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Can I use a transistor to switch fluctuating voltage? Answered

I have a connection which I need to have switched on/off  with current from another source. Can I use a transistor to allow current/voltage which fluctuates from 10mV to 3.3V, and serves as a data connection? I have struggled to find anything which is small and easily purchased that would do as I need. I am coming down to the wire in this project, so any help is greatly appreciated. -Sterling

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XP1 (author)2013-05-14

Thanks guys for all the help. I ended up using a reed relay for a temporary proof of concept. It was actually for a high school project, so in either regard, thanks a lot. I just needed to figure out if such a small relay existed, or if a transistor would work. My uncovered research was a little fuzzy.

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XP1 (author)Josehf Murchison2013-05-04

I know, but it is DC current.

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Josehf Murchison (author)XP12013-05-05

Triacs work AC & DC they are more stable with fluxing current then a SCR and transistors should be run at Q more than sat.

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user

Triacs work from 0 to + and 0 to – AC and DC what he wants is definitely not a transistor as it will load the line he wants a triac photocoupler like a MOC3010 this way the control circuit never loads the circuit being controlled.

See the datasheet here: http://www.maxim4u.com/

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user

HOW ? The forward voltage drop of a triac is 500mV, he wants to pass 500mA at 10mV.

Your triac optocoupler has a maximum current of 50mA and a forward volt drop of 1200mV.

Also, how do you turn it off ?

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user

I don’t know what datasheet you are reading; mine says 1 amp load max.

And if you don't know how to turn it off??

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user

Wrong column, mine says 100mA at 25C, 50mA at 70 C

Its the wrong part for the job, don't keep advocating it. Its not suitable.

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user

You are reading the emitter specks, (The LED) however in an optocoupler it is the detector specks that carries the load and that is 1 Amp.

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user

Nope. Mine says 50mA in the LED, 100mA in the triac. AND IT STILL WON'T SWITCH 10mV.

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user

Show me 50 mA or 100 mA anywhere on these datasheets.

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user

You appear to confuse peak repetitive surge current with RMS continuous current - considerably lower.

http://pdf1.alldatasheet.com/datasheet-pdf/view/35282/QT/MOC3010.html

Your move.

Now find me the bit where you switch 10mV @ 500mA with it.

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user

His circuit.

“I have a connection which I need to have switched on/off with current from another source. Can I use a transistor to allow current/voltage which fluctuates from 10mV to 3.3V, and serves as a data connection? I have struggled to find anything which is small and easily purchased that would do as I need. I am coming down to the wire in this project, so any help is greatly appreciated. –Sterling”

It is a digital information transmission line.

10 mV is 0

3.3 volts is 1

And 500 mA controls the number of IC it can transmit the information too.

And If you want to know how I know ask him.

It is a standard assignment in collage.

And midterms are coming up.

He is asking for help or guidance.

The reason not to use a transistor it loads the circuit corrupting the data.

The reason not to use a reed switch is it acts like an inductor corrupting the data.

The reason not to use a relay is power surges and it can corrupt the data.

The reason not to use a SCR a large voltage drop.

The reason not to use a Phototransistor is Q and sat.

Leaves you with a tryac opticcoupler or photocoupler.

You can go to Radio Shack and buy an IR LED and matching photo transistor and do the same thing but the optocouler is more impressive for a higher grade.

Depending on the teacher.

With an optocoupler or photocoupler you can control the circuit from any source including fiberoptics.

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user

What a lot you have read into the question without actually asking anything.

You clearly have little experience switching high speed signals. Reed relays work at at least Mhz, the right ones switch Ghz signals.

A triac has the same drop independent of its size, as the datasheet you misread has shown. So it won't switch 10mV.

The RIGHT transistor design WON'T load the circuit, that's what buffers do, so the simplest solution for the problem AS presented is....a reed relay

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iceng (author)steveastrouk2013-05-06
My PDF document by Fairchild dated 2009

" ©2005 Fairchild Semiconductor Corporation
www.fairchildsemi.com
MOC301XM, MOC302XM Rev. 1.0.2 "

has a peak repetitive surge current of 1Amp for 1ms 120peakpulss/sec
for the internal  opto-triac.

It also says
"This optoisolator should not be used to drive a load directly. It is intended to be a trigger device only".

There is un-surprisingly no mention of a continuous triac forward current.

A

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RavensCraft (author)2013-05-04

Not sure exactly what you have in mind.
Maybe this link will help:
http://www.ni.com/white-paper/2774/en

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XP1 (author)RavensCraft2013-05-05

I tried a reed relay to no avail.

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steveastrouk (author)XP12013-05-05

? Why not ? Its an ideal method, if its fast enough.

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XP1 (author)steveastrouk2013-05-05

They may be fast enough, but if I want to minimize the circuit, they cannot really be shrunk down to the size of say a transistor, or TRIAC.

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steveastrouk (author)XP12013-05-05

Oh yes they can. And a triac won't work.
http://reedrelay.net/micro-relay.html

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XP1 (author)steveastrouk2013-05-05

Oh, alright. I did not realize. Well I built a test circuit and the relay seemed to function as I hope, so I suppose that will work for the foreseeable future. Thanks everybody, sorry for the constant rephrasing of the questions, but I just could not work it out.

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iceng (author)2013-05-04

As Murchison said the T word
but it won't work below 1250mV

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XP1 (author)iceng2013-05-04

So, a transistor would not function?

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iceng (author)XP12013-05-04

A Triac is a transistor...

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XP1 (author)iceng2013-05-05

Sorry, I am new to electronics and have no idea what I am doing.

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steveastrouk (author)XP12013-05-05
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steveastrouk (author)2013-05-05

How much current are you switching ?

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