BS170 Transistor Burnt Up?

I am working with a BS170 Transistor to turn on/off a series of LEDs using a microprocessor to control the gate. It seems that my transistor has stopped working as it does not ever turn off. Does this mean the transistor is burnt up?


Hi Daniel

Probably YES.

How many LEDs and more importantly how much current. The BS170 has an absolute maximum of 500mA but to operate it at this level is not advised . One of the normal uses for this device is to switch relays at 200 to 300mA.

Another important point. Are you turning the device fully on or are you trying to control current and therefore brightness by turning the BS170 partially on. If you are controlling brightness the far better approach is use the PWM method and turn the Mosfet fully on.

Note the data sheet figures are quoted with a Gate source voltage of 10V but I could not find a voltage for fully on, just a threshold voltage of 2V which is where the device starts to conduct.

It is quite possible that if you are driving the gate with only 5V it is not turning on fully and would be quickly destroyed

I have used this device myself but only in higher (12-24V) circuits where providing enough gate voltage has not been an issue.
Cheers Bob


Hi Daniel
Just had a look at Element 14 Data sheet which has a bit more info. Everything still suggests 10V gate/Source voltage. If you are only driving it with 5V you are very likely cooking the device. You should maybe look for a "Logic Level device which will turn fully on at 5V

Core have some, a handy little board is SKU CEO4538 by Freetronics. Everything you need is on the board just connect it up.
Cheers Bob


Hey Bob and Daniel,

With that one being out of stock at the moment you could also make it yourself with an N-channel MOSFET and some resistors.

Could you even use a motor driver if you dont need too much current?


Hi Liam
That Freetronics board has been out of stock for some time now, I wonder if it is going to be “retired”.
Useful little board to lash up for experimenting I have a couple with a header fitted to one so I can connect easily.

Lets not get too carried away. Core have a couple of N channel devices which I think can operate with 3.3V or 5V gate input. All you need are a couple of resistors. 1k and 10k.
Cheers Bob