Simple Switch

[jfalcon]

I am using the output pin of a MC68HC908AB32 to toggle a NPN transistor MMBT4401. The Base is biased with a 4.7K resistor and has a 10K pull down prior to the base of the the NPN. The emitter is tied to ground and the collector is tied to the base of a PNP MMBT4403 transistor. The emitter is tied to 5V and the collector will pass the 5V when it is turned on. My problem is that I am getting 3.5V on the collector when the input on the emitter is 5V. Once I toggle the input to the NPN high this should turn on the PNP and turn on the 5V that I need. Once the circuit is turned on the voltge from the 5V regulator starts to drop. Why is this happening? Need help ASAP.


Regards,

Jfalcon

 

[ScottyUK]

It sounds like you are driving the NPN into conduction and using the collector to pull down the base of the PNP. If you do this you are effectively applying a short through the combined 0.7V base-emitter volt drop of the PNP plus the collector-emitter voltdrop the conducting NPN. Add a resistor between the base of the PNP and the collector of the NPN and see if that improves things. Start with maybe 1k resistor or so.


----------------------------------

Start each day with a smile. Get it over with.

 

[BrianG]

If I understand your circuit description correctly you have no current limiting resistor between the collector of the 4401 and the base of the 4403. TWhen the 4401 turns on this is effectively a short circuit across the 5V rails, causing large currents to flow - which is why your 5V regulator output drops.

Also, you have no "shunt" resistor acros the 4403 base-emitter junction, thus any leakeage current in the 4401 will tend to turn on the 4403 even when it's supposed to be "off". This is probably where your 3.5V is coming from - unless its because you have already damaged both transistors with the excessive short circuit current described above.