Electrical Engineering - 2014-08-23

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2:30 AM

I'm looking for a part, and as I can't really ask if anyone knows what part I need in the normal EE.SE, I asked where I should ask in the EE.SE Meta. I was directed here to ask.

I need a MOSFET. I am looking for a MOSFET that can carry a current of 3A and switches on when a current of about 15mA to the gate passes through it. If it helps, I can direct to my EE.SE question that led to me needing to find that part.

Any help would be appreciated

@CoilKid The MOSFET gate won't pass a dc current. It's controlled by voltage. You need to say how much voltage you have available to drive the gate. Current only comes into the question if you need to switch the MOSFET very quickly.

Have you checked the selection tables at On Semi, IR, etc.?

2:46 AM

A few mV then?, and no. Still kinda new to EE. Like I said, I can direct to my original question if it would help.

0

Q: How to design a power switching circuit using a photo-interrupter

Okay. I'm completely at a loss here. I was trying to design a switching circuit for a 3A load (Solenoid) using a photo-interrupter. I got to about here: But I have no idea what to look for in my parts. I plan to use these as the current limiters, and I have to use this as the photo-interrupter....

circuit phototransistor coil

That's my original question.

I think I knew how to use a transistor(kinda) and find the one I need, but no idea about MOSFETs

@CoilKid A few mV is not enough to switch a MOSFET. 500 mV maybe, making some compromises on otherspecs. 1 V, okay. 2-3 V even better.

Let me check the specs again for my photointerrupter

Transistor Collector Current 20mA

Photo Transistor Collector-emitter Voltage 30V

Photo Transistor Emitter-collector Voltage 5V

That's for the phototransistor built into the photointerrupter

What voltage are you using to supply the collector?

In the circuit in your post, if you replace the "transistor" with a MOSFET, you can provide up to 12 V of gate drive. That is much better than a few mV.

I think the max voltage for the phototransistor would be:
Photo Transistor Collector-emitter Voltage 30V

And the max current would be 20mA

I think

I'm still unclear as to if I'm reading the spec sheet right

Look at this:

user image

Where they have an optocoupler you have an optointerrupter.

Q1 is the MOSFET.

3:01 AM

So, I suppose the question is, can I set the current limiter to limit the current to <20mA (So I don't blow out the phototransistor) and still set the voltage to 12V?

L1 and D1 are the load --- they have some inductive device --- I dunno what you have.

I have to use a photo-interrupter though

@CoilKid A photo interrupter is an optcoupler with a slot through it to let you break the beam.

Huh

Okay, admittedly, I did not know that

Can you give a link to a datasheet for your optointerruptor?

3:03 AM

The specs are part of the web listing, not a separate sheet, but I can link the page. Would that work?

ebay.com/itm/…

I can't find any other photo-interrupters with a gap of 5/16", so I think I have to use the HY860H

Anyway, from what I can tell, I need to attach the MOSFET gate to the wire before the phototransistor. And that means, I can't run a current/voltage that would blow out the phototransistor. Right? So I have to find a MOSFET that can pass a current of 3A, the gate has to operate on <20mA, but I think I can go up to 30V on the voltage?

Does that sound right?

Omron EE-SX1070 is 8mm, pretty close to 5/16, and available from DigiKey (but I dunno if you're in the US, so that might not matter to you)

Well, I'm putting a clear acrylic pipe with an outer diameter of 5/16" in the gap.

Does the circuit I posted work for you? It turns the load on when the slot is blocked.

Where's the power source for it?

@CoilKid You connect a power supply between the triangle at the top and the ground symbol at the bottom.

Like the 12 V in your post.

3:12 AM

Like where R2 is?

R1 and R2 are the current limitting.

@CoilKid No, across the whole circuit.

Ah, so those would be my current limiters?

@CoilKid R1 and R2 limit current.

*I should mention that I have adjustable current/voltage regulators

That's what I meant by current limiters

@CoilKid That's probably overkill. A couple of resistors will work fine.

3:14 AM

I don't remember what they're called, but they work by switching on/off in the mH range

@CoilKid I'm not sure what you're talking about. mH is a measure of indcutance.

Well, I did some calculations, and I asked some questions on EE.SE, and it ould want a couple W

...

megaHertz?

@CoilKid "MHz"

"mHz" would be millihertz

OOOHH...

Yeah

I mean mHz :P

wait, let me check

Anyway, don't try to use a switching supply for a current limiter here. Just get a couple of resistors.

3:17 AM

They would get too hot

Pretty sure

@CoilKid You're only running 50 mA through one and 20 mA through the other. It's only the load branch that draws 3 A.

I am putting 11.4A in, the load MUST have an 3A

That sound about right

The load has to have a constant supply of 3A

@CoilKid You don't (usually) force a current into a circuit. You provide a voltage, and the circuit takes as much current as it needs.

@CoilKid Well why are you switching it on and off then?

This is basically a short-circuit.

Or a big resistor

I'm building a solenoid

So I think it would draw as much as the wire would allow

I really don't want it to melt

@CoilKid Doesn't the coil have some internal resistance? What is the rated voltage on the coil?

3:20 AM

Uh... Lemme check, but its less than 2 Ohms.

1.213Ω

@CoilKid BTW, that makes your application even closer to the circuit I posted. Note the diode. You will need that to avoid blowing up your MOSFET because a solenoid is an inductive load.

I suppose that's true...

@CoilKid So add 3 ohms in series. 12 V / 4.2 ohms ~~ 3 A.

11.4, and I don't know the output current

For that you will need a beefy resistor, but it is not impossible to get a 15 or 20 W resistor.

3:22 AM

Its a battery

@CoilKid As long as it's more than 3 A, you're fine.

Read what I said before
> You don't (usually) force a current into a circuit. You provide a voltage, and the circuit takes as much current as it needs.

Also this:

83

Q: Choosing power supply, how to get the voltage and current ratings?

Power supplies are available in a wide range of voltage and current ratings. If I have a device that has specific voltage and current ratings, how do those relate to the power ratings I need to specify? What if I don't know the device's specs, but am replacing a previous power supply with parti...

power-supply voltage current

If the current goes up, the resistance doesn't change right? Meaning the current on the other end of the resistor will be higher than you want.

Not using a PS

Using a battery

Not a choice for me

If you don't increase the voltage, the current will not go up.

Hmm

@CoilKid As far as understanding the ratings, it doesn't matter. A battery provides a more-or-less fixed voltage, and the circuit draws the current it needs.

3:24 AM

Wouldn't change as the battery drains?

The battery just has to be rated for at least that much current.

@CoilKid Yes. And with a 3 A load you'd better be using a hefty battery.

A stack of car batteries or something.

(or similar 12 V batteries)

Well, as I said earlier I'm (designing and )building a solenoid. I can change the current, but I'd like it to STAY at that current until the battery dies.

Hmm I can do 1A

For the coils

This is getting confusing...

Let me restart.

Okay

I am building a solenoid, which runs somewhere between 1 and 3A(Haven't decided which yet) I am running this off a model battery(small solenoid)(I should double check the batteries specs also.)

I need switching for the coil

I was planning to use a photo-interrupter, with a transistor to switch

Someone said I should use a MOSFET

And here I am :/

user image

L1 is the solenoid.

And that would work, with the battery between the ground, and the optotransistor?

@CoilKid With the battery between ground and the topmost node in the circuit.

3:31 AM

So battery + to the arrow on top, and - to the ground?

Yes

Okay

Also, again, this powers the load when the slot is blocked.

Now, how would I choose an optocoupler and MOSFET that works for me?

Also, what's the circle with the arching wire inside?

Next to the diode?

@CoilKid The interrupter you found on EBay is probably okay.

3:33 AM

Just adjust the voltage/current?

For the FET, check the selection tables at On Semi or International Rectifier.

@CoilKid That I don't know. Maybe a fuse?

I'm still unclear as to the specs for the FET I'm looking for.

So something that can pass 1A, but what should the gate be?

Also, if I run of 11.4V, would that mean I need to look for parts that can take 11.4V in this circuit?

*off

@CoilKid The gate will go up to 12 V.

Usually you worry about trying to drive the gate with a low voltage (1 V or 3 V). With 12 V it's easy.

You need to look at the maximum drain-source voltage. Get something with at least 20 or 30 V rating.

Wait, I think the interrupter I chose passes current when its NOT blocked

Meaning the MOSFET would switch, right?

Oh, wait

"During operation, output phototransistor switch from ON state to OFF state when object is inserted into gap between emitter and sensor."

3:37 AM

The MOSFET gets current when the the optotransistor is blocked, meaning that the gate gets 11.4V...

Okaye

*Okay

@CoilKid I would say, when the slot isn't blocked, the phototransistor pulls the gate low. But I think you've got it.

So, something that switches at 11.4V, can pass 11.4V @1A?

And what's the resistor in front of the MOSFET for?

Switches at 11 V (easy) and can pass 1 A (maybe 2 A to give margin) and can withstand 12 V when off.

Hmm

Okay

And R2 changes the current so the opto- can take it. Right? So what's R3 for?

@CoilKid Just limiting current during turn-off transient. Probably keeps things from bouncing. I don't think it's critical (but I don't design this type of circuit often).

3:40 AM

Huh...

So what would I set that to? Any idea?

@CoilKid R2 lets the opto pull down the gate voltage. The Ptransistor itself will limit current.

@CoilKid How quickly do you need to switch from on to off or off to on?

Uh, ASAP?

Basically as fast as I feasibly can without super-expensive parts

This is for a circular accelerator (Physics project)

coil on when beam broken, off when beam intact

generally

So, would I need R3 at all?

*@ThePhoton So, would I need R3 at all?

@CoilKid You need to figure out the real spec. If it's nanoseconds, you need to consider getting fancier. If it's 10 ms, it's easy. In between, gotta do some engineering.

well, ms should be fine

@CoilKid Once you choose a FET, check the gate capacitance. Call it Cg. Make R3 * Cg about 1/4 to 1/8 of the turn off time you're targetting.

3:45 AM

10ms, that's what, 1/1000th second * 10? or 1/100th second?

Okay

Also make sure Cg * 12 V / 20 mA is less than the turn-off time you want.

Sorry, gotta go.

Okay, thanks for all the help!

1 hour later…

5:06 AM

morning

2 hours later…

abdullah kahraman

7:22 AM

Hi

@abdullahkahraman ho

2 hours later…

9:13 AM

Hello?

@deep morning

9:36 AM

Morning

Do you have a minute?

user image

I have this circuit, but I cant get it to work as I want it too.

I need the components to turn on when it is dark, and off when it is light

@deep Won't work without at least a transistor. The required currents will be too large for the LDR

what about a FET?

@deep physicsatweb.com/rev.php?id=Electric%20Circuits&rid=25 Take a look at this example

The T in FET is short for transistor. A FET will work, a BJT too. Just depends on the exact circuit you use

ok

should the components be in series or parrallel?

@deep velleman.eu/downloads/0/user/usermanual_edu01_uk.pdf

@deep they should be in parallel, but you also need a series resistor for your LED

@deep If you are lucky, you might be able to use your own circuit when you remove the LED, but it won't be very reliable.

9:52 AM

oh ok

@deep The linked pdf isn't bad at all. It comes with a experiment kit when you order it online.

project 9 is what you want, but it is good to work through te doc from start

oh ok

user image

like that?

10:12 AM

@deep You should remove the wire from transitor emitter to ground. I shorts your load

I realised

anything apart from that?

@deep there is a reasonable chance that it may just work

It is not perfect, but it is certainly a good start

i put a 750R resisistor between the battery and variable resistor

750 ohm? that is not a stanard value. where did you find it? Or are you just simulating?

simulating

10:16 AM

ok

so you can measure the voltage at base and at emitter :o)

do i just need to change the values of the resistors now?

yes i can

the advantage of simulation is that you can't blow up the components :)

yh :)

if your potmeter has too low value you may destroy the LDR. So indeed a series resistor is a good plan

10:35 AM

oh ok

/me is out

hope you manage from here

2 hours later…

12:38 PM

@Funkyguy Yes, it has been removed.

Hi all

1 hour later…

abdullah kahraman

2:01 PM

@Roh Hi

2:25 PM

@abdullahkahraman Hey, seems we have a guy from Israel!

2 hours later…

4:27 PM

I was told I could ask more specific questions here, then on the forum? I've checked several electronics websites that were suggested to me, but I don't know the terminology. I have a list of requirements in plaintext for my part, but I don't know what it would be listed as. Can anyone help?

5:03 PM

So... I don't suppose anyone knows the terminology for some MOSFET specifications?

@CoilKid The other way around. On the stack you ask specific questions, here you can also ask less specific (more hand waving) questions

Yeah

But I was told I can't ask for part on the forum.

*parts

Anyway, do you know the terminology for MOSFET specs?

@CoilKid some, but I am on my way out.

Okay

shoot one

5:14 PM

I guess I can probably get away with asking about the terminology on the forum

Uh, The switching voltage?

What would that be for a MOSFET?

hmmm

Oh that is a term you made up, right?

From what I can tell, they don't say "switching voltage"

I don't know

It's the voltage through the gate that lets the MOSFET pass current

But I don't know what I would look for that means that on a spec sheet

There is this thing called "threshold voltage", but that is not really "switching". It is more the voltage that the MOSFET slightly starts to conduct. There are images in the datasheet that should help you finding the value you need. It has V(GS) vs V(DS) or R for various currents.

I am using the MOSFET as a switch...

Its probably the threshold voltage as you say

A MOSFET is not a switch, unfortunately.

It can saturate, that is the behaviour you're looking for.

5:17 PM

No, but I use it as one in this: tesla3.com/free_websites/zpe_lindemann_attraction_lighty1.gif

No the threshold voltage is probably lower than what you call "switching voltage", as it is defined for a current in microamp range.

Hmm...

I'm looking for the name of whatever voltage passes through R3, and into the MOSFET

Do you think this is the kind of thing I should ask as a questioon on the forum?

*question

I guess you could ask that if you have a specific transistor with a good datasheet (containing graphs), the currents you expect and preferably a circuit diagram

I think that should be a fair question

Explain your problem, like you explained to me.

Unfortunately I have to leave now.

Type you later @CoilKid

I don't have a specific FET in mind. I have some specs, but I don't know what my would be on a datasheet

*they

*I don't know what they would be written as on a dtatasheet

what is order magnitude of current you want to switch?

and order magnitude voltage

do a parametric search on digikey

or look at your favorite webshop and see which ones they have in stock and find the datasheet with it

eg small current (abt 100mA) could be a BS170

larger current could be a IRF530, say 5A, probably more

(top of mind, could be IRF531)

but check the datasheets.

do a proposal and explain why you picked that one. Then someone will explain if you are right or not, possibly point out an alternative

anyways

/out

5:36 PM

Yeah, but I don't know what to call the specs :/

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