CRT monitor radiation measurement project

[regulaz]

page 16/17 provide good details of electric fields https://www.electro-tech-online.com/custompdfs/2013/01/wcms_107821.pdf

is there a similar document for lcds?

 

[ronsimpson]

now the only question remains what is this 300 millivac for crt and 1000 millivac for LCD that is being shown from the rad screen ground? is it spurious radiation?

I think you put a 'rad screen' in front of a CRT and a LCD and you measure 'volts' from screen to ground.
I think you can do this with any product with a transformer or switching power supply and you will measure volts AC.
This depends on what type of shielding is in a product.

A mili-volt meter is very sensitive. I just used a meter and a piece of blank PCB material and I measure milli-volts when the meter is near the wires in the wall. My body makes a voltage. (low or high depending where I stand in the room)

I can measure 60 and 120hz noise every where. I see 50khz noise from the printer. 80khz and 200mhz from the PC. The LCD monitor makes noise at 100khz for the power supply and RF noise from the display. My phone makes noise that the PC speakers pick up. Your AC meter is only reading noise at low frequency. I use a spectrum analyzer to see "volts" at different frequencies. There is several sources at 2.5ghz, 5ghz. I can see the cell tower transmitting. There are 30 frequencies from 530 to 1575khz and from 87.7 to 108.0 MHz. Across the road some one is transmitting at 200mhz.

 

[regulaz]

The LCD monitor makes noise at 100khz for the power supply and RF noise from the display.

since everything is producing its own hum, then the bottom line is, is it worth it to use a rad screen esp if someone is working 8-9 hours in front of the screen? (as the ilo pdf n previous link shows the measured values too)

you have a spectrum analyzer of all things! goodness, can you please see if indeed lcds are producing more elf/vlf/rf than a crt like sony trinitron 21"? your inputs will be invaluable.

that should settle it.

 

[Nigel Goodwin]

since everything is producing its own hum, then the bottom line is, is it worth it to use a rad screen esp if someone is working 8-9 hours in front of the screen? (as the ilo pdf n previous link shows the measured values too)

you have a spectrum analyzer of all things! goodness, can you please see if indeed lcds are producing more elf/vlf/rf than a crt like sony trinitron 21"? your inputs will be invaluable.

that should settle it.

As it would still be measuring 'nothing' then it's pointless - you can't measure any imagined 'radiation' in that way.

Why are you at all bothered about this?, it's obviously true that 'a little knowledge is a dangerous thing'

Incidentally - the main 'fears' with CRT's were XRAY radiation, with TV's having circuitry to disable the set if the EHT goes too high (which could cause XRAY emissions)

 

[ronsimpson]

All CRT and LCD monitors I designed meet FCC a or FCC b specs and were testes at certified labs.

 

[unclejed613]

let me see if i've got this right... you're putting a conductive screen in front of the face of the CRT and you're measuring the voltage picked up by the screen? if so, then what you are picking up is capacitively coupled noise from the electron beam and it's beam current variations. since you are measuring this with instruments with high input impedance (10MEGohms for a standard DMM), very small currents are showing up as very high voltages, but very little energy is actually there. if you measure with an old-fashioned analog meter, i'm sure the voltages would show up as being MUCH lower. try putting a 10K resistor across the DMM input and see how much voltage develops across a "loaded" meter VS an (almost) open-circuit meter. i think you will find that these "huge" voltages are just a result of very tiny currents developing a voltage across a very large input impedance.

any small changes in the path that these leakage currents take to ground equals large changes in the voltage you are measuring.

 

[ronsimpson]

When we talk about radiation form from a CRT monitor they are condenser about X-rays or low frequency magnet fields.
X-rays have been talked about above.

The deflection yoke uses magnet fields to move the beam. There are fields at the vertical and horizontal rates. V=60 to 85hz (more or less) and H=31.6khz for VGA and up to 120khz depending on the monitor. Using an electric razor or electric tooth brush is far worse than a monitor. 60 hz fields come from transformers. The power lines on the road cause fields. I don't know anyone has been hurt by a magnetic field form a monitor but, just in case, we built yokes with low radiation (out the front) of monitors.

I don't think the tests we are talking about will pick up a AC magnetic field. LCD monitors do not use magnetic fields to make the picture.

I think you should get large DC voltages by moving the shield closer or farther away from the monitor.

Your monitor has 25 to 30kV only a inch or two away from your shield. I think you should get readings on the meter just from static build up.

 

[regulaz]

As it would still be measuring 'nothing' then it's pointless - you can't measure any imagined 'radiation' in that way.

Why are you at all bothered about this?, it's obviously true that 'a little knowledge is a dangerous thing'

Incidentally - the main 'fears' with CRT's were XRAY radiation, with TV's having circuitry to disable the set if the EHT goes too high (which could cause XRAY emissions)

i was moved to botheration after reading this report https://www.electro-tech-online.com/custompdfs/2013/01/SAGE20first20interim20assessment.pdf it shows the imaginary fields being harmful to an extent.

and also the previous ilo pdf.

 

[regulaz]

I don't think the tests we are talking about will pick up a AC magnetic field. LCD monitors do not use magnetic fields to make the picture.

I think you should get large DC voltages by moving the shield closer or farther away from the monitor.

Your monitor has 25 to 30kV only a inch or two away from your shield. I think you should get readings on the meter just from static build up.

i understand lcd doesnt use magnetic fields to make the picture however it could still have magnetic fields from its operation i suppose. When i bring in a compass in front of the lcd screen it is easily spun in different directions showing their is some magnetic field. I suppose that field could be measured.

I tried with the DC voltage setting but didnt get anything on either lcd or crt somehow.

i suppose it is static likely but wasnt sony trinitron 21" monitor designed for no static build up on the screen, i cant feel any static on the screen when i touch it.

im glad there is an electronics forum to have these elementary questions answered!

 

[regulaz]

let me see if i've got this right... you're putting a conductive screen in front of the face of the CRT and you're measuring the voltage picked up by the screen? if so, then what you are picking up is capacitively coupled noise from the electron beam and it's beam current variations. since you are measuring this with instruments with high input impedance (10MEGohms for a standard DMM), very small currents are showing up as very high voltages, but very little energy is actually there. if you measure with an old-fashioned analog meter, i'm sure the voltages would show up as being MUCH lower. try putting a 10K resistor across the DMM input and see how much voltage develops across a "loaded" meter VS an (almost) open-circuit meter. i think you will find that these "huge" voltages are just a result of very tiny currents developing a voltage across a very large input impedance.

any small changes in the path that these leakage currents take to ground equals large changes in the voltage you are measuring.

that is a very good explanation the kind i was looking for, that resolves the CRT question. Now what is the1000 millivolts being shown on the LCD coming from? the internal electronics, it doesnt have such a powerful gun inside still the screen is catching 3 times more volt AC than compared to CRT.

 

[alec_t]

Now what is the1000 millivolts being shown on the LCD coming from?

If the LCD backlight is the fluorescent lamp type it has a high voltage supply and therefore creates an electrostatic field, which is probably what you are detecting at the screen via capacitive coupling.