Member postings for Robert Atkinson 2

Dave,

LEDs do make fairly good low frequency noise sources. .I've seen them used for that in the past.

Robert G8RPI.

Hi Maurice,

Unfortunatly I'm pretty busy with the day job (mostly from home) and the workshop is full of stuff due to some re-rranging and decorating for SWMBO. so no time. Additionally I can't replacae a 1929 car.

However Tim's recent posts indicate it might have been mechanical failure of connections. I've not seen the car or LEDs and I guess we may never know for sure. I'm not convinced by the broken connection theory though.

Tim,
The LED I sent is a wide angle type. It's not a quality issue, they make them with different viewing angles. The one I sent was left over from a backlight modification on a digital meter.

Robert G8RPI.

Maurice,
Interesting experiment, but again producing very narrow spikes.

Tim,
Picoscope is a trade name of Pico Technology (picotech, www.picotech.com ) for a range of oscilloscopes that use a personal computer for a display and processing. As well as "ordinary" 'scopes thay do an automotive range, www.picoauto.com that include very comprehensive vehicle diagnostics. For example you can check compression just by clipping a current sensor on the battery lead and crankng with igntion off.
They are great units, I have several, including some their early ones, and a full 4 channel automotive kit. There are cheaper similar units but Pico are probably the best.

Dave,
Did you see my earlier explanation of how partial junction damage could produce the effect seen by Tim in the failed LED directly connected to a 6V supply. I've blown up LEDs in the past when pushing the limits under pulse operation and seen this kind of effect. Under a microscope you could see the damaged area and a curve tracer showed a distinct change in the forward voltage characteristic.

Robert G8RPI.

There is nothing on that page I can see that suggests 100 ohms is correct for a single white LED and 12 volt supply.

The closest is 100 ohms for TWO white LEDs on 9 volts and that is for 30mA not the 20mA of Tims LEDs.
I would never design for any component to be at ait's Absoute Maximum rating(s) under nominal conditions.
I broke my first LED in 1974 at the age of 12, the lead fractured close to the body. That LED cost me about 2 weeks pocket money.

Maurice,

None of the LED specifications or suppliers linked to in this thread are the ones being used by Tim in his car. A typical 3mm white LED has a absolute maximum current rating of 20 mA We know that Tims LEDs hve a 470 Ohm series resistor. with 9 V across the resistor and 3 V across the LED (typical for white LED) you get 19 mA (9/470) for 12V supply. At 13.8 V this rises to 23mA (10.8/470 voltge across the LED is fairly constant), so out of specification. I would not expect this 15% overload to kill the LED. It is almost certainly NEGATIVE SPIKES generated when the engine is running by dynamo, ignition or the starter motor turning off that are killing the LED.

I'm stopping replying on this thread until either Tim reports back on the parts I sent or SOD sent or somone nearer to tim goes and puts an oscilloscope on the car.

Robert G8RPI.

Hi Michael,

While this is a nominally UK seller the item and listing are typical of the chinese sellers. Just because this seller is only listing 12V LEDs does not mean the photo is of the actual item they are selling. These sellers often "share" photos. Even professonal online catalogs like Farnell do not use correct images. I'm an electronics designer but a couple of weeks ago I asked why a mechanical designer used a flanged riv-nut (not something he had used before), causing a panel gap, instead of a countersunk one. He said Farnell (the supplier I suggested) didn't do countersunk riv-nuts. I gave him the stock number and he pointed out the picture on the web site was of a flanged one. You can't trust photos in adverts.

The fact that these LEDs are rated at 12 V and seem to be running at the absolute maximum current, is probably part of the issue. The described failures still point to negative spikes being the cause.

Robert G8RPI.

The ebay sellers generally offer a choice of voltage ratings for these., The pictured resistor may be for the 5V version. Tim said he measured 470 ohms. A 470 ohm resistor is consistent with a 20mA, 12V white LED set-up when you allow for a 3V drop across the LED.

Robert G8RPI.

My employers apprentice scheme has been running for 100 years. They pick the best of many applicants and it dosn't matter if you are going to be a fitter, designer, mechanical or electrical, the course is the same They all do hand fitting and time in the design office, commercial and shop floor as well as college. Our chief engineer started as an apprentice.

Not all schemes are the same.

Good point Maurice,

Positive earth per se should not make any difference, all else being equal. However if the modern Dynamo regulator works on positive or negative earth (or is based on a negative earth basic design) that could affect the level of spikes generated by it's operation on negative earth.
Additionally a conversion from 6 to 12 V may make any spikes generated by the dynamo work.

Tim,
is the car positive or neagative earth? Was it converted from 6 to 12 V and or positive to negative earth.

Robert G8RPI.

Year after our gas water heater was fitted (most work by me, connected and signed off by competent plumber) we needed an inspection and certificate as we were hosting an exchange student. Known plumber was not avilable, so booked a local who was. I leaft the manual on the counter for him, he moved it aside..
1/ could not get cover off.
2/ Him "where's the thermostat?" Me "there isn't one" Him " there must be one for the boiler", Me "it's not a boiler it's a water heater"
3/ told me he could not sign it off as we didn't have a fixed vent in the kitchen. I explained that as it was a sealed balanced flue design it didn't need a fixed vent and if it did the cooker hood met the requirement.

Friend of SWMBO complaining that the had a slight leak (like excessive damp down a downstairs wall( and two plumbers had looked but failed to find the problem or didn't want to be bothered. As SWMBO was visting them in a cople of days I said I'd have look. Sure enough wet wall (side B), what's above - tap end of bath side A of wall. Go upstairs and look no leak under bath and cold pipe goes through bigish hole in wall reach in as far a possible but no water. Go in bedroom (side B of wall) where one plumber refused to lift carpet unless given full job without quote. and ther is a cut floorboard, Lift it and there is a mist underneath. seens to be coming from a "yorkshire" solder joint T. Closer inspection shows it's near the joint at hacksaw "strike" where the original fitter slipped cutting the pipe. The thinned wall eventually cracked. It was all a bit tight with pipes wall and joist so I cut the end off a capiliary straight coupler, cut an anged slot in it, cleaned and lightly fluxed it and the pipe, slipped it over the cut in the pipe and soldered it up. Total time 40 minutes! You could call hat a bodge But I bet it will last as long as the other plumbing in he house.

Robert G8RPI

Unfortunatly it only proves the spikes from your ignition don't kill your LEDs We have no high speed measurements of the power quality on Tims 1929 car. While the spikes form your igniton are impressive, they are narrow so lower total energy. Also you are using a quality 5 mm / 30 mA LED which will almost certainly have a larger chip than Tim's unknown 3mm ebay LEDs.
I do note that the LED you used has a maximum reverse voltage rating of 5A and a ESD HBM rating of 250 V. This gives an ESD energy limit of 3.2 uJ (1/2CV2). A 10V negative spike from inductive swiching (maybe that solid state dynamo regulator?) 60us wide would equal this with a 470R resistor with (crude) assumption of 5V across LED and 5V across resistor.

We DO NOT KNOW the quality of the power supply on Tims car.

For reference, if this LED was to meet the power surge and spike standard used on a 12 V light aircraft it would have to be tested to withstand +16 V for 5 minutes, +24 V for 1 s, +40 V for 0.1 s, 50 off 10 us pulses of 600 V positive and 50 off negative (RTCA DO10G sections 16 and 17).
A typical automotive "load-dump" specification (for alternator equipped cars) resqures resistance to 10 pulses of + 90 V about 0.2s wide (ISO 16750).

Robert G8RPI.

Hi,
This indicates that the LED junction has been damaged. Just connecting a LED to 6 V without a resistor is going to damage it. Do you have a milliammeter on a multimeter you can measure the current laken by the LED when directly on the power supply? I'm pretty sure it is more than 20 mA..

Likely failure mode for this LED is one area of the junction has gone low resistance (short circuit). With the 470 ohm resistor this prevents the voltage at the junction reaching the emission threshold. With direct connection to supply ther is enough current to drive the voltage across the junction above the emission threshold. It won't last long like this hough.

A useful check would be to measure the voltage drop across the diode (powered off) in both directions with your multimeter in diode test mose if it has one.

Robert G8RPI.

Edited By Robert Atkinson 2 on 18/08/2020 15:22:43

If you are refering to the "mobile Plant" it is (was at least, it was about 16 years ago). The response of the copper indicated to me that he was aware of the "loophole" and annoyed that I had found it. If Mobile plant had "storage" commonsense is that it would be for items required for operation of the plant. It would be a matter for the courts to determine if a marginal case was mobile plant or trailer. The GTCP-85 filled the whole Sankey Chssis and only had storage for the air delivery hose so not much argument.

My pet motoring peeves are:

LED replacement bulbs (not applicable to vintage vehicles Tim) beeing promoted and espcially those who say "i've got them and my car pased the MOT so it must be OK"

Deletion of emissions control equipment on modern diesels. For example EGR valve blanking n DPF removal. Supposed to be checked by MOT but the testes not even allowed to pull the plastic cover off the top of the engine so how is he supposed to know? Agian lots of people proting this on car forums and denying that it's illegal.

If you wnat to know the law on trailers etc go to a carnival and ask a driver. Their packed up rides and caravans often push the regulations to the mm and kg limits
Trailer laws don't apply to "mobile plant" so when looking to transport one of my larger gas turbines on a Sankey trailer (don't meet current regulations) beind the land rover I just took the trailer body off and bolted the engine to the chassis - mobile plant (compressor / generator) problem solved. And I did get asked once, soon as I said mobile plant the copper backed off muttering to himself.

Robert G8RPI.

SOD,
I agree that forward spikes are likely not the issue unless there is something elso going on. I've used LED's in strobe type applications with pulse currents orders of magnitude higher than rated continious current. The forward pulse limt seems to be the bond wire fusing . Need ot watch out for stray inductance though. Reverse voltage is a differen matter. The shorter wavelength LEDs are more susceptible to reverse voltage.

Off topic but interesting, I once looked at white LEDs for a fast strobe application (fluorescence decay measurement) . I thought the phosphor on the LEDs would have a long persitance but somewhat surprisingly it didn't. The fall of light output was actually much faster than even a standard xenon flash tube arrangement. Trouble was gaps in the phosphor's emission spectrum, some at critical points. Ended up witha xenon tube with a SCR across it to turn the tube off quickly. SCR triggered by tube firing pulse via a coaxial delay line.

Robert G8RPI.

The regulator may not, itself, produce spikes but the way it controls the dynamo might cuse the dynamo to do so. I'm not saying it does, but it might. They will not have tested the regulator with every make and model of dynamo.
Again just assumptions. I wish I was closer, I'd be on the car with test equipment.

Robert G8RPI.

Hi Dave,

What colour LED? 1.5 V Vf seems low for a white LED. These are gnerally a blue or near UV LED with phosphor on top of the chip.

Red ones are a lot more robust.

Robert G8RPI.

While this is an interesting demonstration of the magnitude of spikes I note your LED had a 820 ohm series resistor. This is a lot higher than the 470 ohm in the OP's. This gives a much more reasonable 11mA current at 12V (assuming 3 V Vf for LED).

I can't see what wiring fault would cause the LED to fail.
As I stated earlier the OP's LEDS seem to be running close t absolute maximum curret rating at 12V.

Robert G8RPI

P.S whats the box of electronics next to the coil?

Edited By Robert Atkinson 2 on 17/08/2020 15:23:50

No,
Anything on a cheap VFD will be capcitive and with poor conduction angle to boot. VFDs turn AC into DC then back into AC. the inductive effects of the motor don't cross the DC link. Better ones have power factor correction either active or a AC reactor (choke).

Robert G8RPI.

Hi Tim,

Bit OT but it has come up - legality of LED bulbs. This does NOT apply to your 1929 Vehicle, but the regulations are clear, all required exterior lights on a modern car (post 1985 at least, some regs apply earlier) must be approved and "E" marked. The "E" mark confirms that the light meets the required standards. These standards include that REPLACEABLE lamps (bulbs / filaments) must also meet certain standards (ECE) and be E marked. There are currently NO ECE standards for replacable LED lamps or LED alternatives to filament lamps. If it is not E marked you can't use it for exterior lighting on a modern car. The odd exception is the reversing light.
So How come some new cars have LED lights? This is because the whole light assembly has been tested, approved and E marked and there are no replacable lamps, bulbs or LEDs. You can for instance buy an LED replacement for a sealled beam uit because it is a complete light. LED DRLs are also available.

Back to spikes,

A resistor zener suppressor has been posted to Tim today

180 ohm resistor and 12 V 1 W zener (found a 1 W before a 500 mW) with a bit of Kapton tape insulaion so you can see how it's wired. I also included a 5 mm "12 V 20 mA" LED that made up this has a 470 ohm series resistor and draws 19 mA at 12V. I noted tha while the catalog listed the LED as 20 mA, the data sheet says 20 mA is absolute maximum. So this is righ on the edge. Conneced to the suppressor circuit it survived 28V input (19 mA through LED) though the resistor and zener run hot at that. It also survived 12V negative for short duration (resistor is overloaded by 20 % at this).

I'm confident this will fix the issue.

Robert G8RPI.