Current source with High side load

Status
Not open for further replies.
H

hemnath

Advanced Member level 3
Joined
Jun 24, 2012
Messages
702
Helped
61
Reputation
120
Reaction score
57
Trophy points
1,308
Location
Chennai
Activity points
6,588
I am designing a 4 to 20mA current source using DAC.

How to calculate the resistor value to control the current. Please help.
 

Attachments

  • KCoCp.jpg
    5.2 KB · Views: 149

Hi,

I´d call it "low side load", because the load is connected to GND (and not to V+)

R2 = (V+ - V_Ref) / I_desired.

Where (V+ - V_Ref) is the voltage across R1.

With this type of current source:
voltage_across_R2 = voltage_across_R1.

Klaus
 
Where's the DAC output signal connected to the circuit?
 

That circuit has the current source Vref referenced to the plus supply, not ground.
I assume you want Vref referenced to ground for the DAC output(?).
 

Found the similar circuit at online.

Which one is most popular and more advantages.
 

Attachments

  • IMG_20171103_113651.jpg
    487.5 KB · Views: 180
  • IMG_20171103_113628.jpg
    468.6 KB · Views: 181

Hi,

The first of post#5 is basically the same as the one of post#1.

The other is better suited to your needs - at least I think so.
Give more informations:
* power supply
* DAC output voltage range
* desired speed / settling time / upper frequency limit
* load resistance range
* desired accuracy, precision
* other requirements

You will find a lot of 4..20mA circuit discussions in this forum.
To ease the design I recommend a VCC referenced load
To improve accuracy I recommend to use a mosfet instead the bjt.

Klaus
 

Power Supply: +15V
DAC Output voltage: 0 to 5V
Load Resistance: 0 to 250 Ohms

I want to connect one end of load to ground. How to control 4 to 20mA with 0 to 5V using pnp? Please help.

- - - Updated - - -
 

You can use the circuit below as a starting point



Iout = Vin*(R3+R5)/(R1*R5)

Some points to consider.

1. You need a differential amplifier topology to make the output current independent from supply voltage variations.
2. A bipolar transistor introduces a considerable error by its base current. The suggested circuit uses a darlington transistor to reduce the error below 1e-4.
3. As shown, the output current is proportional to input control voltage. To introduce 4 mA offset, an additional reference voltage and respective circuit modification is required.
 

Hi,

For 4..20mA outputs I recommend to add some headroom. I often go to 0..24mA.
The adjustment of 4.00mA and 20.00mA can be easily made by software. Without drift with time and temperature.

Klaus
 

This circuit using one common dual op-amp can work for you.

You can do away with the first op-amp if your 0-5 V source can sink about 400 uA when at zero.
 

Attachments

  • 420_5V.png
    38.9 KB · Views: 191

Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…