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# computer Christmas lights

#### MrDEB

##### Well-Known Member
years ago I assembled a Compurterized Christmas light display but since sold the entire setup.
I recall the SSR or triac boards used an opti-isolator, couple resistors but instead of using a computer, I want to just use a PIC to drive the opti_isolator. Just started looking for leftover plans and parts but figure would ask first.

Doing a simulation using TINA I see about .10 volt increase in GATE voltage using the diagram in post#75

I don't think you need TINA to tell you that 10000/(10000+130) * VDD is going to give you about 1/10 of a volt drop.

every little bit helps
If it's that close you may have problems.

you lost me with math. I am not very good at anything concerning math.
Get lot with decimals.
Been contemplating this project and contemplating looking at using SMD on the MOSFETS. wOULD LOWER THE PHYSICAL HEIGHT OF ASSEMBLED BOARD. Sorry about the caps
Need to research the IRL520 in SMD.

Using a different battery pack. The final design uses a 7805 for Vcc
So you're using a battery to supply the "5V VDD"?
What does it's output voltage measure when you have it hooked up?

With the 320/10K voltage divider you were using at one point, that should pretty much work with the IRF520 down to about 4.3V or so. It'd probably work with a battery voltage lower than that, but it needs to put >4V on the drain to guarantee it meets the datasheet specs.

Be careful with the SMD mosfets. Smaller packages typically can't handle a lot of power.

..... SMD on the MOSFETS. wOULD LOWER THE PHYSICAL HEIGHT OF ASSEMBLED BOARD. Sorry about the caps

You're not typing in ink. You can fix it and even edit after it's posted.

getting lazy in my old age
looking at a smd irl520
suggested use the irl520nl
also looking at using a smaller gate resistor. suggested 10 ohm - 100 ohm

IRL520NL is obsolete.

As long as the gate resistor is after the pull down it's basically only for protection.
It will effect the switching speed as it charges the gate capacitance, but that's about it.

I ordered some IRL520.s from Jameco
designed pcb smaller.
onword I guess

You don't need to find an smd version of the IRL520. There are many thousands of different mosfets.

What are the load specs of the LEDs you're driving?

The load specs vary with each channel (20 channels at 12 volts) but keeping the load a max of 1 amp per channel.
My plan is to mount the MOSFETs with the leads bent at a 90 degree angle then put the mosfets on the back of the pcb. Could even mount using the board as a heat sink.

If you want surface mount mosfets, check out Taydaelectronics.com. A couple good choices might be* these two.

N-channel
SOT-23 package
Vds breakdown > 12v
Gate threshold voltage low enough
Drain – Source resistance low

And the all-important, IN STOCK

*These suggested parts appear to meet your needs. It's up to you to check the datasheets to be certain. I take no responsibility for any decisions you make about them.

Thanks will look at them both.
Was attempting to build a "keepout"
area using the IRL520 but the Diptrace has several choices so traces will not go under the TO220 package after bending over the tabs. Choices are KEEPOUT, PASTE, MASK, and COURTYARD.
Need to look at the two that popcorn posted if they will work as desired.

They both look promising. Now to try my hand at figuring out if I exceed 2 watts of power
P=Isquared * Rds
Looking at using .5amps per MOSFET with lots of room for error at 12volts'

Sigh

What did I do wrong now?

Figuring out the power dissipation is just a "do", not a "write about and save it for later." It should be much much quicker just to do it than to type a sentence about doing it.

Units must be volts (not mV), amps (not mA), Ohms (not milli-Ohms), etc.

And just to clarify, above did you mean 0.5 amps or 5 amps?

500MA
I am needing to configure the current draw but using the one LED/strip I have it shows .800 or 800ma per string. Plan to cut up the strings into sections and apply to my artwork. THEN measure each strings current draw.
Using the figure of 1 amp per string (uncut) should be enough room for error.
The present project has 12 segments (cut strings of different lengths) so just dividing 800ma divided by 12 should be 60 Ma if my math is correct?

Are you cutting one string into 12 pieces, or cutting 12 strings into shorter lengths?

Let's say you're connecting a string that draws 1 amp to one mosfet. As you stated above,

P = I^2 × R, where R = Rds of the mosfet.

Rds worst case of the two mosfets I listed is 51milli-ohm if the gate voltage is 4.5v. But let's go for the "most worst" case when it's 69milli-ohms for a lower gate voltage.

P = 1^2 x 69milli-ohms × (1 ohm / 1000 milli-ohm)

-->

P = 1 × 69/1000 ohms = 0.069 watts

This is a simple calculation. Takes a minute to do it. If you just don't get it, here's a calculator for you

Thanks for explaining it better.
I copied n Pasted your explanation then printed it out.
So using the posted MOSFET then each MOSFET is using ,069 watts of power?
Then either of the 2 you posted should be safe to use?

One thing you should do is use large copper areas around each mosfet to provide heatsinking via the pcb. You may want to operate at higher current levels at some point so design for it.

The question, in your picture it appears the GATE is the only part of the MOSFET? that has the heat sink (copper area).
On my board, all my traces are .700mm.
I need to research and find out how to place a copper area under each MOSFET.

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