Tuesday, December 12, 2006

Wanted - antistatic or conductive road surface material or coating

I have a number of client who's customers suffer shocks when they touch a car park ticket machine. In most cases this is due to the electrostatic characteristics of the road surface materials. The solution is far from easy.

I am looking for a conductive coating material that can be applied to, or instead of it, asphalt or epoxy coatings on car park approach road surfaces.

Another possibility might be to fit conductive speed bumps before the barrier - I am also looking for any manufacturer who may be interested in supplying such things.

If you can help, please get in touch via my web site.

Thursday, November 30, 2006

Kelvin electrostatic generator generates static from water

I have been told of a simple apparatus that can generate an electrical potential out of drops of water. Can you give me any guidance as to where I can find details of the construction and the underlying science. I am told that it is called a storm in a teacup.

You may be thinking of the Kelvin electrostatic generator. This takes water from one container and drops it inoto two others. There is a system of loops and wires connected to the "output" cans which feeds back voltages to the input, resulting in the output cans charging to a high positive and negative voltages respectively
If you search on google you will find other references.

Can rubbing the contacts damage a DIMM module?

A friend told me to clean the finger contacs of a DIMM memory module with an eraser :))).. I told him don't do it that way because you will get esd damage.... am I wrong????

To be honest, I don't know the answer to this. I think the main thing is that you should wear a wrist strap to ground your body while handling the module. If you are not doing this the risk is far greater that ESD from your body will damage the module.

If you are wearing a grounded wrist strap while handling the board, and working on a grounded work surface (if you are using a work surface), then I think the ESD risk from static generation as you rub the contacts is probably small.
There must be a better way to clean the contacts, but I don't know it.

Wednesday, June 14, 2006

Did I damage my RAM stick?

I'm recently going to upgrade my computer with a 200GB HD and 1GB RAM. I've been "messing" for months with the inside of my computer, but something happened yesterday. I opened it and took out both RAM sticks (256 + 128). Did some other stuff also, like cleaning and moving some cables, in order to make room for the HDD. When I closed my computer, opened it, it went directly from the POST screen to a black one. Then windows couldn\'t start because of a file missing. Then I couldn't start the Repair console or the Linux Live CD. It turned out to be the 128MB RAM stick. I took it out and it worked. I have some questions though.
1. Certainly it was damaged by static right?
2. Is there a chance that I ruined something else, including the RAM slot? I ran a 2h stress test and no errors came by.

We can rarely say for definite that ESD is the cause of damage unless expensive failure analysis confirms it to be so. However it certainly sounds as if it is possible, even likely. ESD can weaken semiconductor components and give later failures - so it's possible that another component could be damaged. You'll only know if it fails.

3. I\'m now buying a wrist strap from amazon now... to avoid more problems. Will this prevent any damage from ESD? I have to clip the wire to the metal chasis of the pc, unplugged?

Connect the wrist strap to the PC chassis and wear it in good contact with the skin of your wrist. For safety, the pc should not be plugged in. Don't touch and electronic parts until you are connected in this way.

I may have to work on a carpeted room... but if I absolutely have to, I'll work in some other room.

If you use the wrist strap it will remove most of the risk of damage, carpets or not. The biggest cause of ESD damage is from a charged person to a component at a different voltage. By connecting to the pc via the wrist strap you are "equipotential bonding" yourself to the pc and there will be no voltage difference between you and it - no voltage difference means no ESD.

Try to keep other sources of static such as polythene or foam packaging well away from the pc while the covers are off. Once the covers are back on, it is well protected.

Tuesday, March 07, 2006

Shocks in winter

As a baby i would give my mother shocks when she picked me up, and if i touched a car and someone was leaning against it they would receive a small shock too. I am now 40yrs old and have had many many static shock free years but at xmas it returned with a vengeance. The shocks i receive closing my car door cause a heavy sensation in the muscle of my arm, sometimes elbow down, sometimes into the upper arm and the pain can last for up to 8hrs... this now extends to receiving shocks from payment machines in car parks, from my allow wheels when checking tyre pressure, from light switches in my cottage, from my lamp. It sounds over dramatic and daft, i know, but it is really getting me down now and i have a certain amount of trepidation about touching some items that cannot be avoided - let alone the shocks i give from time to time to fellow colleagues. Brushing my hair is a theatrical act in itself.

Have you read my on-line article ? Most shocks of this type are due to the person becoming charged up with static electricity because of the materials of their shoes, floor material and furnishing materials. This often becomes worse in winter due to dry air conditions.

It's hard to say what can help in any particular case without investigation of the particular case. Sometimes choosing different types of shoes can help. Unfortunately the real cure may be in changing floor or furniture materials which is often not practical or expensive.

These problems often become less or disappear in spring when the weather becomes warmer and damper.

Tuesday, February 21, 2006

Why does static electricity increase in cold weather?

I'm hoping you could give me some information about why static electricity increases in the cold weather?

The reason static is more problematic in cold weather is because of low humidity of the air. "Relative humidity" is the percentage of moisture held in the air compared to the maximum it could hold at that temperature. So 50% rh means the air has only half the amount of moisture it could hold. It turns out that static is promoted if rh drops below about 30%.

Cold air can hold less moisture than warm air. So cold outside air at say 0oC and 100% rh is taken into a building and heated up to make it comfortable. The relative humidity drops by one half for every 10oC rise in temperature - so if no moisture is added, the air will be 50%rh at 10oC and 25%rh if heated to 20oC. 25% is certainly dry enough to promote static electricity!.

The main factors conributing to static electricity indoors are floor covering and shoe sole materials, and furniture covers materials, and dry air conditions.

Thursday, February 02, 2006

Grounding through ESD footwear and flooring

We have a very peculiar problem while using the ESD ( Static Dissipative ) Shoes. The the pass band - lower limit on the tester is 0.75M Ohm and the upper limit is 35M Ohm) some shoes pass and some fail. Are the limits set on the footwear tester ok or should we increase the upper limit?

The limits on your tester are correct for measuring resistance from the person's body to ground via the footwear and flooring, for a person who is handling ESD susceptible parts. This is recommended by IEC 61340-5-1 to be between 750 k ohm and 35 M ohm.

However the limits might not be correct for testing footwear as worn, to a metal plate electrode. Under the IEC 61340-5-1 standard the range limit for this is between 100k ohm per shoe, or 50 k ohm testing both shoes together, and 10^8 ohms. Footwear can be within this range (and so compliant with the standard) and yet fail your test.

For primary grounding of personel (in other words, the main means of grounding people who are handling ESD susceptible parts) the resistance for shoes needs to be lower than the maximum required resistance for grounding through footwear and flooring. There must be some margin for the resistance of the floor. Also, the floor resistance needs to be low enough to achieve a resistance to ground less than 35 M ohm - this will require floor resistance considerably less than 10^9 ohm allowed by the standard. In this case the best procedure in my view is to measure the resistance from person's body to ground while they stand on the floor - this is recommended to be between 35 M ohm and 750 k ohm. This measurement, and compliance with this resistance range, is not specifically required by the standard.

In summary:
1) 61340-5-1 allows footwear and floors to have higher resistance than would be needed to achieve 35 M ohm resistance from a person's body to ground
2) In order to achieve 35 M ohm resistance from body to ground you would have to choose footwear and flooring types that would reliably give low enough resistance
3) It is recommended, but not required, by 61340-5-1 that the resistance from a persons body to ground should be between 750 k ohm and 35 M ohm when grounded by footwear and flooring - if they are relying on this as the primary means of grounding while they are handling ESD susceptible components.

Monday, January 23, 2006

What is the average static shock (in volts) that you get from touching a doorknob?

I cannot answer this question directly but I can say that body voltages can vary from zero up to 35000 V and above. In general you don't feel shocks unless your body voltage is above about 3000-4000 V. Getting out of a car, about 8000-10000 V is not uncommon. In general the likelihood of achieving a certain body voltage decreases as the voltage increases, so voltages of a few hundred volts are "normal", and a few thousand volts not uncommon, but tens of thousands of volts are more unusual.

Thursday, January 05, 2006

How much of a charge can a human body build up from static electricity?

This question is a bit like "How long is a piece of string?" The answer depends on various factors and circumstances. We tend to think in terms of voltage rather than charge, because as the saying goes, "It's volts that jolts" - in other words voltage, rather than charge, causes us to feel a shock. About 3000-4000 Volts on the body will cause us to feel a shock when we touch some object. A shock felt when getting out of the car may be caused by a body voltage of about 10000 V. Body voltages up to 35000 V have been reported, but I see no reason that higher voltages might not be possible in exceptional circumstances. Body voltages up to 10 kV (10000 V) are commonly responsible for the shocks we feel in daily life, and are largely caused by insulating flooring, footwear and furnishings. For more information on this see my article on Static shocks and how to avoid them.