Hi this is my first post and i was after some advise i am quite new to electronics and am really just teaching myself however i have a circuit i am building from a schematic i found on the internet
**broken link removed**
its an eprom programmer and i have all the components but i dont have T1 (bc558c) i only have a B version
looking up on the datasheet it is only the Hfe is different between the B and C
b = 200 - 450
c = 420 - 800
now this dosent mean much to me but as far as i can ascertain it is the amount of current needed to activate the transistor.
so by my reckoning its going to need a lot of current to switch on T1
now as i say i am very new to this and my next bit may be way of target but i cant understand why there is a 6.8k resistor from the paralell port then a 2.2k pullup resistor after it?
can i alter the resistance on either of these to give me the right current to activate a BC558B insead of a C?
any help appreciated
Craig
**broken link removed**
its an eprom programmer and i have all the components but i dont have T1 (bc558c) i only have a B version
looking up on the datasheet it is only the Hfe is different between the B and C
b = 200 - 450
c = 420 - 800
now this dosent mean much to me but as far as i can ascertain it is the amount of current needed to activate the transistor.
so by my reckoning its going to need a lot of current to switch on T1
now as i say i am very new to this and my next bit may be way of target but i cant understand why there is a 6.8k resistor from the paralell port then a 2.2k pullup resistor after it?
can i alter the resistance on either of these to give me the right current to activate a BC558B insead of a C?
any help appreciated
Craig