Resistor Power

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RustyH

Senior Member
Hi Everyone,

Ive never really adventured down the route of SMD components, but I want to build so more compact circuits.

I plan to to turn a 12v 1A load on with a SMD MOSFET N-Channel 800V 1.8A DPAK Transistor.
Going off the "Interfacing Circuits" manual of Picaxe, I need a 10k pull down resistor between the picaxe pin connected to gate and 0v.

What Im not sure on is how to work out the power rating of the resistor I need??

Thanks
 
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inglewoodpete

Senior Member
The highest voltage that your PICAXE can output is 5 volts (assuming a 5v power supply). You want to use a 10k ohm resistor.

E is electromotive force, commonly called volts. (In physics, V is for veloscity.)

Ohm's law is I = E / R

Power is calculated P = E x I

Doing some algebra, you can end up with P = E2 / R

So the resistor must have an absolute minimum rating of ((5 x 5) / 10,000)W = (25 / 10) mW = 2.5mW

Try and find the smallest resistor you can!

I'd use a 1/4W
 
R

RustyH

Senior Member
perfect, thank you for the help

Is there an SMT transistor that can handle 1 amp with PWM? or would I be better using the fet mentioned above?
 
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Dippy

Moderator
There are tons ;)
Have a look on a site of a big supplier like Farnell or RS.
Put MOSFET into the first search box and then after the first zillion options appear use their 'parametric' options i.e. select the parameters you want.


Be aware about MOSFET gate capacitance which will impact on your PWM frequency unless you use a driver.
If you choose a lower voltage and lower current (e.g. 30V @ 2Amps or less) you will, if you read the data sheet, see lower gate capacitance/charge values as you reduce the , for example, current rating.
Check also the gate voltage required to switch it on e.g. logic levels.
Beware the quoted 'threshold' values can be deceptive to newbies.
A MOSFET is not a magic switch and needs some design thought.
Is your 1A load reactive? (e.g. a relay).


What is your electronics experience 'level'?
If you are worried and need more advice you will have to describe your proposed circuit in detail and, ideally, post a schematic.
 
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RustyH

Senior Member
Thanks,

Why is the gate capacitance so important? (is that the same as input capacitance)

Im driving an LED array
 
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RustyH

Senior Member
Thanks, Ok, so to summerise , the Gate Capacitance can effect the efficiency of fast switching due to it requiring charging / discharging. So for fast switching, you need as low a capacitance as possible. However, the the lower the capacitance can also introduce noise.


What gate capacitance is recommended to pwm drives?
 
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MPep

Senior Member
RustyH said:
Thanks,

Why is the gate capacitance so important? (is that the same as input capacitance)

Im driving an LED array
Click to expand...
It takes current to charge to capacitor (the gate capacitance in this case).
Therefore an applied voltage (PICAXE output pin high voltage, 5V) is required to source current which takes time to charge and discharge the said capacitor.

Now this is more important in switch-mode power supplies and the like, but where the MOSFET is part of a H-bridge configuration, you need to discharge gates (and therefore switch OFF the MOSFET) as fast as possible, otherwise it is possible that both MOSFETs on the same side are ON at the same time!!!! This then creates a short-circuit with some interesting results.

When the MOSFET is not either fully ON or OFF, it is in a linear region. This implies that there is both voltage across the MOSFET as well as current flowing through it, thereby dissipating power, ie it gets warm. This is power wastage, and therefore counteracts the whole concept of PWM.

For these reasons there are Gate Driver ICs

As far as driving a LED string, you'll most likely end up having only a single MOSFET, usually as a low side driver. The charge and discharge times generally don't matter too much.

Hope this helps somewhat :D
 
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