Circuits FAQ

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Sunday, November 17, 2019

Guard rings in strain gauge measurement

Hi... Can you please tell me as to what is a guard circuit used in bridges?

Mail from SN

The very high impedance amps for bridge circuits have inputs in tera ohms and bias in pico amps. The sensitive bridge circuits must be able to detect a 1 uV difference when a 100 gm weight is placed on the cantilever. So it is very sensitive.

The power lines and other logic lines close to chip high impedance pins may leak pico amps to nano amps of currents, humid-air or PCB contamination or power pins being too close, the leakage currents may show as full scale reading without input.

Read also driven gaurd - a better method

" The basic principle is to protect the sensitive wire with a surrounding guard that is held at the same voltage as the wire, if they are at the same voltage then there will be no current flow."

Read also driven gaurd - a better method

The guard rings around these sensitive pins or shielding will act as return paths for the leaking nano amps.Even ionic discharge from a sharp pin with high voltage can imbalance bridge. As it is like an Electrometer.

Read more here. guard rings in strain gauge measurement

The Strain Gage

Strain gage transducers usually employ four strain gage elements electrically connected to form a Wheatstone bridge circuit

Electronic Electroscope
This simple circuit can detect the invisible fields of voltage which surround all electrified objects. It acts as an electronic "electroscope."

Guarded resistance bridge
By using a guarded scanner and two sources to form a guarded resistance bridge, measurements from 100KW to 10GW can be made with excellent accuracy. This system provides a simple yet effective way to activate both the high and low guard circuits.

Compact DWDM laser Temperature Control with the MAX8521

The guard ring functions as follows. Because the DAC output voltage is similar to the op amp's inverting input voltage there is essentially no potential difference between the guard ring and summing junction and its associated components inside the ring. The ring intercepts outside surface leakage currents where they are shunted by the low impedance of the DAC.


Saturday, November 16, 2019

Lighting a LED String - Serial Drive

Hi there, my name is steve. I would like to ask you about the best way or the most efficient way to supply LEDs. there are 36 small LEDs or 72 small LEDs (5mm).

could I just put then in series the 36 LEDs and a diode, resistor and plug in to AC power line (110)

(An email i got some time back)

Answer - Lighting a LED String - Serial Drive

Remember that the LED for lighting or otherwise needs to be protected from over current on Forward Bias and High Voltage connection in Reverse Bias.

That means you could use a 3 A Diode like 1N5408 to prevent wrong polarity connection. Then all you need a constant current source with a voltage output a little above the product of the Number of LEDs and Forward Drop of a LED.

5 LEDs in Series with a forward drop of 3 each means 5 x 3 = 12. You need a 15 Volts Regulated Supply with a current limit of around 30mA or more. See the LED datasheet. Go beyond limits you lose your LEDs the same day or months later.

Use SMPS designs for smaller sizes and it has to be Robust to Mains Power Supply Variations. Use a Surge Protection Assembly from a Reputed and Quality Supplier. (this surge part spoils the reputation of your Lighting Product and affects the entire LED lighting Business)

Read these pages to learn more....

Lighting a LED String - Serial Drive

The Perfect LED Light -  by Garry Petrie

"The perfect LED light is designed for cavers who want an efficient, uniform and constant bright light for caving. This article contains some very specific technical information and it is assumed the reader has a basic understanding of electronics. To build the light, the reader also must have some experience soldering, drilling, wiring and cutting small objects."

A linear Constant Current LED Lamp Dimmer

"This simple Linear circuit provides continuously variable regulated current (~25-400mA) from a 4-6 Volt source.  I chose a linear design for simplicity, reliability, ease of repair, and to avoid switching EMI in my Cave Radios.  The circuit requires only 0.2V headroom above the parallel LED Array voltage to provide regulation at maximum current. "

Friday, November 15, 2019

Electrometer, Gauss Meter and EMF Meter

Hey folks, I'm brand new here and to tinkering with electronics in general. I'm looking into building my own electroscope.

My question is: What's the difference between a static field and an EMF field? There seem to be detectors for both, so I wanted to know what the difference is, and what the relationship between the two is.

Question by BCM USA

delabs Answer

Static Field could be Electrical or Magnetic. Electric Charge as a static Field can be measured using Electrometer

Electrometer, Gauss Meter and EMF Meter

Simple Electrometer - A trivial JFET electrometer isn't a new idea, there are millions of designs for them out on the Internet, this is just yet another. This one is slightly unique in that it is a bridge configuration that makes it ultra-sensitive, much more so than a gold-leaf electroscope, but less than a vacuum tube or electrometer grade FET device.

Static Magnetic Field is measured using A magnetometer  also be called a gaussmeter. Gauss_meter

"Magnetometers are measurement instruments used for two general purposes: to measure the magnetization of a magnetic material like a ferromagnet, or to measure the strength and, in some cases, the direction of the magnetic field at a point in space."

EMF is the field of a Moving Magnet, an AC field. This Moving Magnet could be simulated by a Sine Wave in a Coil. Use a EMF Meter for this. EMF_meter

See some Instruments that are used here - AlphaLab Gauss Meter and EMF Detectors