walters said:
Thanks guys alot
What im confussed about is this i seen products and circuit use +9 volts but have different current rates examples
+9 volts and the current is 200ma
+9 volts and the current is 600ma
+9 volts and the current is 1250ma
+9 volts and the current is 2000ma
How can i have the same voltage +9 but the current draw is different?
Wouldn't the power supply have to have a different current rating?
because its the same voltage +9 but the current is different
As I said it is ecinomics!!!!
IF a product is known from bench-tests to ONLY draw 200mA why waste money,size and time providing a supply that CAN (doesn't mean it WILL) 2000mA it makes no sense.
AS to a designer setting a supply at 9V and then winding up the current limit that is a NO-NO!!!!!!! best-case the cct never powers up and you start fault-finding a fault that is none-existant (TRACO DC
C are very fussy at power-up) worst-case IF you have FPGA's on your board then they are now DEAD!!!
AS I said you get a feeling for how much current your design will draw from going over the datasheets and summing all the currents up
Say I have a TLE2021 on my board
https://www.electro-tech-online.com/custompdfs/2005/11/tle2021.pdf (page 8)
From the datasheet it says:
OUTPUT CURRENT: +/-20mA
TOTAL CURRENT INTO +VCC : 80mA
TOTAL CURRENT INTO -VCC : 80mA
INPUT CURRENT (EACH INPUT) : 1mA
The input current can be ignored since that is current from the prev chip.
So summing up the currents:
80+80+20 = 180mA from +/-15V so WORSE-CASE it needs 90mA limit on the +rail and 90mA on the -rail
So I set the current limit at 250mA, why? becuase there will be inrush (to charge caps). I could set the current limit at 2A BUT if their is a design-fault (a short somewhere or cross-wiring) then the Power-suppply will just feed that short and 2A will do alot of damage as opose to 250mA
SO I turn on the supply, current limit kicks in (and the voltage collapses) I quickly turn it off and go over the board.
Find a bad wiring and correct. Power it up again and this time it draws 30mA great!!!!! So i design a PSU to deliver 50mA or the nearest amp level that is easy on the maths since I am lazy