Slightly confused: transistors....

Thread Starter


Joined Jul 22, 2006
Hi all,

I'm using a Luxdrive 3021 Buckpuck (datasheet here) I'll be dimming this with PWM generated from a Picaxe.

I'll be connecting the Picaxe and buckpuck in the same way as shown in fig14 of the datasheet:

This will be first time I've 'dabbled' with transistors so I'm really just seeking help to make sure I've calculated the values for both resistors correctly - the diagram above list 5K but that's a 'non standard' value and quite expensive when I searched on Farnell!

My workings out are as follows:

The datasheet for the buckpuck tells me that the impedance of the Control pin is 1K, so the voltage from the Ref pin is 5V which makes the current 5mA

I'll also be using a 2N4403 transistor as listed in the above diagram - As I don't know any better!! :rolleyes:
According the the 2N4403 datasheet the hfe could be somewhere between 60 and 100 So I've taken the lowest figure....

Using the calculations on

R1 = Supply Voltage / ( Maximum Current Required / Minimum HFE * 1.3 )


5/(.005/60*1.3) = 46,153 - 46.4KR being the nearest value I can see.

R2= 10*R1 so that gives me a 464KR value

But if I use the calculations here my calculations are somewhat different!!

Here, R1 = (Vc *hfe ) / ( 5 * Ic )
so, ( 5*60 ) / ( 5*0.005) = 12KR

But the site doesn't mention any value for R2

This is why I'm confused!!

Which of these calculations are correct or am I barking up the wrong tree? :confused::confused:

Also while I'm here, the frequency I'll use for PWM will be approx 4Khz ( with a duty cycle of between 40-100% ) - so I'll be under the maximum frequency for the buckpuck. If I'm reading the transistor's datasheet correctly am I right in saying that I'm well within it's capability?

Many thanks

Thread Starter


Joined Jul 22, 2006
Thanks for the reply.

I was going to use that layout as described in fig 15 - but I decided against it. There's going to be 2 drivers - one on each side of my motorcycle and I'm very limited on space on where I can hide the drivers but I want to be able to dim both sets of LEDs at the same time when I use the indicators - the Picaxe is going to be tucked away behind the dash.

I did also consider using the setup in fig 13 but I think the cable lengths are too long ( the datasheet recommends that all cables to the LEDs and to the CTRL pin are as close to 6" as possible ) so I figured the method using transistors would be the way to go....


Joined Mar 6, 2008
I think that the resistors aren't really critical there. If i'm seeing this correctly, the resitor at the right is just to make sure the transistor remains cut when the logic input is not connected. Doesn't really do much in other case, as the emitter-base diode sets the voltage there to 5v - 0,7v = 4.3v when the input is conected to gnd, and no current can flow when the input is connected to 5v. This resistor can easily be bigger.

The other resistor is to make the transistor saturate when connected to gnd. If one side of this resistor is at 4.3v, and the other is at 0v, a 860 uA current will flow. Multiplied by 60, this allows up to 51 mA to flow tru the collector-emitter junction. This is much more than the 5 mA you need. You could optimize the current consumption by choosing a bigger base resistor, as you first calculated it.

Ron H

Joined Apr 14, 2005
Vce(sat) for most transistors is spec'ed at a forced beta (Ic/Ib) of 10. This is true of the 2N4403. See Fig. 16 of the datasheet. Since the collector current is 5mA, you need a base current of ≈500uA. More base current is almost always OK if speed or power dissipation is not an issue. A little less is usually OK, but I personally wouldn't go below Ic/Ib=20. As Norfindel pointed out, the base-emitter resistor (Rbe) value is noncritical. The current through it will be ≈0.7V/Rbe.
I probably would make both of them 10k. You can get 10k resistors at a barber shop.:D