Metrology grade Lighting - I am in the dark

[beanflying]

So it's 10pm 25-26C in my shack and I am playing with a newly acquired toy a Fluke 335D before tucking it in for the night.

There is an absolute change of 2-3PPM measurable and repeatable every time I turn on one of my Bench LED's that is well over 1M away (same mains circuit). Quite likely it is the power supply causing hash but as I move toward more accurate measurements what is the conventional wisdom on Lighting for those last few PPM of measurements? My main overhead strip LED's don't change the numbers at all on or off (drivers are further away).

Stick with LED's and move to Linear drivers or just better SMPS with some EMI shielding or filtering, look at alternate lights or look at isolation of instruments behind filtered UPS supplies?

Out of Cal 34401A and untweaked and unstickered 335D has to be a Fluke 

[branadic]

To say it with the words of Jessie J, Flashlight

-branadic-

[Kjelt]

Is it a spike or continuous deviation?
If it is a spike you could try a professional driver with for instance DALI or other digital interface since these are 24/7 powered on.
You should not get a spike on switch on in that case from charging the empty bus capacitor.

[Kleinstein]

Some of the LED and fluorecent lamps can produce quite some EMI problems. With the LEDs there are quite different types of current regulation - some are well behaved while others can be messy. Old instruments tend to care less about a possible effect of RF signals (especially the mobile phone signals) -  so in part it could also be the 335D to blame.

Chances are that here both ends are at fault: the lamps with high emissions and the 335D being too sensitive as a radio receiver..

[TiN]

Try shielded wiring, or at least twist the leads together.

[beanflying]

Is it a spike or continuous deviation?
If it is a spike you could try a professional driver with for instance DALI or other digital interface since these are 24/7 powered on.
You should not get a spike on switch on in that case from charging the empty bus capacitor.

Stayed stable at those numbers on or off for a period of minutes until turned or on or off with an instant change up or down.

The 335D only turned up today and I haven't cracked it open or looked at the circuit diagrams to see if it has any powerline input shielding but it is effectively double cased by it's outer enclosure and internal paneling.

Swapping out the over bench LED SMPS is the logical start I guess.

[Echo88]

Changing the SMPS-LED-Driver against a linear power supply with snubbers at the rectifier, to avoid any EMI. Or change LED-Lighting against Halogen-Lamps, which will still produce short measurement spikes due to the lamp-inrush current when powered on.

[beanflying]

Try shielded wiring, or at least twist the leads together.

Certainly not ideal wiring and I need to make some better sets of test leads for the high voltage anyway. I need to chase up some multicore shielded for my other HP 740B Null meter rebuild as well.

[Dr. Frank]

We had that discussion over in the Super LTZ1000 thread.
There, the original LTZ circuit was affected by any SMPSU, including 'modern' LED illumination.
Latter lamps don't have any EMC shielding, and are REALLY noisy, and I really wonder how these ever get a compliance certificate.

The x kHz EMC radiation is induced into the supply and output cables of the instruments, even twisting and shielding does not help.
That RF is then rectified in the LTZ transistors, which leads to a shift of the oven temperature, or caused frequent spikes and dips in the output voltage, in the range of several µV, or ppm.

The successor of the 332/335, the 5440/42, is practically immune to such EMC radiation, as well as the 3458A, when 10V are applied from the calibrator.

Therefore, I assume that the old design of the 332/335 is susceptible to disturbance, similar to the LTZ circuit.

Latter can probably be hardened against EMC as well, by adding some blocking capacitors parallel to these sensitive pn junctions, but I can hardly guess, which one.

The possible circuits, like the reference, the chopper amp, and the other module, are so big, that they can easily pick up any disturbance from the output jacks.
You may try to identify the root cause by directly measuring the +15V reference voltage with and w/o LED illumination.
That's accessible inside, labelled as Master Ref Test. Please do that testing in the 10V range only!

As the easiest solution, what I also have done in my lab, I recommend to remove all SMPSUs, and use regular incandescent, or fluorescent lamps running directly on 50/60Hz mains only. Where you can't avoid SM PSUs, at least watch out for EMC 'stinkers' inside your PCs / laptops, which also might radiate excessively, especially if after years the snubber capacitor has died.

By these two measures, i.e. no SMPSUs, and use of blocking capacitors, it is now possible to make sensitive and stable DCV measurements in the sub 0.1ppm range.

Frank

[David Hess]

Latter lamps don't have any EMC shielding, and are REALLY noisy, and I really wonder how these ever get a compliance certificate.

They often do not meet compliance requirements and very occasionally our FCC requires them to be removed or the user faces fines.

As the easiest solution, what I also have done in my lab, I recommend to remove all SMPSUs, and use regular incandescent, or fluorescent lamps running directly on 50/60Hz mains only.

For lighting, that has been our surefire solution in the past but finding passive fluorescent ballasts has become more difficult.  I actually prefer the high frequency electronic ballasts but not if they spew noise.

I prefer linear fluorescent lamps to incandescent lamps because of softer shadows and less heating although a noise free variable transformer can be used to dim incandescent lamps.

[beanflying]

Thanks All

Back is the shack this morning Meter is still locked at 10.00002 at 20 degrees so looks like both are drifting nicely together if much at all.

I had a daylight look at my bench lights which are Flexible ones to COB array's the offending power supply runs one of them and the other running two seems to not change the reading at all or at least below the 34401's ability to see it. I dropped some clip on ferrite cores which helped but it was still noticeable on the meter. more like 1 flicking 2 PPM. So I will junk both and go to a single linear one over the bench.

I tried upsetting the 335D with the phone on WiFi and straight to 4G sending data and no change even with the phone between the sloppy wiring .

Also quick check of the Fluke shows only a pair of 0.1 uf Caps to earth as suppression so I will likely add something better there too. The Entire Master reference is fed off the +25V rail and there doesn't seem to much localised suppression on the heater or reference circuit so I will have a look at that some time when I have the covers off and check the Capacitors on the supply rails too.

As to Bulb style lamps with suspect enclosed drivers nearest one is 20m+ away. In Australia we see a mixed bag from cruddy cap droppers to fairly nice ones from Philips and the like.

[CatalinaWOW]

For line conducted noise I have made some power boxes with appropriate filters in both legs of the line.  Somewhat equivalent filtering is available in some commercial power strips.  Obviously won't help if it isn't conducted noise, and even if it is conducted it can't stop everything, but it is worth a try.

[PTR_1275]

A battery and a halogen light globe globe

No noise, but then it gives off UV light and extra heat.

[beanflying]

Next thing you will have us using Soy candles to test by, the added Heat might however be of benefit to you Northerners at the present Pleasant 35C starting to cool with the Sea Breeze up a whole 5PPM in 8 hours and 15C ambient increase which is about all 34401A based on other testing so looks like the Fluke is a keeper.

Found this EMS D506 supply in my collection I don't a plan for it any more. Just need to wind the voltage down to 17V and drop it in an enclosure with an EMI suppressor. Specced at <5mV and linear should solve/minimise my issues.

[beanflying]

Just a quick follow up. Removed problem SMPS and the other one over the bench and as I 'cant see' a problem from the overhead strip LED's I will leave them alone but switch them off prior to anything critical with Calibrations and the Bench lights are adequate pointed at the Calibrators.

Side issue tried tripping up the output of the 335D by running the Valhalla under it. With no leads no change in live/standby (range of voltages and Frequencies) with leads wrapped around the DC output as expected all over the place. The reverse was also true with AC out being unchanged by the Fluke live/standby.

[David Hess]

Next thing you will have us using Soy candles to test by, ...

I have thought about designing a small linear audio power amplifier to drive low power 120 volt AC lighting for dimming where I do not have a variac handy.

[Kjelt]

We finally have lightsources that can run on low voltage dc, why does everyone keep going back to use high voltage AC ? 

[Gyro]

Agreed, I'd go for LED light fittings with decent diffusers and a transformer linear supply with well slugged rectifiers.

[David Hess]

We finally have lightsources that can run on low voltage dc, why does everyone keep going back to use high voltage AC ? 

That is no solution if the lamp contains a DC to DC converter for ballasting.  All you have done is removed the AC to DC conversion stage which was not the problem anyway.

Actual low voltage DC with resistive ballasting would of course work unless the DC source is noisy and with high efficiency, this allow acceptable copper losses.

[Kjelt]

That is no solution if the lamp contains a DC to DC converter for ballasting.  All you have done is removed the AC to DC conversion stage which was not the problem anyway.

The whole problem is that people are still using AC dimmers to dim low voltage DC lighting. The ballast should take care of that by means of protocols or even simple dc voltage (0-10V) control.
The reality is that mr Jones still wants to replace their 1900s tech bulbs with modern LED lights and also keep their 1970s tech dimmers.

[David Hess]

That is no solution if the lamp contains a DC to DC converter for ballasting.  All you have done is removed the AC to DC conversion stage which was not the problem anyway.

The whole problem is that people are still using AC dimmers to dim low voltage DC lighting. The ballast should take care of that by means of protocols or even simple dc voltage (0-10V) control.
The reality is that mr Jones still wants to replace their 1900s tech bulbs with modern LED lights and also keep their 1970s tech dimmers.

No, that is not the whole problem.  AC phase control dimmers are noisy but even without them, many LED and fluorescent lamp electronic ballasts are completely unacceptable as far as EMI.  I have used a variac to dim dimmable LED bulbs before and it hardly made any difference as far as EMI.  The problem is the cheap electronic ballasts.

LED bulbs with passive ballasts and a variac should be noise free but you will have to build it yourself.

[Kjelt]

From a ballast manufacturer point of view it is way much easier and better to design a DC ballast with control signals than an AC mains operated ballast with distorted waveforms which can be at least 6 different versions with distortion where the microcontroller has to figure out what should be done.
Not too mention the high peaks (380V) that the input circuit has to tolerate, the other overvoltage conditions etc. etc.
I have seen both ballasts and about half the size of the ballast could be saved with an DC input and the life expectancy would also increase dramatically.

[tkamiya]

All of my lighting in the area are either LED or compact flourscent.  However, I have dedicated wiring coming into lab from breaker panel JUST for equipment.  It goes through a giant UPS and goes to every rack.  Lighting and general use sockets are totally separate. 

No problem so far.