# 2N3904 driving FOD814 - currents...

Discussion in 'The Projects Forum' started by daman, May 25, 2016.

1. ### daman Thread Starter New Member

Jun 14, 2012
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Hi,

I have a small project and I need to use 2N3904 to drive FOD814 (which is then used to drive some load).
Here's the circuit:

Here's my math (and please be lenient/understanding...):

At 2N3904:
Vbe (base-emitter) = 0.7V
Ib (base) = (3.3-07)/10k = 0.26mA
Vce (collector-emitter) = 0.2V

As for FOD814 part:
Ir1 = (12V - Vd - Vce) / R1 = (12V - 2.1V - 0.2V) / 1000 = 9.7mA
Ir2 = (12V - Vfod - Vce) / R2 = (12V - 1.2V - 0.2V) / 1000 = 10.6mA

So Ic (collector current) = 20.3mA

Considering hFE = 60 I will need to increase Ib (eg. changing R3 to 3k4) to be able to achieve 20mA at the collector.

Now - at 20mA FODs CTR is about 110% so I should be able to drive 20mA load at the OPTO_OUT...?

Does that make _any_sense...?

Thanks,

2. ### ronv AAC Fanatic!

Nov 12, 2008
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Hmm, could get a little complicated.
Let's work backwards.
The curve for the 814 CTR is a typical curve at 25C. It is also with the voltage from collector to emitter still at 5 volts - so not turned on hard. I see another curve for when it is saturated but they don't say which model it is for. Bottom line is I think I would drive it harder. Maybe 40 ma.
The only current that counts towards CTR is the current thru R2 and the opto, so you need to adjust for that then add the current thru the LED.
A similar thing for the 3904 as far a saturation voltage. HFE is calculated with a volt from collector to emitter. To get it down into saturation you need to drive the base harder. Usually a gain of 10 is used for this. So if you want 40ma and .2 volts drive it with 4 ma.

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3. ### DickCappels Moderator

Aug 21, 2008
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It all makes sense, but it would be a good idea, as ronv suggest, to over-drive both Q1 and U6 by a considerable margin.

The gains of Q1 and the transistor inside U6 are temperature dependent and the CTR of U6 will also decrease as the LED ages.

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4. ### daman Thread Starter New Member

Jun 14, 2012
15
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Great - thanks!
So based on what you're saying I need to change R3 to something close to 1k and R2 to around 270Ohm - right...?

5. ### ronv AAC Fanatic!

Nov 12, 2008
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Yes, maybe even 620 for R3.

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6. ### ErnieM AAC Fanatic!

Apr 24, 2011
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Did anyone mention R1 is not doing anything useful here?

Last edited: May 26, 2016
7. ### daman Thread Starter New Member

Jun 14, 2012
15
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Limits LED current...?

8. ### DickCappels Moderator

Aug 21, 2008
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According to the datasheet, the absolute maximum current for the Optoisolator's LED is 50 ma.

9. ### daman Thread Starter New Member

Jun 14, 2012
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I meant D9 (as when Q1 is ON)

10. ### ErnieM AAC Fanatic!

Apr 24, 2011
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Oops... you are correct. I missed the led reading the squeezed image on my iPad.