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5V TTL switch?

Speedy78

Speedy78

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Cant seem to find whst I'm looking for on the googles. Im sure its out there I'm just not searching by its correct name. What I need is a TTL with a 5V signal to switch 11.1V and just under 2A. Its for an RC helicopter. I have found PWM switches that plug into my receiver but I dont want to do that. I want it switched off the arduino based board on the helo more specifically the output with an off of 0V and an on of 5V. This way I can set it up to be used or not via the program and dont have to unplug it to not use it. Would like it to be small and using a MOSFET if able but im not sure if it could handle the 2A or not so may need to throw a relay.

So whst is this called. TTL switch is giving me things I dont want and rc switch is the PWM type that would eat a channel

Speedy.
 





T

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Just look up a MOSFET switch. Basically you'd connect the 5V TTL signal to it's gate (With a pulldown resistor, 10K will do), hook up the Source pin to ground, then the Drain pin to the negative of your load, then load positive straight to positive 11.1V.

Just look for logic level N channel MOSFETs, you can get them up to 100A or so even in a TO-220 package.

tran21.png


Don't worry about resistor Rin, and Rgs can be 10K. Don't need the flywheel diode either.

MOSFET's need a pulldown resistor on their gates since they have slight gate capacitance, so sometimes the MOSFET can turn itself on without an actual logic input.
 
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Speedy78

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Alright, I have the circuit working except one thing. MOSFET gets really really hot and starts producing a small amount of smoke. MOSFET in question Am I doing something wrong or is this the wrong component for the job?

Edit: Just went and double checked my current draw at 11.1V for the two drivers running the LED and its 1.67A @ 11.1V
 
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T

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I don't think that's a logic level MOSFET, you might not be driving it hard enough to fully switch it on.
 
Speedy78

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The LED apears to fully come on when it is switched on. Hmm. Its the only one radioshack had that fit the description. Might have to order the appropriate one.
 
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The LED will come on fine, but because you aren't completely switching the MOSFET, it's resistance is higher than it would be when it's fully switched, thus wasting energy in heating it.
 
Speedy78

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What needs to change then? I can power the gate from a PSU up to 18V if needed
 
djQUAN

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Yes, you're under driving the gate. Since it is not a logic level mosfet, you'd need over 10V to switch it on completely. Looking at Fig7 in the linked datasheet, giving it 5V of gate voltage lets you conduct to only a little over 1A at 25C.

In circuits I design, I usually over rate mosfets to about 10X the expected draw. the IRF510 (as others in the DIY electronics field has said) is a small signal device stuck inside a TO220 case. IRF540 should work a little better if you can find it but you'd still need over 10V to turn it on.
 
Speedy78

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Couldn't try it with 10V on the gate. The MOSFET seems to be allowing current to flow with or without a voltage on the gate. I take it I fried this one? Is there a part number specifically I should be looking for the will fully turn on with 5V?
 
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Do you have a pulldown resistor on the gate to gnd? A MOSFET's gate is very capacitive and even touching it with your finger can turn it on.
 
djQUAN

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You probably toasted it when you said it smoked during testing.
 
Speedy78

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Ok new mosfet and ran 10V into the gate with absolutely no heat generated by thr mosfet. So I take it I need more thsn 5V to switch it. Now my question is, can I easily turn the 5V signal into 10V or so with small components?

Edit: Something like this seems like its what I need. Not sure if I can fit all of this neatly on a small board from radioshack or not though.
 
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djQUAN

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Speedy78 said:
Ok new mosfet and ran 10V into the gate with absolutely no heat generated by thr mosfet. So I take it I need more thsn 5V to switch it. Now my question is, can I easily turn the 5V signal into 10V or so with small components?

Edit: Something like this seems like its what I need. Not sure if I can fit all of this neatly on a small board from radioshack or not though.
Click to expand...

how about something like this? http://cds.linear.com/docs/en/datasheet/lt1044.pdf

mosfets don't need much current to turn on. There is a voltage doubler circuit with not much parts on that PDF.
 
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