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Topic: 48 V signal though opto isolator into Arduino (Read 1 time) previous topic - next topic

stealthtransam

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

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

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.

Professor Chaos

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

stealthtransam

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

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

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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Professor Chaos

#7
Oct 07, 2010, 10:19 pm Last 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

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 pm Last 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.
Panucatt Devices

stealthtransam

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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