Thank you to all for taking the time to view my thread. I have been working with NPN transistors as of late, I am just learning the fundamentals here, and had a few things that I was looking for clarification on. I do not have a schematic but hope the provided info will be enough for you to be able to provide some assistance. I am using a fairchild bu337-25 npn transistor, the supply voltage , for base and collector sides, is thru a 5v reg which is giving me 4.97 at the reg. output (the power supply to the volt reg. is set at 10v. from a extech c.v. dc power supply).I am trying to set up the npn to function as a switch. I understand that at cutoff the Vb will be less that Ve and Vc. I also know that in saturation that Vb is greater than Vc and greater than Ve ( greater than the .7 drop threshold). I understand that my Vce will be around .2v at saturation.
On a breadboard I built the circuit up in the following manner:
I used two resistors in parallel(98.9 ohm and 98.2 ohm = 42.7 ohm) to achieve a Ic of 92.3mA. (The current I want in the collector.)
The Beta (Hfe) for the bu337-25 is 160.
I calculated my Ib using the formula: Ib=Ic/hfe which came out to .0005768A
I then calculated my Rb using the formula: Rb=4.97-.7/.0005768 which gave me 7402.9 ohms for Rb.
The closest I could come to the Rb value was 7170 ohms.
This value made my current in the Vb .0005857A which should sat. the npn more. This changes my beta to .0923/.0005857 = 157.58.
After I built this circuit and probed it w/ my meter I found the following values:
Vb: 4.2v, Vbe .733v (it is above the threshold), and Vbc of .607v.
Vce was .125v.
All of which met my requirements of Vb being larger than Vc and Ve.
I made some alterations to the circuit and changed my Rb to 10,475 ohms which gave me a Ib of .000397A(actual meter reading) calculated to .0004009A. I was looking to get my Vce up to the .2v that I had read it had to be. I achieved a value of .206v for Vce. These also met all my requirements. But the beta was 232.49.
I understand that you want to add 5-10% to your Ib to really get the npn into saturation.
In the data sheet I am confused when they give you the beta of 160 as a min and let's say 400 as a max. This I do not understand because I have calculated values out that gave me a Vce of .120 and the beta was around 157.58 which is under 160 on the datasheet so is this still in saturation? DO I try to get the npn into saturation with 106 beta or 400 beta or somewhere in between?
How much Vce should I try for? Is .120 enough?
The graph for the Vce vs Ic doesn't seem to give me the correct numbers vs. what I see on the breadboard.
What is the best way to look at this in terms of saturating the transistor to use it as a switch?
Thanks for your time and help.
-Val
On a breadboard I built the circuit up in the following manner:
I used two resistors in parallel(98.9 ohm and 98.2 ohm = 42.7 ohm) to achieve a Ic of 92.3mA. (The current I want in the collector.)
The Beta (Hfe) for the bu337-25 is 160.
I calculated my Ib using the formula: Ib=Ic/hfe which came out to .0005768A
I then calculated my Rb using the formula: Rb=4.97-.7/.0005768 which gave me 7402.9 ohms for Rb.
The closest I could come to the Rb value was 7170 ohms.
This value made my current in the Vb .0005857A which should sat. the npn more. This changes my beta to .0923/.0005857 = 157.58.
After I built this circuit and probed it w/ my meter I found the following values:
Vb: 4.2v, Vbe .733v (it is above the threshold), and Vbc of .607v.
Vce was .125v.
All of which met my requirements of Vb being larger than Vc and Ve.
I made some alterations to the circuit and changed my Rb to 10,475 ohms which gave me a Ib of .000397A(actual meter reading) calculated to .0004009A. I was looking to get my Vce up to the .2v that I had read it had to be. I achieved a value of .206v for Vce. These also met all my requirements. But the beta was 232.49.
I understand that you want to add 5-10% to your Ib to really get the npn into saturation.
In the data sheet I am confused when they give you the beta of 160 as a min and let's say 400 as a max. This I do not understand because I have calculated values out that gave me a Vce of .120 and the beta was around 157.58 which is under 160 on the datasheet so is this still in saturation? DO I try to get the npn into saturation with 106 beta or 400 beta or somewhere in between?
How much Vce should I try for? Is .120 enough?
The graph for the Vce vs Ic doesn't seem to give me the correct numbers vs. what I see on the breadboard.
What is the best way to look at this in terms of saturating the transistor to use it as a switch?
Thanks for your time and help.
-Val