Resistance through a circuit

Thread Starter

jerseyguy1996

Joined Feb 2, 2008
214
I am building the circuit on page 5 of this appnote:

http://focus.ti.com/lit/ug/sluu023b/sluu023b.pdf

I have not included resistors R16, R15, or R17 in my circuit because they are not necessary for my application.

Looking at the schematic I can only see one path for current to take when all of the transistors and are off and that would be from:

+VDC-->D6B-->R21-->R14-->R12-->R20-->Ground

I add those up and I get a resistance 331K but when I check the resistance of my circuit across VDC+ and VDC- I get a resistance of only 500 ohms. I'm scared to plug this thing in until I know for sure what the resistance should be. Would the PNP transistors and P-Channel FET be conducting when measuring resistance with an ohm meter?
 

designnut

Joined Apr 21, 2011
33
You are measuring the input diodes that are inherent to the circuit. are you sure you don't need the resistors, sounds like you are a real beginner.
 

Thread Starter

jerseyguy1996

Joined Feb 2, 2008
214
You are measuring the input diodes that are inherent to the circuit. are you sure you don't need the resistors, sounds like you are a real beginner.
Look closely at the schematic. Those resistors aren't connected to anything if you don't have a jumper connecting them across JP5.
 

Thread Starter

jerseyguy1996

Joined Feb 2, 2008
214
You are measuring the input diodes that are inherent to the circuit. are you sure you don't need the resistors, sounds like you are a real beginner.
Not sure what you mean by measuring the input diodes. Shouldn't current flow from VDC+ through D6B (I used D6B instead of D6A) through R21 through R14 through R12 through R20 and back to ground?
 

Thread Starter

jerseyguy1996

Joined Feb 2, 2008
214
Well found the problem. I had the circuit already hooked into the power supply and I was reading the resistance cross the V+ and COM of the power supply. Actually I guess I was reading the parallel combination of the resistance of the power supply and the resistance of the circuit.
 
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