Question: so i'm going to use IRF530 as a LED driver for my project, but as soon as i open datasheet, i can't find K for calculating current Id. so how to calculate Id to make sure i'm supplying enough current for 10-15 LED @ 1 watt (350mA) and not overrated it? i've heard from other source that you just don't use K. i mean if that was true...how to precisely control current that passed MOSFET?

thanks in advance
 [![Circuit Example ](https://i.stack.imgur.com/aPV0u.png)](https://i.stack.imgur.com/aPV0u.png)

Answer: 
This is a bad way to control LEDs. For a start you have them all in parallel - have you done no research at all about LEDs? LEDs need individual current limiting/sharing resistors when placed in parallel or one of them hogs all the current and blows up after a short while then, the current that was to be shared among 4 LEDs is now inflicted on three and, the one that has a slightly lower forward volt drop, takes most of that current and blows.

You can see where this is going - basically after a few minutes or seconds you have no more working LEDs.

You have a 12 V supply and four LEDs which I assume to be standard 2V, 20mA types. So, put them all in series and calculate a current limiting resistor to go in series with them.

If you want to control LED current to control brightness then try googling current sources like this one: -

[![enter image description here](https://i.stack.imgur.com/JqWup.png)](https://i.stack.imgur.com/JqWup.png)

Varying R2 can give you some control on brightness. Of course you could consider a more elegant solution involving a proper LED control chip: -

[![enter image description here](https://i.stack.imgur.com/Zxa2J.png)](https://i.stack.imgur.com/Zxa2J.png)
What's 'k'? I assume from the context you mean transconductance, that is the Ids per Vgs ratio.

This parameter varies from FET to FET, and with temperature, and with voltage, so it's not good to assume it's stable and to try to program the LED current by dead reckoning from Vgs, much as you wouldn't take a BJT's \$\beta\$ to be constant and try to do the same thing by base current.

What you *should* do is to add a source resistor Rs to sample the source current. Sense the voltage across it with an opamp or a transistor to control Vgs, or to really cheapskate it, keep the present circuit diagram you have, and the voltage developed across Rs effectively applies negative feedback onto Vgs, to provide some stabilisation against transconductance variations.

**BEWARE** IRF530, and almost every other FET in the world, are **switching** FETs, not linear FETs. In the circuit you have drawn, you will be using the FET in a linear mode to dissipate power. These FETs are built from multiple cells, that share dissipation OK if they are fully on, or fully off, and you can use them to their rated power. If in the linear region, they become thermally unstable after a short time (seconds, milliseconds, not microseconds which is why they can switch fast), and fail to share the dissipation, causing local thermal runaway and device failure. If you stick to <10% of its rated dissipation, and keep it cool, so well heatsunk, you will probably be OK. BJTs do not have this problem. You can buy linear rated FETs for audio amplifiers, but they are mucho denarios and difficult to find.