maxim 7219 driving 7 segment displays with transistors and mosfets
jonw
Posts: 67
Hi all:·· I have a stamp BS-2 communicating with a maxim 7219 LED driver.
·The 7219 is driving a two digit CA 7 segment display.
·As per Maxim website application note 1196 Driving with higher Voltages and currents.
····························· My· Questions are:
····· Q1, I want to measure the segment current (so as to not damage it). I tried to use a 1 ohm resistor
·in series with the segment and measure the voltage drop on the 1 ohm R with a scope, but the difference
·in the two voltages were non conclusive.· Any help here··· Settings on the scope?? or can I increase the resistor· to 2 or 3 ohms?
···· ·Q2, I am getting confused about the transistor V c-e· I am using a tip 122· and I measure aprox 3 volts. No matter·how I adjust the R set to drive it.
·But what I am wondering is the only other thing in line with the collector circuit is the segment.· Which is· a 2 volt dropping non ohmic device
·( ie it will allways have· 2 volts on it so will not the Vce allways be 3 V)·
··( ie if I add a series dropping resistor· ((as·I suspect I will need·) how does it affect this question)?··· any comments· to help my thinking pattern.
··· EDIT >>>·· Q3· What is the procedure to tell if a mosfet is fully on ( ie Can you measure across it like V c-e of a transistor? )
·································
Post Edited (jonw) : 12/23/2006 2:02:35 AM GMT
·The 7219 is driving a two digit CA 7 segment display.
·As per Maxim website application note 1196 Driving with higher Voltages and currents.
····························· My· Questions are:
····· Q1, I want to measure the segment current (so as to not damage it). I tried to use a 1 ohm resistor
·in series with the segment and measure the voltage drop on the 1 ohm R with a scope, but the difference
·in the two voltages were non conclusive.· Any help here··· Settings on the scope?? or can I increase the resistor· to 2 or 3 ohms?
···· ·Q2, I am getting confused about the transistor V c-e· I am using a tip 122· and I measure aprox 3 volts. No matter·how I adjust the R set to drive it.
·But what I am wondering is the only other thing in line with the collector circuit is the segment.· Which is· a 2 volt dropping non ohmic device
·( ie it will allways have· 2 volts on it so will not the Vce allways be 3 V)·
··( ie if I add a series dropping resistor· ((as·I suspect I will need·) how does it affect this question)?··· any comments· to help my thinking pattern.
··· EDIT >>>·· Q3· What is the procedure to tell if a mosfet is fully on ( ie Can you measure across it like V c-e of a transistor? )
·································
Post Edited (jonw) : 12/23/2006 2:02:35 AM GMT