Channeling 5000v via resistor

Hi all,

I am working on a project in which I need to channel 5000v to power an LED light. I am fairly new to this all and was wondering what kind of resistor I would need to not fry everything. Assuming the LED is persay 30mA wouldn't that mean I would need a 166666 ohm resistor? I'm sorry if this is a dumb question, I'm just trying to piece together the puzzle.

Thanks much

kmaamari:
Hi all,

I am working on a project in which I need to channel 5000v to power an LED light. I am fairly new to this all and was wondering what kind of resistor I would need to not fry everything. Assuming the LED is persay 30mA wouldn't that mean I would need a 166666 ohm resistor? I'm sorry if this is a dumb question, I'm just trying to piece together the puzzle.

Thanks much

Please clarify! 5000v to me means 5000 volts. IF this is true, you really need to understand you will need many high value resistors to avoid arcing from one side of a resistor to the other. Surely you mean something else.

Paul

I do mean 5000 volts. The project involves checking the voltage of an electric fence.

kmaamari:
I do mean 5000 volts. The project involves checking the voltage of an electric fence.

Wouldn't a neon lamp be better.

Search for a special resistor suitable for high voltage. If you find one, it will be several inches long because of the voltages involved.

Is an Arduino project? If not, why ask the question here?

Paul

Are you sure the fence can put-out 30mA?

5kV @ 30mA is also 150W. That's why you don't often see an LED & resistor on an AC power line (120 or 220VAC)... You need a power-resistor and the resistor wastes more power than LED uses.

I doubt this will work. Most electric fences check for faults before outputing voltage and an led would be seen as a huge fault by the electric fence controller. I've seen grass (which has a very high resistance) cause electric fences to go into fault.

I think you should should use a voltage sensor to sense the voltage on the fence, then use that to control your led. I'm suprised the control doesn't already have an alarm...

There are professional devices which do this.

The simple ones have a resistive divider of many Mohms driving a series of neons.

The posh ones have a similar high value divider driving an a/d which gives a voltage readout on a display.

They both use special resistors and split pcb designs to avoid tracking.

Typical output of a fencer is up to about 10kV in a ~1mS pulse with energies up to about 12J

Mind your fingers.

Allan

Sorry, allow me to backtrack here a second and give a better explanation. The premise of this project is to check the the voltage of an electric fence. If the fence is running normally, 5000v, a green light should light up. If the fence is running at anything less than 5000v, a red light will light up.

Furthermore, the electric fence sends out these 5000v charges in 5 second pulses, so I need the arduino in order to implement a pulse sensor.

Basically, if I were to explain the situation it is something like this (I know this is incorrect code, it’s just to give an idea):

if (pulse<=5s){
if(voltage<5000v){
light up red light
}
}
else if (pulse>5s){
light up blue light
}
else {
light up green light
}

sorry about the confusion guys, I’m just trying to figure out if there’s anyway possible to do this.

The software is the least of your problems.

Typical fencers put out a pulse as described above about once per second. I've worked on them.
They discharge a large capacitor into a step-up transformer.

Allan.

edit - to answer your question - yes, it has been done, so it can be done. The high voltage handling and the peak capture are the tricky bits.

I would use a few turns of insulated wire around the fence wire to act as a pulse transformer. Then you have nice small isolated voltages to play with.

Electric fences generally give a pulse of 10 to 20 microseconds followed by 500 uS of ringing.

You will probably need a peak hold circuit to get a meaningful measurment.
Circuit will need to be very high impedence also.

Paul_KD7HB:
Wouldn't a neon lamp be better

Paul

Yes.you can use photodetectors to read the neons.

Grumpy_Mike:
I would use a few turns of insulated wire around the fence wire to act as a pulse transformer. Then you have nice small isolated voltages to play with.

More capacitive probe than transformer perhaps, but yes that would pick a pulse. Note sure 5kV rated
insulated wire is easy to come by though, you don't want to find one day the insulation has degraded
and is letting 5kV back to your sensing circuit!

I think that amplification will be required though.

Even if you can capture the 20 uS pulse i am not sure there will be enough energy to charge the input capacitance of the arduino.

I had that problem with a circuit for a photo flash,

How did you solve it?

I did not solve it properly.
It massivley underread but i calibrated the software.
Even a 10 meg DMm significantly underread/loaded the circuit.
I think an amp with 100 meg input impedence would be needed.

More energy from a fencing circuit i think per pulse so may not be needed.

Later i solved it by using a custom charging ic which had a digital charge indication output.

Quote boardburner2:

Electric fences generally give a pulse of 10 to 20 microseconds followed by 500 uS of ringing.

Since the driving capacitor is say 100uF charged to about 300 volts, a discharge in 20uS implies a peak current of

I = C dv/dt or 1500 amps

( this is for a 4.5 joule fencer - a typical largish one) Seems an awful lot

The primary inductance of the transformer it drives is typically about 1/2 mH, and simulating the resultant pulse provides the enclosed.

My estimations and measurements of a pulsewidth of ~1mS are more realistic.

The lower trace shows the current, peaking at about -50A.

Allanedsfence1.pdf (40.2 KB)

edsfence1.pdf (40.2 KB)

This is similar to what i remember on MY fencer.

https://cdn.instructables.com/FD2/G4SG/H0A2HD9R/FD2G4SGH0A2HD9R.MEDIUM.gif

There could be wide variations though.

The waveform from my flash circuit was much faster and more challenging.
FWIW that used an air gapped torroid of a few microhenries running at about 800 KHz.

EDIT
from memory a capacitive divider seemed to work better but finding suitable value caps for use at 10 KV may be more challenging.

Never tried it but, it may be possible to do it electrostatically. This would present no load to the fencer unit.