#507 Single-stage Delay Switch
Comparing BJT and MOSFETs for creating a simple turn-off delay with discrete components.
Here’s a quick demo..
Adding an RC timing component to a single-stage low-side control circuit is a simple way of creating a “turn off delay”.
The delay is proportional to the RC time constant, but the actual behaviour is quite different depending on whether an NPN BJT or n-channel MOSFET is used as the active control component.
In both cases, a capacitor is charged during the “on” period. When power is disconnected (switch off):
- an NPN BJT will gradually turn off as the capacitor is discharged via the resistor and B-E junction. The current will reduce with voltage, producing a prolonged “fade out” effect.
- an n-channel MOSFET will turn off as the gate voltage drops, but the ohmic region will be relatively small, producing a sharper off transition
I was inspired to whip up a quick demonstration after seeing learnelectronics’s “How to make a simple delay circuit” video:
To compare the BJT and MOSFET behaviour, I put both on a breadboard, triggered by a DPDT switch. I used an 2N3904 NPN transistor, and a 2N7000 n-FET, though the parts selection is not critical.
For visually testing the circuit, the delay is set rather slow with R=1MΩ and C=1µF, for a time constant of 1s.
Actually on a breadboard:
To capture behaviour with an oscilloscope, I switched to R=10kΩ and C=1µF for a time constant of 10ms.
In the following trace, the channels are all offset by -2V and set for 1V/division. Channels are assigned:
- CH1 (Yellow) - MOSFET switch: MOSFET drain
- CH2 (Blue) - MOSFET switch: capacitor anode/MOSFET gate
- CH3 (Red) - BJT switch: BJT collector
- CH4 (Green) - BJT switch: capacitor anode
Points to note:
- n-channel MOSFET switch: when capacitor voltage drops to ~2V, the FET rapidly switches off within about 6ms
- NPN BJT switch: when capacitor voltage drops to 0.9V, the BJT fades over more than 100ms
The Obligatory OCD Build
I felt like flowing some solder, so made the circuit permanent in a very literal transposition of the circuit to a freeform build mounted on a bit of cardboard: