing the lot of them when I finally DO use them for a prolonged period? [I've just tested them while building.]
Can someone tell me what's probably happening? Have I bought 3v LEDS?
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Luckily you are running them off dry cells, which act as though they have an internal resistor built in, and limit the current they can deliver. With a single LED on two dry cells I would expect it to be rather bright, and the life expectancy to be reduced, but several in parallel will cause the available current to be spread out.
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Luckily you are running them off dry cells, which act as though they have an internal resistor built in, and limit the current they can deliver. With a single LED on two dry cells I would expect it to be rather bright, and the life expectancy to be reduced, but several in parallel will cause the available current to be spread out.
Good.. a bit of dimness is ok.. it's not like I'm hunting the High seas Fleet every night, is it.
By the way, my maths is terible.. it's 17 LEDs and it now needs 4 more (just found out accurately where they go... wish me luck soldering wire as thick as human hair! (I'll need it) I'll probably end up tieing the wire joint!!