ok so i found a bunch of the 2n3904, the are rated for max of 40 volts, and 6 volts on the emitter, so i think i should be ok to proceed with the protyping. http://www.fairchildsemi.com/ds/2N%2F2N3904.pdf, thats the data sheet, which field on that would tell me the min power needed to switch the transistor?
Thanks for the help

josh

 

6V is the base-emitter reverse (negative) voltage maximum rating which of no concern in this application, since you will never reverse bias the base.

The current needed to switch the transistor ON is determined by its current gain (Ic/Ib). To turn on a transistor with low collector-emitter voltage, a current gain of 10 is typically used (see Vce(sat) data on pg.2 of the data sheet) . Thus the 7mA collector current for this application would require a base current of 0.7mA. With a 5V control signal and a typical 0.65V base-emitter voltage (see Vbe(sat) data on data sheet), the base resistor required value is (5-0.65)/0.7mA = 6.2k ohm.

The base resistor value is not critical and can easily be +/- 20% of nominal with no significant change in operation.

 

I build the circuit, and connected everything, sketeched up my schematic, i put it all on the breadboard, i only had a 4.7 Kohm resistor so i went with that i think it might be causing the problem when i apply 5 v nothing happens but i noticed that when i touch the base pin or the resistor attached to this base pin the relay turns on. other then the value of my resistor what could be causeing this to happen.
thanks josh

 

It doesn't make sense. The resistor value should not be a problem.

Where are you applying the 5V? It should be connected to one end of the base resistor and the other resistor end connected to the transistor base. If so, then touching the base or base resistor should have no effect. Sounds like something is floating or you're missing a ground connection, perhaps between the 5V supply and the relay circuit? Remember the current always has to have a return path to the source.

Please attach a schematic of how you've connected it.

 

That is the basic lay out i used. the i had the resistor connected to the base, and the 5 volt supply connected to the resistor, the ground from the relay and the supply shared the emiitter, and the 24 v control line to the collector.

i used a 5volt power supply to sub for the microcontroller at the moment, i just used a momentary switch in line with the base

thanks josh

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The circuit looks correct. If everything is properly connected, touching any of the leads should have no effect. Sounds like a bad connection somewhere.

 

ok so i will double check all the connections make sure there all solid, i ordered a bunch of new transistors off ebay, possible mine got mixed up in storage. so ill give this a shot when the new transistors arrive ( i got the 2n3904 NPN transistors)

i guess ill do some work on my darlinton LED drivers untill those arrive.

thanks
josh