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[on:]

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?

[Reply #1 on:]

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

[Reply #2 on:]

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.

[Reply #3 on:]

Darlington optocoupler?

[Reply #4 on:]

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.

[Reply #5 on:]

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.  

[Reply #6 on:]

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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[Reply #7 on:]

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.

[Reply #8 on:]

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

[Reply #9 on:]

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.

[Reply #10 on:]

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.