A couple general electronic questions.

[jbelectric777]

Good Morning: Just a few basic questions, all feedback is appreciated:

1) Is it better to switch +V or -0V, I ask because some shematics switch + and some switch -0 in my field we always and only switch the hot leg so I am slowly learning in electronics either can be switched via , transistors, IC's etc.

2) If time = RXC does that calculation reflect the 2/3 trigger voltage for a 555 timer or transistor or does that calculation need to be figured different. For example if my source is 9V then my trigger is 2/3 (6V) would I find R & C by using the 9V value or the 6V value this way i could calculate exactly what size resistor I should use for a trigger pulse of 1.0s based on T=RXC and using a 1000uf capacitor is it a 10K resistor for 1 second ?

3) Is there a set trigger for a transistor if I just want it to turn on and off, I learned there are other types of electronic switching devices I could use but a standard on / off switching what is best to use and what is the trigger value. (Example: I dont want my IC's to see the current draw of a very small miniture relay so I want to switch another source from either a transistor or other device but I want it to trigger not amplify) Thanks in advance. Jim

 

[Hero999]

1. It doesn't matter but if you have a bipolar supply then you need to switch both the +V and -V lines.
2. It doesn't matter since the capacitor slowly charges up to the same voltage as the power supply and changing the power also changes the trigger voltage. The formula is the same for all power supply voltages, look on the datasheet it would tell you otherwise.
3. What do you mean? Are you talking about a switch that switched the load? Small loads (<200mA) can be connected directly to the output of the 555 whilst larger loads will requirea relay or a bjt or MOSFET transistor.

 

[mneary]

1) Often, 0V is switched because N-channel and NPN devices have tended to be cheaper. There's no safety issue, as there would be in high voltage.

2) RxC is an approximation for the time to trigger a 555, since the 555 2/3 trigger point is very close to voltage at which we define one RC time constant.

3) Transistors have a "current gain" or Hfe, which is the ratio of output/input current. If you give a transistor an input of 1 mA, and it has an Hfe of 200, then it will conduct up to 200 mA. Here, it gets complicated. To minimize power dissipation you want to "saturate" the transistor, and the saturated Hfe is generally a lot lower.

 

[ljcox]

2) If time = RXC does that calculation reflect the 2/3 trigger voltage for a 555 timer or transistor or does that calculation need to be figured different. For example if my source is 9V then my trigger is 2/3 (6V) would I find R & C by using the 9V value or the 6V value this way i could calculate exactly what size resistor I should use for a trigger pulse of 1.0s based on T=RXC and using a 1000uf capacitor is it a 10K resistor for 1 second ?

Jim,
R*C is the time constant. It is usually indicated by the Greek letter Tor (like a T with curves)

If a capacitor is charging via a resistor to a 9 Volt source, it will reach about 63% of 9 Volt after 1 time constant. The 555 has an upper trigger level of about 67% (2/3 as you said) of the supply voltage.

So T = R*C is a reasonable approximation to the pulse length generated by a 555.

When a capacitor is discharging via a resistor the voltage across it decays to about 34% after one time constant.

 

[Hero999]

Sorry, I cocked up the list tags in my previous post so I've corrected it for you.

1000:mu:F is far too large for a 555, also when used with 10k it will give an 11 second delay not 1 second.

For a delay of 1 second use 10:mu:F capacitor and a 100K resistor or a 1:mu:F capacitor and a 1:mu:F capacitor. Also note that most electrolytics have a tollerence of +/-20% so your delay will be between 0.88s and 1.32s.