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### Topic: 48 V signal though opto isolator into Arduino (Read 2730 times)previous topic - next topic

#### stealthtransam

##### Oct 01, 2010, 08:06 pm
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?

#### mowcius

#1
##### Oct 01, 2010, 09:28 pm
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

#### stealthtransam

#2
##### Oct 01, 2010, 09:45 pm
a current limiting resistor is causing too much heat =(  I need at least 40ma though the circuit to kick on the opto.  I'll post a schematic in a bit.

#### lefstin

#3
##### Oct 01, 2010, 10:44 pmLast Edit: Oct 01, 2010, 10:50 pm by lefstin Reason: 1
Darlington optocoupler?

#### stealthtransam

#4
##### Oct 07, 2010, 08:06 pm
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.

#### stealthtransam

#5
##### Oct 07, 2010, 09:10 pm
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.

#### RuggedCircuits

#6
##### Oct 07, 2010, 09:57 pm
Maybe you want to consider an isolator with lower current demands, like a digital isolator:

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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#### lefstin

#7
##### Oct 07, 2010, 10:19 pmLast Edit: Oct 07, 2010, 10:19 pm by lefstin Reason: 1
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.

#### stealthtransam

#8
##### Oct 07, 2010, 10:48 pm
Here is the circuit i'm using.  Each isolator is only pulling 1.6mA now and the power dissipation is only 76.8mW each.

#### royco

#9
##### Oct 08, 2010, 08:07 pmLast Edit: Oct 08, 2010, 08:08 pm by royco Reason: 1
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.

#### stealthtransam

#10
##### Oct 09, 2010, 07:25 pm
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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