Hi,
Need to build a Circuit that can measure from 0 to 20A DC
have 24-28 V DC
Will have readouts at 0-20A and 24-28V
Planing to use a microchip ex 16f690.
Any hint on doing that,
The programming on microchip i think i can do, just the "input" i need taking care of

 

I don't think PIC adc can take 24-28 volts. So you will have to scale it down, or to use external adc that can take 24-28 volts.

These panel meters can measure 0-20 A currents: http://www.digikey.com/product-sear...ee=0&rohs=0&quantity=&ptm=0&fid=0&pageSize=25
Looks like the cheapest is 50 USD.

You can do second panel meter for the voltages.

 

I will use a MCU, not the panels.
I will have to do some stuff with the readout, ex sound alarm if we dont charge enough, or to Little time ect.

 

I will use a MCU, not the panels.
I will have to do some stuff with the readout, ex sound alarm if we dont charge enough, or to Little time ect.

Some of the panels might have outputs that could be connected to the MCU.

Otherwise you will need voltage ADC that can take 24-28 volts. That can be done in two ways:
A) buy ADC that takes that voltage
B) build a circuit that will scale down the voltage to the level that your ADC can take (usually voltage divider with some buffer op-amps)

You will need current ADC. Also two ways to do it:
A) buy current ADC (I personally never looked for one)
B) transform current into voltage and feed it to voltage ADC
B1) transimpendance amplifier converts current into voltage
B2) run current through resistor (20 A is A LOT OF CURRENT, you will need one of those big power resistors) and take the voltage measurement across the resistor, your basic application of Ohm's Law.

There are also other options. One of them is voltage to frequency converter and maybe current to frequency converter. What this device does it takes voltage/current as input and outputs square or sinusoidal wave of specific frequency. For example, say you input 10 volts, 10 Hz voltage signal comes out. You can then feed this signal of frequency X to the MCU and program MCU to act as frequency detector. So MCU detects 10 Hz signal and that tells it that the origianl input was 10 volt.

 

Using the voltage divider to reducing the voltage.
You can in series with a current shunt resistor as this and to measuring the voltage, and calculating I=V/R.

 

Google "high side current sense" and you will find some good devices from TI or Mamim and more that will convert a current on the positive line to a ground referenced voltage a PIC can easily convert with it's A2D.

For the voltage all you need is a resistive divider, and some over voltage protection would also be nice.

 

Hello,

You can also use a hall effect sensor for measuring the current.
http://www.scienceshareware.com/how-to-measure-AC-DC-current-with-a-hall-effect-clamp-.htm

Bertus

 

An appropriate shunt resistor (few mOhms) and a high-side current monitor IC (e.g. ZXCT1009) will measure DC current and make it so that a 5V ADC can read it.

A two-resistor voltage divider will make it so that the 5V ADC can read a higher DC voltage.