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TIP120 MOSFET to power a high current build with a low current switch????

xXDUNNXx

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How do I do this! I have google'd and done so much research. I am doing a high power 445 build with a micro flexdrive in a WL e2 and it will be pulling 3A at one point. I cant have 3A going through a 50mA rated switch!!!

I've come to the conclusion that a MOSFET will do the trick...but how do I choose the right one?

Will a TIP120 work.....?
 
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Cyparagon

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TIP120 is a Darlington, not a MOSFET. Ideally you want a bigger switch. This is far simpler than a "relay" design. If you have your mind set on a MOSFET, use one with proper rated current, and low rds value.
 

lasersbee

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How do I do this! I have google'd and done so much research. I am doing a high power 445 build with a micro flexdrive in a WL e2 and it will be pulling 3A at one point. I cant have 3A going through a 50mA rated switch!!!

I've come to the conclusion that a MOSFET will do the trick...but how do I choose the right one?

Will a TIP120 work.....?
You couldn't have done that much Google Research...
I just entered "mosfet switch circuit" and got >9,200,000
hits...:whistle:


Jerry
 

Sigurthr

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There are also many threads here about dimming or pulsing a laser, all of which use a transistor as a low current controlled high current switch, which is what you need. Btw, keep in mind a MOSFET is a voltage controlled switch, it does require low current at the gate though, but is isn't current controlled.
 

lasersbee

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There are also many threads here about dimming or pulsing a laser, all of which use a transistor as a low current controlled high current switch, which is what you need. Btw, keep in mind a MOSFET is a voltage controlled switch, it does require low current at the gate though, but is isn't current controlled.
A MOSFET would be better for is ON/OFF only application
to replace his 50mA rated switch..
A transistor would drop the battery voltage across it much
more than a Mosfet.


Jerry
 

Sigurthr

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Sorry Jerry, I meant transistor in the broad sense, not as in BJT's. After all, a MOSFET is a Metal Oxide Semiconductor Field-Effect Transistor. You are correct though, BJT's will drop 1.5V.
 

lasersbee

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Sorry Jerry, I meant transistor in the broad sense, not as in BJT's. After all, a MOSFET is a Metal Oxide Semiconductor Field-Effect Transistor. You are correct though, BJT's will drop 1.5V.
No need to apologize....;)
If he wanted variable output control I would
also agree to using a transistor circuit...

Jerry
 

xXDUNNXx

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Lol guys...thanks for the pointers...I have done quite some research. I want to know what specs of the transistors do what...I figured someone on here may have some better descriptions that are less technical than the ones found online. I need this to be an on/off switch, that is all...the least voltage dropped, the better.

I just need to know how to read the specs of the transistors. How do I know what current will run through the switch via the specs? I've looked up many different ways to find answers to my questions but nothing has been remotely relevant. :(
 

Sigurthr

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Vgs(th) is the threshold where the transistor turns on. Apply more than this voltage to the Gate to turn it on.

Rds(on) is the resistance between Drain and Source, which affects the voltage drop and current limit at a certain voltage of the transistor.

Vds(max) is the maximum voltage the transistor can switch. Also known as the breakdown voltage.

Ids is the maximum current the transistor can handle.

Pmax (or "Power Dissipation") is the maximum power the transistor can handle. Multiply the voltage you'll be feeding the D/S side by the current it will be carying to find the P it will be operating at.
 

xXDUNNXx

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That is half the battle! Thank you...now how do I tell what current will run through the switch that needs to be limited at ~50mA?

It would be

[ V+ to switch, base ]
[ V+ to collector ]
[ Emitter/Drain to V+ of the flexdrive ]

Right?

Case will be negative.
 

Sigurthr

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If you picture the transistor with base (gate) on the left and collector (drain) on top right and emitter (source) on bottom right:

you can place the load in between collector/drain and Vcc
or
you can place the load in between emitter/source and Ground

It doesn't matter which way.

To find the current needed at the Base you determine the current through CE by measuring how much current the flexdrive draws when set up. Then divide that current by the hFE(min) (also called "Beta" or current gain) on the spec sheet. This should be the current needed at the Base. To apply this much current at the base you determine the voltage you'll have going to the base and use I = V/R to find the resistance needed, then put that resistance (or slightly less for slightly more current) between Vcc and Base. This is for a BJT though and not a Mosfet, BJT's aren't great for your needs here.

I'm not sure how you'd determine the current draw at the Gate for a Mosfet based on the current needed through the DS, a Mosfet is a transconductance device, meaning that it converts voltage to current. AFAIK as long as the current flowing to the Gate is over the threshold needed it doesn't matter how much is actually available, it won't affect the output. I'm not sure how you'd find the threshold current, but it's probably called Igs(th).
 

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Got it! now p vs n type MOSFET...n type to switch ground AFTER the flexdrive and p type to switch the power BEFORE the flexdrive...

Which is a better idea...switching the flexdrives input voltage...right?
 

lasersbee

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Got it! now p vs n type MOSFET...n type to switch ground AFTER the flexdrive and p type to switch the power BEFORE the flexdrive...

Which is a better idea...switching the flexdrives input voltage...right?
NEVER EVER switch the output power of the flexdrives...
unless you want to make a pile of burnt out drives...


Jerry
 
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Cyparagon

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^He means the negative input as opposed to the positive input. (I think)
 




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