Help with transistor, will this blow up?

[Charger]

Hello all!

I am making a device using a picaxe (in this case 18m2+) and a few transistors that allow the picaxe to operate a pre-existing switch that would normally be operated manually. I am using an NPN 2N3904 transistor to turn a ground-triggered load (20 mA) on and off. My concern is, if I operate the switch and connect the path to ground, will it cause any issues for the transistor? What about switch on and transistor on? I have included the simplified schematic for visual aid:

 

[westaust55]

Having the switch in parallel with the transistor does not present any problems.

 

[AlbertZ]

Hello all!

I am making a device using a picaxe (in this case 18m2+) and a few transistors that allow the picaxe to operate a pre-existing switch that would normally be operated manually. I am using an NPN 2N3904 transistor to turn a ground-triggered load (20 mA) on and off. My concern is, if I operate the switch and connect the path to ground, will it cause any issues for the transistor? What about switch on and transistor on? I have included the simplified schematic for visual aid:

I would put a current limiting resistor in series with the collector circuit (say 10k) just to be on the safe side. What kind of load is drawing only 20 mA?

 

[John West]

10K Ohms is much too high a value for the collector. Assuming the OP is pulling 12 V to ground, 10K would limit the closed circuit to about 1 mA of "pull-down" current, not the 20 mA specified. I say let the base resistor (value not specified) and the transistor's Hfe control the collector current.

 

[MartinM57]

Welcome to the forum!

No collector resistor needed, as above, but as you'll come to realise if you hang out here for long, more details of the load would be helpful...you might even be able to sink the current straight through a PICAXE pin, with no transistor needed

 

[MFB]

Agree, no collector resistor needed. I would suggest using a base resistor of not more than 10K to insure that even a low gain transistor will turn fully on and pull the collector voltage to near ground. However, most transistors will have reasonable current gain (say x50) at 20mA, but its worth remembering that gain can decrease at high currents. Which can lead to excessive transistor heating if the collector is not driven to saturation (eg. near ground).

 

[Charger]

Thank you all for your replies!

No collector resistor needed, as above, but as you'll come to realise if you hang out here for long, more details of the load would be helpful...you might even be able to sink the current straight through a PICAXE pin, with no transistor needed

I am making a remote entry system for a 1988 Toyota Celica. The picaxe will be used to control a door lock module to operate the door locks. The "load" is a just a low current IC inside the module that looks for the switch to be toggled to 'locked' or 'unlocked', pulling either wire to ground and then powers the door lock relays. The 'low current IC' in the door lock module has 'ND' like a manufacturer's name and the part number "MH1450 (next line) 9H01A" on it; searching produces no results. Due to the unknown nature of the 'load' I wanted to use two transistors (one for lock, one for unlock) in order to protect the picaxe. Diagram I am splicing into shown here.

Initial bench testing has proved successful. Only giving the door lock module 12v, ground, and connecting lock/unlock trigger wires to the corresponding transistors on the breadboard. Giving 5v to the transistor simulates having a floating pin to the door lock module, which means the manual switch is at rest. Giving 0v to either (but never both) transistors simulate pulling the trigger wire to ground causing the door lock module to lock or unlock the car. This setup works, however I have not tested this in the car yet, with the switches and how it will respond to the added picaxe and supporting electronics, but from what I see here it shouldn't become an issue.

As for the base current limiting resistor I am currently using a 330 ohm resistor, which I know is too low. Another part of my circuit needs to turn on a relay (to disconnect the passenger's door lock motor in order to only unlock the driver's door) and I had simply used the same resistor I had calculated for that relay. I will be testing a more proper resistor soon, which leads me to my next question...

My data sheet lists a min Hfe (60) and a max Hfe (300), which do I use or how do I take this into account? Data sheet here:
https://www.sparkfun.com/datasheets/Components/2N3904.pdf

Thank you all for your help! I look forward to posting a detailed thread on this project once it is working and installed in the car!

 

[premelec]

Minimum Hfe would be ok - you just want it to turn on not be in a linear region... the main factor as to if you can drive the module directly from a PICAXE is what the open circuit voltage across the contacts is - you can measure that and the operating current with a multimeter [voltage OC current Closed Circuit]. If you have less than V+ of PICAXE open circuit and less than 20 ma closed that could be driven without the transistor added...