makeshift very high resistance resistors

[multichnl]

This is not for the unexperienced. And may be dangerous to people which have not worked with high voltage. Try this trick. Place a 1/4 watt 10 meg carbon resistor in series with another resistor of higher wattage of around 100k in series with a high voltage power supply. Put an ammeter in series, (this will be your current to figure the value of resistance you will ultimately have) but make sure it is on the ground side of the circuit where the 100k resistor is, and the high voltage power supply can be grounded on that side. When the current lowers to the value to which calculates to the ohmmage you need, remove the high voltage from the circuit. (The current you need to make your resistor R=E/I). But you need to know the voltage supplied by the high voltage supply. It may be beneficial to arc the connection by touching the h.v. source and drawing an arc to limit the current, then get closer as the current decreases. The 100k resistor is only a precaution. To be extra safe put some plexiglas over the components in case of minute flying debree.

 

[Hero999]

It's easy to calculate the resistance of resistors in series just add them together.

If anyone has enough experiance with electricity they'll kno ohms law so they'll be able to calculate the resistor values without doing any experimenting.

 

[multichnl]

It's easy to calculate the resistance of resistors in series just add them together.

If anyone has enough experiance with electricity they'll kno ohms law so they'll be able to calculate the resistor values without doing any experimenting.

The Idea is the 10 meg resisor is being altered to the value you desire. Presently unknown. The changed value of the 10 meg can be up to and greater than 1 gigohm.

 

[Papabravo]

Nonsense! Voltage and current are related to the resistance which is the sum of the resistors in series. Where did you come up with this bumpf!

 

[Hero999]

Seconded, that's total rubbish!

I wouldn't play around with high voltages if I were you, you obviously don't have enough experiance!

I would recommend you study ohms law and read a few articles on electrical safety before you even consider it.

 

[ericgibbs]

hi,
It must be at least 40 years ago since I last heard of this method of increasing the value of a resistor by electrically overstressing it.

The problem was that they were inherently unstable with time and used to drift in value, lots!!!. It was used with the old carbon style resistors.

 

[Nigel Goodwin]

hi,
It must be at least 40 years ago since I last heard of this method of increasing the value of a resistor by electrically overstressing it.

The problem was that they were inherently unstable with time and used to drift in value, lots!!!. It was used with the old carbon style resistors.

Next we'll be suggesting using triangular files to trim the value!

 

[ericgibbs]

Next we'll be suggesting using triangular files to trim the value!

Nigel,
A play on words!
Long long ago in a galaxy far far away, we used to use a triangular file [engineering type]
to cut a 'V' notch in carbon resistors in order to trim to a higher value!..

Be careful what you say, in another Universe it could be true.

 

[Nigel Goodwin]

Nigel,
A play on words!
Long long ago in a galaxy far far away, we used to use a triangular file [engineering type]
to cut a 'V' notch in carbon resistors in order to trim to a higher value!..

Be careful what you say, in another Universe it could be true.

I already knew it was true, I'm not as old as you, but I'm no 'spring chicken' either!

 

[Optikon]

The Idea is the 10 meg resisor is being altered to the value you desire. Presently unknown. The changed value of the 10 meg can be up to and greater than 1 gigohm.

Yeah, this is a terrible idea. Not only presently unknown, it will remain unknown in the future too. All the way up to the point when it fails altogether. Sometimes debugging circuits is hard enough let alone introducing something has hokie-pokie as this.

 

[Hero999]

I take back what I said as I'm obviously too young to have heard of the method. Nevertheless it sounds a stupid idea, I suppose you could reduce a self healing capacitor's value by overvoltaging it which is also pretty silly.

 

[mneary]

@multichnl:
Where, pray tell, would I use an unstable gigohm resistor?

 

[multichnl]

Why I'd use it in an

@multichnl:
Where, pray tell, would I use an unstable gigohm resistor?

I am using it in an op amp for a very high input impedance non inverting input connected to a ground reference . It remains stable because it is subjected to negligible current. If you look at the resistor carefully you wouldn't know that it changed in value. There is no discoloration.
Why say more? Some of you guys speak before reading.

 

[Leftyretro]

@multichnl:
Where, pray tell, would I use an unstable gigohm resistor?

Texas..........

 

[dknguyen]

I am using it in an op amp for a very high input impedance non inverting input connected to a ground reference . It remains stable because it is subjected to negligible current. If you look at the resistor carefully you wouldn't know that it changed in value. There is no discoloration.
Why say more? Some of you guys speak before reading.

What do you mean it stays stable because there is negligble ground current? That would just make it ultra sensitive to noise especially since anything could induce a current at least comparable to the current flowing (plus the noise from such a large resistor itself).

 

[Optikon]

I am using it in an op amp for a very high input impedance non inverting input connected to a ground reference . It remains stable because it is subjected to negligible current. If you look at the resistor carefully you wouldn't know that it changed in value. There is no discoloration.
Why say more? Some of you guys speak before reading.

You cannot discern what happens to that resistor material at the molecular level by looking at it (for discoloration)!!

Just because it carries low current doesnt mean didly-do-squat. It remains stressed by the voltage across it. And since its material properties have changed due to your torture test, it may eventually go out of spec (failed)

If you say you do not care if it changes value or eventually fails, why use it all all then? why not just put a finger nail clipping there to make a 10G-Ohm..

Really, what you have done to the resistor is BAD. Don't do it. Go buy a suitable value. The only trick is the deception that your part is still usable/good.

 

[john1]

Hi,

I used to alter some of the older carbon types by filing them, but not
making a vee-notch, it was by making a flat along one side up to the
connection at each end.
Not much of a flat, this was only to make minor adjustment to the
value.
I suppose a vee-notch would make a very local resistance change, it
might also make it more likely to break at that place.

John

 

[Hero999]

Want a high value resistor?

Just leave a gap in the tracks on the PCB and shade between them with a pencil.

If you want an amplifier with a high input impedance use a TL081 as a unity gain buffer, it will be more stable than a bodged amplifier made with destroyed resistors.

 

[j.p.bill]

If you want a gig resistor, Digi-Key has them for $4.00 and change. Not much danger there, plus they are 1% tolerance. You can get up to 5 gigs.

The file technique was to start with a 2 watt 1 ohm resistor and cut a notch until the desired higher value was reached. Then you put corona dope on the cut to keep moisture out.

 

[mramos1]

If you want a gig resistor, Digi-Key has them for $4.00 and change. Not much danger there, plus they are 1% tolerance. You can get up to 5 gigs.

The file technique was to start with a 2 watt 1 ohm resistor and cut a notch until the desired higher value was reached. Then you put corona dope on the cut to keep moisture out.

I have not heard corona dope in MANY MANY years.. It still around these days?

So, if a finger nail is 10gigohms, try a would a dirty finger nail for 5gigohms, could save $4, just trim it with a nail file