Transitor (switch mode) calculator problem

I have set myself up a transistor calculator in an Excel spread sheet as follows:


A B C D E F G

=E22/C22 =(B22-D22)/F22

BC327 is on line 22

What am I doing wrong here with my formulas because I am a factor of 10 out on the base current?

With a 270R resistor in series with a 555 pin 3 and connected to the base of my BC327, the current I am measuring (multimeter in series with resistor) 3.15mA compared to my calculation that says it should beabout 31mA.

Sorry but the forum system is not recognizing my space formating.

IB= and RB= columns contain the formulas below the image of my table

if the 555 is in astable mode the multimeter reading maynot be correct since it is not pure dc

picgak said:

if the 555 is in astable mode the multimeter reading maynot be correct since it is not pure dc

Click to expand...

Oh - forgot about that. The duty cycle is 10% too.

I found this any: http://www.kpsec.freeuk.com/trancirc.htm#ic

My formulas are not appropriate when an IC is involved.

After modifying my spread sheet with the forumals on this web page I come up with a base resistor value of about 80R to get a CE current of about 600mA

My multimeter now says that the base current is about 7.67mA which alone is some way into saturation according to the graph in the datasheet for the BC327.

Isn't there a formula (based on the duty cycle) that you can use to calculate the approximate real current when the DC is pulsed through the transistor?

- - - Updated - - -

I am wrapped - I figured it out!

I have re-calculated the resistor values to make the following circuit work with a salvaged 2SC4458 in place of the original Darlington. I got about a 0.5cm arc with two ignition coils in series......but my PC power supply is only delivering perhaps an amp or two to the coils.

**broken link removed**

I feel bit more confident now that I have my head a little further around transistors......at least in this circuit situation.

So one key thing I think I have learned is that, even in switching mode, the collector current is still dependant on the hFE. Originally I had thought that this was really only relevant when using the transistor in amplifier mode. So I more or less understand why the original designer of this circuit stated to me that I cannot use any old salvaged power transistor in this circuit.

Good work. Getting an arc can be called success.

It might pay to experiment with the coil, to discover its operating characteristics. For instance, if you reduce the operating frequency, it might let the current ramp up some more through the coil.

boylesg said:

Oh - forgot about that. The duty cycle is 10% too.


My formulas are not appropriate when an IC is involved.

After modifying my spread sheet with the forumals on this web page I come up with a base resistor value of about 80R to get a CE current of about 600mA

My multimeter now says that the base current is about 7.67mA

Isn't there a formula (based on the duty cycle) that you can use to calculate the approximate real current when the DC is pulsed through the transistor?

Click to expand...

Good work boylesg
your multimeter is showing approximately 10% of the actual current if it is a 10% dutycycle circuit.