BC558b or c??

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

It will make no difference for your application, don't worry about it!.

The currents used are all only small, no worries at all.

So I dont need to change anything? just substitute the C for a B

sorry to be a pain but could you/anyone explain the calculations? (just so i dont have to ask the question again about a different circuit i will just be able calclate)

Much appreciated

Craig

Grist_E said:

So I dont need to change anything? just substitute the C for a B

sorry to be a pain but could you/anyone explain the calculations? (just so i dont have to ask the question again about a different circuit i will just be able calclate)

Click to expand...

I haven't done any calculations, the transistors are only used as switches, so you simply stick lot's more current in the base than you need for the fairly small collector current required in this application. You generally design a circuit so as not to require specific devices, and a simple switch like this is very much like that.

Assuming you wanted to calculate it?, and personally I wouldn't bother, you first need to know the current required from the collector. Divide that by the minimum gain at that current - this gives you the minimum base current you need, multiply that by at least five - and calculate the base reistor for that current.

Woah!!! Thankyou for that (it will probably take me an hour to understand your response but i can now work it out)

Much appreciated