LED Problems

[David Marks 2Participant @davidmarks2]

[David Marks 2Participant @davidmarks2]

I have spent a frustrating time trying to wire up an LED. The first one declined to illuminate at all. The second lights intermittently i.e about 5 seconds on and the same off, with a slight flicker of light when in the off mode. The LED is White running from a 6V battery with a 150 ohm resistor on the +ve terminal. Has anyone any idea of the cause of the problem? One odd thing that I have noted is that my 6V gel-cell batteries are providing about 7.7V.

[Dave MilbournParticipant @davemilbourn48782]

Resistor should be nearer 220R if the supply voltage is 7.7v. I'd check that last value, BTW – I've never seen a 6v SLA give above 6.8v when fully charged.
Are you certain that this is a standard LED and not, for example, a flashing one or one that's already fitted with an internal ballast resistance i.e. intended for a higher voltage?

DM

[David Marks 2Participant @davidmarks2]

David

The LEDs are standard i.e. as purchased from Component Shop a week or so back. I will re-check the battery voltage tomorrow. Strangely, I left the LED connected to the battery whilst I had my evening meal and…..when I returned the LED was permanently illuminated.

[David Marks 2Participant @davidmarks2]

Update – I re-checked the battery voltage using the same digital multimeter as yesterday and obtained the same reading of 7.7V. I then used my analogue unit and obtained a reading of 5.5 V. The latter is about the figure I was expecting. I had a boat-less year last year due to illness so the batteries have sat on the shelf for some time. I therefore need to get my digital unit calibrated in some way. Additionally the offending LED is performing correctly.

[Dave MilbournParticipant @davemilbourn48782]

David

Sounds like your DMM might need a new battery. It seems it's the only thing that isn't working properly. You can buy a decent new one for less than £20 if you don't need it calibrating to industrial spec.

DM

[David Marks 2Participant @davidmarks2]

David – Strangely, I tend to use the analogue unit most of the time but for some reason the Resistance range is not functioning correctly so I used the DMM that my son uses for his `car projects'. What is the best way of telling that the battery is in need of renewal?

Many thanks for your help once again

[Dave MilbournParticipant @davemilbourn48782]

How much is a new battery??? Change it and see if it improves. If it doesn't then remove the new battery, bin the DMM and make your son pay for the new meter.

DM

[Bob WilsonParticipant @bobwilson59101]

I have found that some digital multimeters give incorrect readings when the battery is low. I have an old analogue AVO for reliable voltage readings of batteries.

LEDs can be affected by voltage (too much) Try increasing the value of the resistor! Recently, I got some yellow LEDs and they lit up with a distinct orange colour. When I increased the value of the resistor, they changed to a definite yellow.

Bob

[David Marks 2Participant @davidmarks2]

Dave/Bob – Regarding the DMM, I changed the battery and found that the voltage reading for the two 6V Gel Cells that I have was just over 6V. There is still some difference between the DMM and the old GEC multimeter that I have which gives around 5.5V. I asked Dave how a layman such as myself can tell when the battery in a DMM is in need of replacement, but possibly I did not pose the question in the correct manner. So I read the instructions………the pristine….never been fired in anger instructions. On my particular DMM (Draper manufacture) a little battery icon appears in the corner of the liquid crystal display. So back in went the old battery and yes there was a battery icon on display. Incidentally I checked the discarded battery with my analogue unit and the voltage was down to 6V as opposed to the required 9V. Hope Mr Mibourn is suitable impressed with all these terms……icon…..liquid crystal display. I am probably incorrect but never mind.

Thank you gentlemen.

[Dave MilbournParticipant @davemilbourn48782]

David
I am truly heartened. I always am when someone discovers that the manual isn't just "that fancy-coloured bit of the packaging I threw away with the rest". Incidentally your use of the technical terms was spot on, too!
Just to show you don't need to fork out umpty-squillion quid on a Fluke Auto-Ranging, back-scratching, breakfast-cooking, NASA-calibrated, alarm clock/radio DMM, I've used one of these for years **LINK** and would thoroughly recommend it.
DM

[Bob WilsonParticipant @bobwilson59101]

David,

Modern digital multimeters are not all they are cracked up to be when you are dealing with voltages of batteries, they will often give you false readings. Take this simple experiment with a 6 Volt sealed Lead Acid battery. The resistor is 1 Megohn (One million ohms).

I connected my old analogue Avometer between A and B and got a reading of 6 Volts.

I connected my digital meter between A and B and also got a reading of 6.0 Volts.

So far, so good.

I connected my Avometer between A and C and got a reading of 1 Volt.

I connected my digital meter between A and C and got a reading of 5.56 Volts.

The old Avo is the more accurate reading, because if you connect a small bulb between A and C, it will not light, even dimly, despite the digital meter saying there is 5.56 Volts there!

The reason for this is that the digital meter has a very high resistance and draws hardly any current, and the voltage drop across the resistor only shows as 0.44 Volts, leaving 5.56 Volts between A and C.

The analogue meter, however, draws more current, and consequently 5 Volts are dropped across the resistor, leaving 1 Volt showing between A and C.

But if you connect a 1.5 Volt bulb between A and C, it will still not light, even dimly, because with the increased current, most of the voltage is then dropped across the resistor leaving virtually nothing between A and C when the bulb is connected. Both types of meter will confirm this!

You can carry out this experiment with any fairly high value resistor.

Bob

[David Marks 2Participant @davidmarks2]

David/Bob – Many thanks for your update and comments. Training wise I am mechanical…..tool and gauge making from mid 1960's. Always worked for MOD and for the final 20 years of my working life was an auditor ISO 9001 and all that jazz. I can recall educating one of my younger colleagues and as an example mentioned the word Avo. The look on this bloke's face told me that he wondered what the hell I was talking about. Indeed checking with contractors (sorry suppliers) of electronic equipments, the vast majority of younger employees had never heard of the device. David mentioned Fluke. I once did an audit (calibration) at a suppliers where they had a piece of electronic test equipment made by a company called Wayne Kerr. When my report went for typing, apparently the ladies in the typing pool were in hysterics!!

[Bob WilsonParticipant @bobwilson59101]

This is an ancient Avometer, and I do use it quite a lot, especially for checking batteries. I do have two digital meters as well that I use for resistance/capacitor testing, or taking voltage readings from circuits where I do not want the resistance of the meter to alter the functioning of the circuit. I got the AVO on a car boot sale for £5.

Bob

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