# Voltage Logic & Transistors

Discussion in 'General Electronics Chat' started by doug3460, Dec 19, 2008.

1. ### doug3460 Thread Starter Active Member

Oct 19, 2008
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0
Question: Is a negative ATX supply rail considered logic low in regard to activating a transistor?

Background: As a budding electronic hobbiest, I must admit I am flustered because I haven't been able grasp a clear understanding of negative DC rails, Zero Volt rails & their relationship in circuits. I understand the swing associated with AC & the concepts of positive grounds used in the past by the automotive industry. But this DC question eludes me.

The answer will help me understand the solution I need to come up with in regard to a Project Forum thread I posted about a fuse circuit indicator for an ATX PS conversion. Having pretty much resolved the positive rail circuit (I hope), the question I last posed regarded the negative rails. I'm stuck until I really grasp this voltage concept. I think the solution lies in an NPN transistor to flip the logic for the PNP, but I'm not confident about it.

As always, thanks for the time & any suggestions.

2. ### beenthere Retired Moderator

Apr 20, 2004
15,808
294
A transistor begins to turn on when the voltage difference from the base to the emitter makes that PN junction forward biased. That is on the order of .7 volts . The polarity depends on the transistor - NPN or PNP. After becoming forward biased, the transistor must have the current in the base limited by a resistor to a safe level, as there is no current limiting in the junction.

For logic, a high is defined by the family the gate belongs to. For many families, +5 volts is a high. A low is 0 volts. Check the data sheet for the correct level for Vcc.

The ATX supply will have several black leads out. They are the grounds, against which all other voltages are referenced to.