Power Relay needed

[Sunnysky]

then do as I said before with separate battery and $5 budget

 

[Sunnysky]

not possible without boost convertor

 

[jngtt]

You dont seem to understand what I've told you.
An Ultrabright Red LED needs 2V, (the old GaAs dim red 1.8)
This means it goes out below 1.8V or barely visible

Your circuit exceeds the recommended 20mA above 22V
Do you understand Ohm's Law?

OK i am willing to go with the 1.8V LED plan, But since the max voltage is 28 VDC, do you think it will be ok to use a voltage regulator?

http://www.newark.com/recom-power/r-78e3-3-1-0/dc-dc-converter-3-3v-1a-sip/dp/51Y4020

But when the voltage drop below 1.5Vdc does the voltage regulator try to regulate the voltage to 3V even the voltage is below 1.5V?

 

[Sunnysky]

No

For same reason. Reg will not operate at low Vin.

Why complicate the design, when an infrequently used indicator will not drain a battery much?

Your high level requirements or objectives are now unclear.

 

[jngtt]

No

For same reason. Reg will not operate at low Vin.

So this method may work for my application?
Since it regulates high voltage to a lower voltage so that the led will not be burnt out.

 

[Sunnysky]

no.
a fixed separate bat. will control LED current independant of 0-28V

 

[jngtt]

no.
a fixed separate bat. will control LED current independant of 0-28V

Ok with a separate battery, how should i tackle this problem?
Keep in mind that I have very little knowledge with electrical designing of circuits.

 

[Sunnysky]

Choose ultrabright LED indicator so you can conserve battery >10Cd @ 20mA, so use 2mA , that is 1000 mcd and plenty bright. cost 0.25

Choose convenient battery 4x AA or 9V Alkaline.

Choose low voltage comparator , any

Choose low voltage 3 or 3,3 V LDO. , any

Choose R ratio with high value R's to make reference voltage = 1.5V to Vin-

Use same R value from Vin+ to 28Vout but add Shottkey diode between LDO and Vin+ to prevent over-voltage on input.

When 28V out is >1.5V on Vin+ then Vout goes low to current limiting R (2mA) to LED cathode and Anode to 3V or 3.3 LDO out.

Using RED UBright Vf=1.9V thus e.g. Rs=(3.3-1.9)V / 2mA= 700Ω or nearest.

9V Battery should last 800mAh at 2mA or 400h of operation. ok?

LDO idle current will be much less.

Add recommended ceramic caps. for LDO and Vin+ Add enable switch for bat.

ok?. if not ok, why are you doing this without training?

 

[jngtt]

ok?. if not ok, why are you doing this without training?

Good Question, ask my boss lol
But thank you very much for this information. i will be going radio shack to get these stuff and test it out.

Thank you again.
Jon

 

[Sunnysky]

I would think Digikey is a better place to choose parts and deliver same day.

 

[jngtt]

I would think Digikey is a better place to choose parts and deliver same day.

BTw do you think you can draw up a diagram for me? it will be easier to follow.

Thanks,
John

 

[Sunnysky]

n

 

[BobK]

Here is a circuit that would work. It uses Linear Technologies parts, but that is because I use their simulator (LTSPICE) and was too lazy to try to add more standard parts.

The very common TL431 can be used in place of the LT1009 voltage reference. Just connect the ref pin to the cathode and otherwise the circuit is the same. Any single supply comparator that can handle 9V and has a push-pull output can be used in place of the LT1716 shown.

The green line is the input voltage, which I have varied from 3V down to 0V.

The blue line is the voltage on the LED, which you can see goes on when the input voltage crosses 1.5V.

The resistor and zener on the input are to protect the rest of the circuitry from the 28V possible input. Any Zener less than 9V but well above 1.5V would be okay. I have used an 8.2V one because that is what was available in the simulator.

You can also use a comparator with an open collector output by swapping the + and - inputs to it and putting the LED between 9V and the output (with resistor of course).

Bob

 

[jngtt]

Thank you very much Bob.