Led circuit help

[cummins2]

Sorry for my handicap in electrical circuits. I have a 24Vac circuit that I need an led to turn on and off at specific voltages. The voltage will start out at 0VAC as voltage rises to 24vac I need the led to turn on at around 21.6vac. If voltage falls I do not want the led to turn off until it reaches 19.5vac. If anybody can help with this problem it would be appreciated.

 

[BrianG]

Hi,

As you seem to be in an idustrial environment from your login handle, you may get (almost) exactly what you want off the shelf if you Google "trip amplifiers". Various companies make them in rugged packages which have standard mounting formats such as DIN rail, etc. You may have to rectify/smooth your a.c. signal to d.c. for an input signal though.

If you really must build this as a home grown circuit you should realise it's failry complex topic and could get difficult to discuss here from first principles. Start by Googling for operational amplifier comparator circuits. There are various ways you can use one or more op-amps with hysteresis to trigger or trip at a certain voltage. You can build a "window detector" with two comparator circuits, which covers the voltage range you want, programmed with different value resistors.

 

[ScottyUK]

Check out monitoring relays such as the Telemecanique RM4-UA** series. It should do exactly what you want.

http://download.telemecanique.com/8...characteristicssetupdiagramsp_en8.1_28472.pdf

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Sometimes I only open my mouth to swap feet...

 

[cummins2]

I have looked into the RM4-UA relays and also using a op-amp comparator circuit. the problem I see using these is I have no supply voltage to power the relay or to give a reference voltage to an op-amp. the only voltage that is present is the analog voltage that I am trying to turn the led on with.

 

[europus2002]

Hi Cummins2,
What is the maximum limit of current draw allowable from your AC signal for this solution ??

 

[OperaHouse]

You could do it with an amplified zener like the TL431 This would drive a PNP transistor which drives the LED and in the bottom legwould be a diode and resistor that would provide positive feedback to the voltage divider to give a high turn on and low turn off. Simple, but would require a lot of fiddeling with values. At least I think that is waht you want....I'm only getting 3/4 of the width of these postings and can't see what I've typed either.

 

[t3chguy]

Would it be possible to use a bridge rectifier to convert the AC to an equivalent DC and then use a window comparator that SETs on the higher value and RESETs on the lower value?