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Is this circuit too good to be true

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Before I order the parts I need can you just verify for me that R1 and R2 should be rated for 2W and R3 and R4 are fine at 1/4W.

thanks
 
Thanks for posting that, it's good to know what the project is going to be used for.

I think we have had a similar thread before, was it you or someone else from that forum?

Which circuit do you want to know the resistor power ratings for?

Use Ohm's law:

P = V²/R

Or:
P = I²R

For example, take R2 on the first circuit, the votlage across will be about 0.6V.

P = 0.6²/0.82 = 0.44W

A 0.5W resistor will do but the voltage might be slightly higher than 0.6V so I'd recommend a 1W resistor.

You can calculate the power or the other resistors used in the circuit but it'll be very small.

Q1 might need to be on a large heat sink.

How much power does it dissipate?

P = VI

What's the voltage across it?

It'll be the supply voltage minus the voltage drops of all the LEDs and 0.6V.

How many LEDs are you powering and what colour are they?

What's the current through Q1?

I've shown you how to calculate that before.
 
R1

58²/100,000 = .03W

R3 and R4 have very low current and voltage so I know they would be fine with 1/4W

Q1

The LED string will be 16 LEDs with a Fv of 3.5V for a total of 56V. The power supply is 58v so 56-58 = 2v across Q1.


P=VI

P = (2)(.750) = 1.5W will be dissipated by Q1

The IRL520 has the ability to dissipate 60W. Is this before total meltdown? What heat dissipation is reasonable for a TO-220 MOSFET to handle without a heatsink? With a small bolt on type?

Did I get these right?
 
The power dissipation in R1 will be lower is it's part of a potential divider or you're using a zener, not that it matters.

You're right about the power dissipation of the transistor.

The practical power dissipation is limited by the heatsink.

According to the datasheet, the thermal resistance of the transistor to ambient, without a heatsink, is 62°C/W so with a power dissipation of 1.5W the temperature rise = 62×1.5 = 93°C.

The maximum operating temperature is 175°C.

If the ambient temperature is 30°C, the temperature of the transistor will be 30+93 = 148°C so no heat sink is required.

However 148°C will be how enough to burn you and melt some plastics so you might opt for a cheap clip on heat sink.
 
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