Wednesday, January 16, 2019

LM3914 Circuit Ground Connections

I am about to build the circuit that you have here: - Millivolt Meter - Bargraph

I am pretty sure that I understand all of it, except for the symbols that you have at the grounds which are little triangles with the letter F next to them. Please excuse my ignorance, but what does that mean? Is that the ground from the tested millivolt source?


One Ground carries the LED Currents and that track has to be thick and go directly to the Voltage Regulator 7805 which provides 5V supply. If this track is looped with the signal ground, millivolts of error may occur in reading and if tracks are thin it may lead to oscillations.

LM3914 Circuit Ground Connections

We have a High Gain Amp  in this circuit that will amplify even these small jumps in mV due to track Resistance in ground loops.

Analog Ground, Digital Ground, Supply Ground, RF Ground and Earthing have to be carefully designed to get accurate performance. PCB Autorouting may ignore these subtle requirements.

Monday, January 14, 2019

ICL7135 PCB Layout Method

I have design a PCB layout for a 4.5 digit voltmeter using the ICL7135 chip, and i have problems with the analog and digital ground. Do you know if somebody have make a PCB for this chip?

I have seen in your schematic a superb circuit- del2003.sch, ICL7135 based auto ranging DVM. Do you know if the PCB layout is avalaible?

I will appreciate to have some help or if someone can send me the PCB layout for the del2003.sch.
Thanking you in advance,

Mail from RL

I am not able to locate my PCB Layouts, but to study about analog and digital grounds for this chip and pcb layout see this Service Manual i found online.

delabs Notes -  Discuss Topic here EE Forum - Analog Ground

Digital Grounds have Switching Noise or Pulses, Interface Grounds have higher currents. Sensor  Low inputs that are not isolated may have potentials and may lift the system ground and also pose hazard. Electrical Earthing and System Grounding are also different things.

Analog Ground and Digital Ground

Keep the analog and digital current return paths separated.  Remember all tracks and wires are resistors and conductors too. connectors have contact resistance. soldered joints (and connectors) have thermometric EMF. Looping Tracks is wrong Radiating tracks is right.

Read this PDF too Grounding in mixed-signal systems demystified, Part 1
Undoubtedly, grounding is one of the most discussed subjects in system design. Though the basic concepts are relatively simple, the implementation is difficult. For linear systems, the ground is the reference against which the signal is based; and, unfortunately,

Wednesday, February 28, 2018

DC Small Motor Control using PWM

message: Greetings, I have a kit motor controller. I works just fine. It uses the NE556. I read some where ( on a NE555 )that I can change one of the capacitors with a variable capacitor to get the full range of frequencies out of the NE555.

 Is it possible to get adjustable frequencies out of the NE556? I have the circuit drawing. Is there any one I can send the diagram to so they could tell me which capacitor I can change? Thank you,


PWM DC Motor Speed Control - This is a circuit for controlling the speed of small DC motors, it works nicely as a speed controller for an HO or N gauge model railroad.

DC Motor Control for Low Power

Timing 555 Motor Speed - It uses a variable frequency 555 astable. The astable triggers a monostable that produces pulses of a fixed width. At lower frequencies, the monostable pulses are infrequent and the output is high about 20% of the time.


Isolation Resistance and Insulation Resistance

message: what is insulation and isolation resistance, why and how it is measured? what are the consequences if it is not measured? 2008

Isolation Resistance and Insulation Resistance are both related to each other. Isolation Resistance could be in Giga Ohms and Tera Ohms and includes Surface Resistivity.

Insulation Resistance will be in the Range of Mega Ohms and Giga Ohms. You could say Isolation Resistance is a Tera Ohm and Giga Ohm Measurement Test.

Isolation Resistance and Insulation Resistance

Les us assume a 5KV Isolation Opto-Coupler on a Glass Epoxy PCB. You could Measure its Isolation or Insulation Resistance and it will be in the range of 1 Tera Ohms in a good PCB.

A good PCB will have a Slot Routed below the Opto Coupler for High Isolation. This will ensure Dust and Grime along with Moisture will not form a Leakage path below opto-coupler.

Use a Megohmmeter for Insulation Resistance of Transformers, Coils and Relays. Use a Teraohm Meter to measure Isolation Resistance in Opto Couplers, PCB, Ceramics and Capacitors.

Megohmmeter is a special type of ohmmeter used to measure the electrical resistance of insulators. Insulating components, for example cable jackets, must be tested for their insulation strength at the time of commissioning and as part of maintenance of high voltage electrical equipment and installations.

6520 Programmable Teraohmmeter makes resistance measurements up to 10 PΩ. Whether used in automated mode via the Guildline developed TeraCal Software or as a stand-alone meter,

Tuesday, February 27, 2018

Digital Interface Conclusion

Here i am listing the last few telegram messages in this Interface discussion. Only Points to remember are listed.

Resistors attenuate voltage or reduce. Current flow has to be limited. The adc may get damaged when voltage above Vcc is applied

Let us assume 4v is applied to a voltage divider of 30k first 10k second. 3v drops across 30k and 1v drops across 10k - This 1v sample can be measured

Values like 10k and 30k are non standard and expensive. Use 1% MFR resistors only

You can go upto 1 mega - Beyond that leakage currents cause errors. A DMM has 10 meg input resistance For the same reason. Not to save battery but to avoid loading the point. Being tested

DMM Amps mode protect

A transistor heats up if it is not fully on or saturated. Ensure it is fully on. - If Vce sat is 0.5 v it is fully conducting -- If Ic is 1A and Vce is 0.5 V then heat dissipated is 1 x 0.5 watts or 500mW

Try out and see the current through motor using DMM carefully with a 10 ohm 5w in series. 10 ohm is to protect DMM at 2A range, In case of short circuit, dmm fuse won't blow. Put dmm, motor and 10 ohm  in series.

Knowing Electrical Basics both theory and practical is important to be a good embedded systems professional. Just dont jump into coding and expect to master the Art of Electronics. 

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