Switching transistor on Microcontroller

[Wamor]

microcontroller transistor switch

Hello,

Sorry if this topic is posted in the wrong forum.
I've got the following. To switch a load with a microcontroller i use a 2n3904 transistor. The collector of this transistor is connected with a 2k2 resistor to +24V.
The base of the transistor is connected with a 22k resistor to a output of the microcontroller. The emmiter of this transistor is connected to ground.
The microcontroller is giving a 10kHz outputsignal.
When i measure on the collector of the transistor i see a switch-on delay of 1us (microcontroller gives output low/high) and i see a switch-off delay of 2us (micrcontroller gives output high/low). This delay is critical in my application and should be at least in the ns-area. (max 100 ns).
Can anybody give me some information how to improve the switch-on and switch-off delay.

Thank you and best regards,

Wamor

 

[kishore2k4]

microcontroller transistor

Hello,

Sorry if this topic is posted in the wrong forum.
I've got the following. To switch a load with a microcontroller i use a 2n3904 transistor. The collector of this transistor is connected with a 2k2 resistor to +24V.
The base of the transistor is connected with a 22k resistor to a output of the microcontroller. The emmiter of this transistor is connected to ground.
The microcontroller is giving a 10kHz outputsignal.
When i measure on the collector of the transistor i see a switch-on delay of 1us (microcontroller gives output low/high) and i see a switch-off delay of 2us (micrcontroller gives output high/low). This delay is critical in my application and should be at least in the ns-area. (max 100 ns).
Can anybody give me some information how to improve the switch-on and switch-off delay.

Thank you and best regards,

Wamor

You should increase the base-current drive. Instead of a 22kohm resistor try something in the range of 2kohm or less. Most microcontrollers can drive 10mA loads so check the datasheet before you decide on the resistor value.

Also the 100ns requirement may not be possible with this transistor. It says that the storage time is 200ns and fall time of 50ns so the turn-off time is 250ns.

 

[FvM]

switching transistor

Generally, you're in the right forum.

I don't think that you should increase the base current. You can speed-up switching by connecting a small capacitor e.g. 50 - 100 pF in parallel to the base resistor.

If you worry about maximum output current, you have to add an low ohmic (e.g. 100 ohm) series resistor with the cap.

The said storage time is specified under particular conditions and the switching can be considerably speeded up. But increasing the static base current also increases the storage charge and slows down switch-off.

The most effective way, to avoid storage charge is unsaturated switching by adding a suitable schottky diode between collector and base.

Are you sure, that the load capacitance doesn't thwart the intended speed?

As another point, you probably won't find considerably faster switching transistors than 2N3904 for 24V operation. You have to go to the RF power section if you really need it.

 

[Wamor]

transistor microcontroller

Hello kishore2k4 and FvM,

Thank you for you help. The information is very usefull.
I still need to check the load capacitance but this is still unknown yet.
At least i got some usefull tips for increasing the switching speed.

Thank you and best regards,

Wamor