How to test esd wrist strap with multimeter

How to test esd wrist strap with multimeter

I want to buy an anti-static kit for handling electronic components (I assume all electronic components should be handled with properly grounded anti-static mats, right?) Some reviews mentioned that the tape I bought "has no continuity".

One of the reviews said.

  

A warning to buyers and all manufacturers of anti-static computer wristbands: before I started assembling my computer and testing this $400 motherboard, I decided, based on past experience, to test whether Rosewill's anti-static wristbands actually worked using a multimeter.  The results were surprising. There is no continuity (connection) between the ALIGATOR clip and the metal pad of the wristband: it is not an anti-static protection! It is not a static protection.

     

I have several questions: 1) Is the bad connection only in the bracelet I just bought or is it in the group? 2) How many people have bought an antistatic bracelet of any brand, put it on thinking it was protected, and then ruined the electrical part thinking it was protected! This is not possible.

     

And to make matters worse, now almost everything is cheap from China (CCS) and it is really difficult for a company to ensure that products from China/Asia are of good quality.

I came across Meitu (just before reading this review) thanks to a video of a water damaged iPhone. The user uses a multimeter to connect one side to something (not seen in the video) and tests both sides of the capacitor to see if it beeps. If both sides beeped, it looked like a faulty capacitor, but if only one side beeped, it meant it wasn't faulty.

So I'm curious.

If the information in the video is true, what exactly is causing the beeping and why is it only heard on one side?

What exactly am I testing and where, on the antistatic wrist strap? The definition of continuity is "continuous flow". So, should I test both ends and different parts of the cable for any problems, or...?

Any information on continuity testing, and any information or reference reading material on electronics would be greatly appreciated, thanks.

Edit: The commentary is this link.

The video is about 26 minutes long and is at this video link/p>.

Unfortunately, the normal part doesn't work, so I can't put it directly into the timestamp.

    

Antistatic continuity protection

Question by XaolingBao

4 replies

17

Brief statement.

A good wrapping system has a high resistance between skin contact and substrate. It is usually 1 MΩ, which is relatively high. This high resistance is for your safety in case a failure somewhere connects you to a dangerous voltage. Even this high resistance is enough to protect your equipment from ESD.

A normal multimeter should be able to measure this resistance. However, most continuity tests will probably report it as an open circuit. This is because many continuity functions look for a resistance less than 100.

Checking the sleeve :

Measure the sleeve : Measure from the skin contact surface of the sleeve (red test line) to the part that is directly connected to ground (black test line). Make sure your meter is set to measure a resistance greater than 1 MΩ, as resistance is not an exact tolerance. Note that in this picture I was careful not to touch any metal surfaces, as this would give a bad reading.

Measure the resistance from the inside of the strip to the outside or the end of the wire. If it is not in the proper range, make sure it is on the high resistance scale of your meter.

In the lab where I work, the resistance is somewhere on the wire (black line). It shows up as 1 MΩ on the pressure box and all the wrist loops have a low resistance connection from the skin end to the pressure port.

New and useful for continuity testing.

The difficulty with the cable and strap is that the continuity check function is always on: it looks like an open circuit (no continuity). With this setup I would expect the meter to show/detect continuity on the front of the wrist, but not on the cable.

Conclusion.

It is impossible to draw conclusions from the report without knowing if the tester only performed continuity tests or if (or when) he measured the resistance. If the tester knew how to measure the circuit correctly, he would have discovered that it was a defective product. It would be useless for the reviewer to attempt to perform a continuity test on a 1 MΩ resistor. The wording of the review leads me to lean towards the latter, but it is impossible to deduce from what is written.

    

W5VO replied that

4

The built-in resistance can be over 10 million ohms and a cheap DMM meter would show this as an open circuit.

Wrist straps should not go through the body at ground level. If they do, there is a risk of electric shock, especially if the AC terminal is accidentally struck with the other hand, the one without the wristband (this will cause the AC to pass through the chest and out of the grounded wrist).

To avoid this danger, wristbands should have a built-in resistor in the megohm range. After all, they are not designed for you to touch a dubious electrical tool barefoot in salt water. Instead, the idea is to subtract DC voltage (which can be up to a thousand volts) from any surface charge to remove from your body.

What is the appropriate maximum value for a series resistor to perform this task? The resistor should discharge its own capacitance significantly in about one second. 0.1 second is probably better, so let's assume this value. The capacitance of the human body with respect to ground is about 100 pF, but in the worst case you could be lying on a concrete floor with steel grounding. This creates a body with a much higher capacitance to ground. Suppose the maximum capacitance of a person is three times as large: 300 pF.

The calculation of the total time constant is simple: T(s) = R (ohms) x C (farads). Or, R = T / C. Thus, R = 0.1 s / 300x10 ^ 12F = max. 3 Megaohms. Higher resistors will still work, but the kilovolts accumulated in the body will be exhausted in a few seconds, not almost immediately.

What is the correct minimum resistance? When you put your finger on a 120VAC line, you really want to produce well under 1 mA. Let's say 0.1 mA is a reasonably safe value for current through the box. Let's say again that 240 VAC is dangerous for the mains voltage. According to Ohm's law, this gives a minimum value of R=V/I=240V/0.0001A=2.4 megohms for the series resistance of the bracelet.

Different manufacturers may take a more cautious approach and the resistance of their bands may be closer to 20 meters than 2 meters. If this is the case, a typical DMM will show an open circuit. However, your high voltage body load experiences the same resistance, which is almost equivalent to a short circuit. In this sense, "friction static" is strange: typical power supply impedances are greater than 1M and can climb into the gigohm range. An ordinary voltmeter can only measure the voltage of a human body when a "short circuit" of 10Megs is applied. To make such high voltage measurements, your voltmeter needs a Zin value of 10G-ohms or higher, not the typical 10M-ohms.