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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.
 

MrDEB

Well-Known Member
I got some time to review what is what.
I connected as per #58 but I have no resistors and the GATE has no connection. NOTE the tab is connected internally to the Drain (#2). By touching the tab with the 5v the LED comes ON. Disconnect and led turns OFF.
I would think the tab is connected to the SOURCE(#3)? so it can be grounderd to a common ground
Something is haywire??
 

For The Popcorn

Active Member
If you have no connection to the gate, why do you expect anything to happen?

Sorry, in post #58, I assumed you had the tiniest understanding of a mosfet switch and the smallest ability to read a schematic. The GATE is connected to +5v to turn the mosfet on.
 

MrDEB

Well-Known Member
the LED comes ON when I touch the DRAIN or the tab. NOTE if I touch the GATE nothing happens.
Using Post #58 as an example but with no connection to GATE. Connect to GATE (5v)nothing..
YES I can read a schematic and YES I have a basic knowledge of Mosfets as a switch.
I purchased from Jamco Electronics part #209226. an off-brand component?
I tried several different mosfets from same bag.
 

For The Popcorn

Active Member
Sorry. I can't help. There are 5 connections to make, and it has to work. Either these connections are wrong, or your parts are bogus.

Have you actually READ the part number on the DEVICE? Jameco does not have a reputation of selling bogus parts but shipping the wrong part can happen.
 

tumbleweed

Active Member
NOTE the tab is connected internally to the Drain (#2). By touching the tab with the 5v the LED comes ON. Disconnect and led turns OFF.
I would think the tab is connected to the SOURCE(#3)? so it can be grounderd to a common ground

Normally the TAB on a TO220 is connected to the middle pin, so in this case it's the DRAIN (pin 2).

If you have it connected as per #58 and put 5V on the TAB you're connecting 7V across the LED (12V - 5V), so it'll probably turn on but be dimmer than normal.
 

MrDEB

Well-Known Member
Well I got it working.
it NEEDED the resistors.
Without the resistors, the led strip comes on just by touching the GATE pin.
IMO the 10k resistor pulls the GATE low.
Now I can finalize my PCB design and order boards.
 

For The Popcorn

Active Member
Your understanding is astonishing, truly.

For an n-channel mosfet, when the gate is at source voltage (i.e., ground), the mosfet is OFF. When the gate is a few volts different than source (i.e., > +4v), the mosfet is ON.

When the gate is connected to a micro port pin (configured as an output), it will either be low (OFF) or high (ON). Therefore, it will work as expected without the pull down resistor.

With the gate floating (i.e., not connected anywhere), touching it may bring it positive enough to turn the mosfet on. If the gate is floating (something you don't want), the pull down resistor keeps the mosfet off. Twenty posts ago, you started with "the mosfet won't turn on", so may this is ¿progress? Ten minutes of research to understand all of this would have saved 4 days of frustration.
 

MrDEB

Well-Known Member
true I was following all suggestions. Post #58 without the resistors fails to work.
After adding the resistors it now works.
Thanks for your help.
 

For The Popcorn

Active Member
You steadfastly refuse to learn anything. My post above explains all the details.
 

MrDEB

Well-Known Member
After I got the mosfet to work as planned I discovered that just touching the unconnected gate does indeed turn on the LED string.
Yes I learned something about mosfets.
THANKS
 

For The Popcorn

Active Member
Yes I learned something about mosfets.
Did you? Do you understand why the pulldown resistor isn't needed when you're using a micro to control the gate?

Did you figure out why the initial circuit you posted and said was accurate didn't work?
 

tumbleweed

Active Member
going to connect a micro to the circuit with the 100k resistor. As I understand it the 10k resistor.
See post#25 RGS

I'm not sure what that means, but if you're using the IRF520 then make sure the resistive divider gives you >4V on the gate when the uC outputs a logic 1 (small series resistor + much larger pulldown resistor).

The IRL520 shown on your original schematic is much more forgiving and would work with lower levels (>2V)
 

MrDEB

Well-Known Member
MY POST#73 i HAVE TOOOO MANY 0 supposed TO BE 10k
the series GATE resistor thinking 320 ohm
got up early and located a perfboard to hold 12 of the MOSFETs.
got one wired and need to test it later.
 

MrDEB

Well-Known Member
It works as planned with the 320 ohm resistor on the gate. It shows 4.02 volts
and yes the MOSFETs are IRF520
Doing some browsing and found this.
The resistor voltage divider has little affect if the gate resistor (130 ohm) is connected AFTER the 10k resistor connection
 

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For The Popcorn

Active Member
The resistor voltage divider has little affect if the gate resistor (130 ohm) is connected AFTER the 10k resistor connection
Well, those two resistors don't form a voltage divider do they?
 

MrDEB

Well-Known Member
I wish I had used the IRL520 but .
Doing a simulation using TINA I see about .10 volt increase in GATE voltage using the diagram in post#75
This might be why the MOSFET failed to turn on in my first attempt. Using a different battery pack. The final design uses a 7805 for Vcc
Wish I had the IRL520's
Just waiting for parts to complete the perf-board'
IF I have a PCB made then will use the IRL520's
 

Pommie

Well-Known Member
Most Helpful Member
I'm assuming that says 0.1V which is much less than any tolerance.

Mike.
Maybe it's point ten volts which is 10 times bigger than point one volts.
 

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