Voltage drop across a BC337 & a BC327

[danrogers]

Hi all. I am working on a project which I need to switch 5v to something that will draw about 200mA.

I first tried a BC337 NPN, but was only measuring a 4.6v output so the device I'm powering won't work (works fine connected to 5V).

Then I looked at the symbol and thought that I should be using a BC327 PNP so I wired one up with the load connected to the collector and the emitter connected to 5V. I also measure 4.6V at the input to the device I want to power

I have tried various base resistances, even a direct 5v & 0v but still can't get them to fully saturate? I thought with 5V (for NPN) or 0V (for PNP) should fully saturate them?

 

[ChrisP58]

Hi all. I am working on a project which I need to switch 5v to something that will draw about 200mA.

I first tried a BC337 NPN, but was only measuring a 4.6v output so the device I'm powering won't work (works fine connected to 5V).

Then I looked at the symbol and thought that I should be using a BC327 PNP so I wired one up with the load connected to the collector and the emitter connected to 5V. I also measure 4.6V at the input to the device I want to power

I have tried various base resistances, even a direct 5v & 0v but still can't get them to fully saturate? I thought with 5V (for NPN) or 0V (for PNP) should fully saturate them?

The data sheet for the BC337 shows a saturation voltage of 0.7V at 500mA, so 0.4V at 200mA doesn't sound too far off. If you need a lower Vsat, you probably need a different transistor.

 

[danrogers]

Oh I see, thanks! I don't know all the terminology yet so, did look at the datasheets but wasn't sure what to look for.

Do you know of a general purpose transistor that I could use with a next to nothing saturation voltage?

 

[ronv]

Maybe something like a 2N2222 - or a small FET

 

[ronsimpson]

It is hard to see what you are doing.
1> You need a base resistor! Do not connect the base to a supply voltage. (OK to connect B to E)
2> I think you have 0.4 volts across C-E. On the data sheet I am looking at a graph "Saturation Region". There are lines for collector current. 10, 100,300,500mA (C-E voltage is up/down) and base current is right left.
There is no line for 200mA so I drew one between 100 and 300mA. It looks like, (with a collector current of 200mA and a saturation voltage of 0.4 volts) you need a base current of 1 to 2mA. With a 5 volt supply, (5V-0.6V=4.4V) So 2200 ohms on the base gives 2mA.

This is for a typical part not a minimum part! I see there are 3 types of transistors. A=100 to 250, B=160 to 400, C=250 to 600 You can see, depending on if you have a A, B or C part and if the part is Maximum or Minimum the gain could be 100 to 600. I don't know what type of part made the graph.

I think with 10mA base current you should get 0.2 to 0.1 saturation voltage.

 

[dr pepper]

A 2n4401 might be an improvement, or even better if you can get one a 2sd965.

Have you thought about using a fet, you ought to be able to get the drop down to a few mv.

 

[Mike odom]

Any bipolar transistor will give you .4V or more at saturation. You need either:

1) Low Rdson resistance FET, or
2) 5.7V source.

 

[Mr RB]

...
... I think with 10mA base current you should get 0.2 to 0.1 saturation voltage.

Agreed, I work with BC337 often and it should be easy enough to get 0.1-0.2v Vce saturation range, at 200mA. Even with less than 10mA base current.

 

[Mike odom]

data sheet shows it is obtainable, but with 20mA base current (Ic/Ib = 10).

 

[danrogers]

OK thank you all, I'd like to try and get it working with optimal base current. How do I calculate a resistor value?

 

[audioguru]

I'd like to try and get it working with optimal base current. How do I calculate a resistor value?

Use Ohm's Law, R= V/I. The datasheet for the transistor shows the base voltage but you did not say what drives the base resistor so we don't know its voltage.

 

[ronv]

If you look at the saturation curves in Ron's data sheet it specs the transistor at a θ of 10. So your base current needs to be 200 ma / 10 = 20 ma. This should give you a Vce of about .1 volt.
You don't say what you are driving it with but lets say it goes to 4.5 volts when you want the transistor to turn on. Also from the curves it shows a Vbe drop of about .8 volts at your current. So you have a drive voltage of 4.5-.8 or3,7 volts. You need 20 ma so the formula would be 3.7/.020 = 185Ω. I think 180 is a standard value.

PS. Start with a new transistor - you probably blew yours when you hooked +5 to the base.

Sorry AG. I'm a slow typer.

 

[danrogers]

Thanks all, I will have a play about later when I get home. I am driving the base from an Arduino digital I/O pin so it should be 5V

 

[Mike odom]

( Vin - Vbase ) / Rbase

 

[audioguru]

I am driving the base from an Arduino digital I/O pin so it should be 5V

No.
An Arduino output pin has a logic high voltage of 5V ONLY WHEN IT HAS NO LOAD.
Since you are using it to source 20mA to a load then its logic high voltage is about 4V.

 

[Mike odom]

spec guarantees 4.2V min. @ 20mA (5V).

 

[danrogers]

thanks all! I might just use a low power relay, sounds like an easier way to supply 5v without dropping any?

 

[dr pepper]

I have to argue with that, an n50d allthough a smd gives around 20mv vce sat at 1 amp.

Maybe a reed relay will do the job for you.

 

[ronv]

Take a look at a logic level FET like this one:

https://www.mouser.com/ds/2/200/irlr8729pbf-91467.pdf

It's on resistance is less than 0.01 milliohms so would only drop 2 mv.

We should ask what your load is because if it's a motor or something it may need more "stuff".

The FET will be easy, just tie the gate to the output of the micro, the source to ground and the drain to one side of your load. The other side of the load to 5 volts.