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Author Topic: 48 V signal though opto isolator into Arduino  (Read 1205 times)
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I'm trying to take some -48V signals into an Arduino.  Using a voltage divider circuit i'm dissipating about 3 watts of power.  Anyone know of a better way to feed 48 V into an optoisolator?
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Quote
Using a voltage divider circuit i'm dissipating about 3 watts of power
Use much larger value resistors in your voltage divider circuit...

But as Richard says, surely a current limiting resistor would suffice?

Mowcius
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a current limiting resistor is causing too much heat smiley-cry  I need at least 40ma though the circuit to kick on the opto.  I'll post a schematic in a bit.
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Darlington optocoupler?
« Last Edit: October 01, 2010, 03:50:15 pm by lefstin » Logged

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here is the circuit.  as you can see I have an input voltage of -48VDC and i'm trying to have that send a high signal to the arudino when the 48 V signal is lost.  the problem is for a circuit that will be in the ON state for most it's life .7Watts of heat across R1 is just too high.

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Because when you have 115 channels of this the total heat on the circuit board will be over 80 Watts dissipated.  And i'm not trying to build a heater =)  I could use relays but i wanted the solid state reliability.  
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Maybe you want to consider an isolator with lower current demands, like a digital isolator:

http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=ADUM7440ARQZ-ND

The input current per channel is only 10uA worst case so you can use a voltage divider with large value resistors to minimize power. The unit does need a ~2mA power supply though, but you will still probably come out ahead as it has 4 channels so it's really only 500uA per channel.

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I repeat, a Darlington optocoupler ought to do the trick for you.

It will saturate with just a few mA or less of input current. For your application you do not have to worry about output current or switching times.
« Last Edit: October 07, 2010, 03:19:46 pm by lefstin » Logged

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Here is the circuit i'm using.  Each isolator is only pulling 1.6mA now and the power dissipation is only 76.8mW each.

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stealthtransam,

What kind of linearity are you getting with the optocoupler? I was thinking of going with the LOC110 which is supposed to be linear but I haven't received it yet. The regular opto couplers suffers from non-linear outputs thus not recommended for ADCs.

The problem I have is being able to measure -48 v systems and +24v systems with one unit. Altho I found a solution for that(relays, voltage inverter), I am still looking for alternate solutions.
« Last Edit: October 08, 2010, 01:08:38 pm by royco » Logged

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I've run the input voltage from 18-54 on both a positive +24V system and  -48V system and still had a stable TTL logic out of it using that 33K resistor.  I know it's under driving the current at that point but it still works great.  

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