Led limiting resistor

[igeorge]

Hello,
I have a line where the current varies from 1 to 5 amps.
I would like to insert a resistor which will turn on an optocoupler rated at 1.5 volts , 10 mA
The issue here is that at 5 amps i need a 0.24ohms resistor , and at 1 amp i need a 1.2 ohm resistor.
Assuming that i use the 1.2 ohms resistor, at 5 amps will generate 6 volts and fry my opto.
I need help to solve this issue, which i do not know how to do it
Regards
Ion

[Demon]

I would start here:

http://www.centralsemi.com/PDFs/selection/leaded/what_is_a_cld.pdf




A Current Limiting Diode, also known as a “Current
Regulating Diode” or a “Constant Current Diode”, performs
quite a unique function. Similar to a zener diode,
which regulates voltage at a particular current, the CLD
limits or regulates current over a wide voltage range.

[igeorge]

Thanks Robert, but you missed the whole point of the question. For constant current ti is not an issue as we use Ohm's law to calculate.
The point is in this particular case that current change from 1 Amp to 5 Amps
I will try to make a sketch and post it soon.
Regards
Ion

[rsocor01]

I will try to make a sketch and post it soon.
Regards
Ion

Yes, posting the schematic will help, so we can have an idea of what you are talking about. Also, what optocoupler are you using?

[igeorge]

Hello,
I do not have any opto in mid yet, but i am flexible on what ever i can use.
Here is a concept of what i need
regards
Ion

[rsocor01]

What is the purpose of R2? I don't quite understand what you are trying to do. Are you trying to measure the current in the heater? Or, are you trying to switch it on/off?

[igeorge]

r2 is to turn on the opto.
all i need is to sense if the heater is burned out when the switch is closed.
I do nt measure current
thanks

[rsocor01]

r2 is to turn on the opto.
all i need is to sense if the heater is burned out when the switch is closed.
I do nt measure current
thanks

What about using a relay to detect if the heater is burned out?

[HankMcSpank]

Hello,
I have a line where the current varies from 1 to 5 amps.
I would like to insert a resistor which will turn on an optocoupler rated at 1.5 volts , 10 mA
The issue here is that at 5 amps i need a 0.24ohms resistor , and at 1 amp i need a 1.2 ohm resistor.
Assuming that i use the 1.2 ohms resistor, at 5 amps will generate 6 volts and fry my opto.
I need help to solve this issue, which i do not know how to do it
Regards
Ion

iF you place a 450R resistor in series with the opto, that'll ensure the voltage doesn't exceed 1.5V/10mA at the opto for 5A through the element ....of course the opto will only then 'get' 300mV if the element current drops to 1A....but without knowing more about the opto blah blah

[mister_e]

Why not using a Current transformer?

[igeorge]

I think Mister_e solution it is something which i will consider, but i have to solve the conversion of 10 mv ac/60 hz to a decent DC voltage so i can use it with a comparator.
As i said, analog it is a mystery for me.
Regards
Ion

[Demon]

I don't understand why the need for complicated design.

Wouldn't an appropriately rated relay do the job just as well (isolate both sides), be easy to install and be able to withstand a lot more?

[igeorge]

I believe that the relay first will not withstand the 5 amps on the coil.
The second issue is that voltage is chopped by the SSR and heat controller, and the relay will come to a point when it will go crazy ON-OFF-ON , and the signal to CPU will be a pulsating one.
Thank you

[Demon]

I believe that the relay first will not withstand the 5 amps on the coil. ...


I would think that would depend on the rating of the relay.



...The second issue is that voltage is chopped by the SSR and heat controller, and the relay will come to a point when it will go crazy ON-OFF-ON , and the signal to CPU will be a pulsating one.
Thank you

That little detail might be a big problem.

[igeorge]

Thank you Demon,
I am testing now a ACS712 and in the scope it looks like a sine wave - 60 HZ, swinging between +1 volts to + 3.5 volts
I will get tomorrow an LTC1966 from Linear Technology , which is an RMS to DC converter and start working from there
Regards
Ion

[rsocor01]

I believe that the relay first will not withstand the 5 amps on the coil.
The second issue is that voltage is chopped by the SSR and heat controller, and the relay will come to a point when it will go crazy ON-OFF-ON , and the signal to CPU will be a pulsating one.
Thank you

You can rectify the AC signal before feeding it to the relay. Add diodes and a capacitor and you are set. Like demon said, you might be making it more complicated than what it is.

[HankMcSpank]

Thank you Demon,
I am testing now a ACS712 and in the scope it looks like a sine wave - 60 HZ, swinging between +1 volts to + 3.5 volts
I will get tomorrow an LTC1966 from Linear Technology , which is an RMS to DC converter and start working from there
Regards
Ion

a liitle more meat on how you anticipate handling the 'trapping' of an element going open cicuit might be useful..... ie the whole signal chain, because I'm left wondering why you're using an RMS to dc converter (and a comparator) for a simple task that a pic can readily do for you.

[igeorge]

The pic is supposed to monitor 16 heaters. Can be done with a multiplexer, but my boss wants a visual LED for each burned heater, so in this case i decided to have 16 separated inputs. He wants the leds to represent the status of the inputs. He does not believe in multiplexing .... and he sign my check also :(

[HankMcSpank]

The pic is supposed to monitor 16 heaters. Can be done with a multiplexer, but my boss wants a visual LED for each burned heater, so in this case i decided to have 16 separated inputs. He wants the leds to represent the status of the inputs. He does not believe in multiplexing .... and he sign my check also :(


No, that bit is clear...what I'm getting at is how are you going to have the PIC 'monitor' (ie what's the PIC gonna be looking for ....a DC level? - therefore ADC, comparator, a small AC signal? therefore ADC to extract a non zero level)....you mentioned comparators....you mentioned RMS to DC converters .....a bit more clarity on your anticipated approach at the PIC itself will help everyone help you towards your end goal.

[igeorge]

Some how i have to bring to pic a level 1 for each good heater and zero for a burned one. They are not enough A/D on the pic to handle 16 channels.
So, what i need is to convert before pic the signal in zero and one . Zero for any current lees than one amp and one for anything above that

[HankMcSpank]

Well, in your shoes, I'd be inclined to use two PICs and just step through the ADC channels to pick out out a 'non AC' signal (ie a dead element), else you're looking at a fair bit of 'gumph' just to put in between your 16 'monitored' signals & the PIC. (ok, so I know your boss said no multiplexing....but would he know that the ADC pins are multiplexed internally anyway?!!)

Also you make no mention of how quick you need to be alerted....is there a 'failed heater response unit' on hot standby looking at the leds, is a minute or two ok, must this be notified within a second etc.....such requirements all help steer the final deployed solution.

[igeorge]

1-2 seconds will be fine

[HankMcSpank]

In which case, I'd not be going dicking about with hw comparators (that said when you think about it, ADC'ing a signal and then comparing against a preset level in software is a comparator...a sw comparator), RMS to DC converters etc....just tap off some reduced AC from each element feed (obviously 5V 'peak to peak' max, but at least here when your element is only drawing 1A there'll still be 1V AC peak to peak - more than enough to work with) ...squirt the 16 tapoffs into a PIC's ADC pins (use two PICs to get around the 'not enough ADC channels' problem). Then step through each pin all sequentially, staying on each pin for long enough to ensure you capture a 'non zero' reading (note: 'zero' here being half your VCC, so for 8 bits, and a VCC of say 5V, 2.5V = 127.....so say any ADC reading less than 110 or more than 144 means there's AC present....if so move on, if not then latch a red led, then move on etc)

[igeorge]

Thank you very much.
I am going now to set up a bread board and try it
Regards
Ion

[Demon]

I'm following where Hank is going, but how will you go from 110V to 5V?

I saw mention of the ACS712 above, I'm just curious on the details of the schematics.