can someone help with RC timing circuits?

Usually the resistor is connected to an active device (transistor, etc.) which turns on the circuit. For those cases where it is connected to a relay, say, it has to be sized to operate the relay and then you have to choose a suitable capacitor. A more modern way is to add a counter circuit and only operate your device after some number of timing cycles.

RC timing circuits normally are applied to signals, where the voltage represents some information (time that a digital state changed, etc.)

Yes.

Yes.

The resistor is between the signal source and the signal output. The capacitor to ground is connected to the output end of the resistor. If you want a power source that follows an RC time constant, you may be able to apply the time constant to the reference voltage the supply regulator copies to produce its output.

Right.

Not if you want an RC time constant. The R in that case would be the resistance of the source feeding the capacitor.

Well, in series with the signal, anyway. RC time constant filters are not often used for power.

If the power source has a significant internal resistance or a current limit, you may be able to get the delay you want by using just a capacitor.

Hey, don't I know you? ;)

The resistor will limit the current. The fact that the resistor limits the current is important to the operation - it literally slows the charging of the cap. So, putting the cap first won't work because the cap charges up without and hinderance. Perhaps use the RC network to control a low current signal (ie; transitor base) and make the transistor switch the higher current. (I know, you've been there before, I'm not much help am I?!)

Most often this technique is only used on chip enables and such, very low current stuff. More complex circuits (microcontrollers and/or SCRs etc.) are employed to "soft start" a power supply, for example.

nifty

You use the "slow" RC to turn on a higher current device. Here's a circuit that illustrates the concept:

+12------------+---------- ---> +12 delayed to circuit | | e\\ /c | | --- PNP | | | \\ +---[33K]---+ | | | /c +---[R]---+---[Zener]---| NPN | 6V \\e [C] | | | Gnd --------+---------------+

Say the R is 10K. The current available at the junction of R and C will be low, about 1.2 mA. However, when the voltage rises above about 6.6 volts, that current will cause the NPN transistor to conduct and drive the PNP transistor on. The PNP can be a power transistor for much higher currents than the puny 1.2 mA available at the R C junction.

This is only 1 implementation - others exist that are better. If you haven't looked into comparators yet, now might be a good time to do that. A comparator works well the delay circuit. You also might want to look into the 555 timer - another worthwhile approach.

Ed

Google for 555 timer circuits.

Thanks for all the repiles. This helps clear a few things up that I had been wondering about. One of the things that got me going about the current issue I saw in a textbook I was looking through at home. It had a design example of a circuit creating a 30s delay before turning on an alarm system. To get the 30s, they used a resistor of something like 390K. Of course, this wouldn't leave a whole lot of current to power the alarm system, so I got thinking....

Anyway, thanks again.

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