How to trigger a transisitor as a switch after capacitor is fully charged

[parvez.designworks]

How to trigger a transisitor as a switch after capacitor is fully charged

Only simple components please

no IC

 

[cowboybob]

The context of your question is confusing.

Can you show us what you've tried thus far?

 

[MikeMl]

Because the Base-Emitter of an NPN Bipolar transistor looks like a forward biased diode (current-operated), where substantial current starts to flow once the voltage reaches ~0.65V, timers based on a RC network do not work well, or the delay must be short ,i.e. during the time the capacitor charges from an initial voltage, typically 0V, to 0.65V.

Using the bipolar transistor puts a lower bound on R; the current through R must be sufficient to turn on the transistor. In practical circuits, R cannot be more than ~100KΩ.

If you replace the NPN with an NFET (voltage operated), then you get much longer, accurate delays. This is because no significant current flows into the Gate-Source of the NFET; all of the current through the timing resistor R goes into charging the capacitor C. The turn-on threshold of an enhancement-mode FET is usually several volts, making for a longer delay period.

Using the FET allows R to be up to tens of megΩ. The upper limit comes from capacitor leakage current.

Try building a timer with a delay of 10 minutes...

 

[JimB]

after capacitor is fully charged

First of all define "fully charged".

Theoretically a capacitor connected to a voltage source through a resistor will never reach the same voltage as the voltage source.

So first you have to define what "fully charged" means in your context.

JimB

 

[crutschow]

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Theoretically a capacitor connected to a voltage source through a resistor will never reach the same voltage as the voltage source.

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I consider when the voltage reaches the thermal noise value of the capacitor, it is fully charged. This is 20 time-constants at room temperature for any capacitor <1000μF.

 

[tcmtech]

I would use a LM431 shunt regulator to trigger the transistor once a specific charge up voltage is reached.