I was going through an old box of assorted resistor values (5%, 1/4 watt) that I've had for a long time, and happened to notice a couple of 5 meg-ohm values. I've never needed anything that large, so I never paid any attention to them. I grabbed my calculator to work out that it would take 250 volts to push even 50 microamps through the thing, which seems odd to me for a low power component. Not having any real experience with high resistor values, I was curious... what sort of circuit would this be used for? Or maybe would have been, once upon a time (the assortment is several years old). Nothing important, just curious.

 

Hello,

In the old days, very high value resistors where used in tube (valve) circuits.
Nowerdays you will find the high value resistors in photo detector circuits using opamps.
See the attached application note for some circuits.

Bertus

 

Not having any real experience with high resistor values, I was curious... what sort of circuit would this be used for?

Resistors in the mid- to high-megohms range aren't used as commonly as lower-value resistors, certainly; but there are still uses for them. Combined with low-leakage capacitors of several μF, they can be used to obtain long time constants for timers (like a TLC555) or a low frequency oscillator. In addition to photo detector usage mentioned above, they're also useful when dealing with signals from piezoelectric sensors.

5 MΩ resistors are by no means the end of the road; my hobby collection includes a lot of 33 MΩ, 47 MΩ, 100 MΩ and even 1000 MΩ resistors, and other oddball values in that range.

 

I replaced a 10 Meg Ohm, 200 Watt resistor once. It was the bleeder resistor for a 15 kV 1.5 Amp Power supply in a piece of equipment. I did a very comprehensive repair, so it didn't break every year. The people before me never fixed the root cause. AC power requirements were 60 Amps, 3 Phase, 208VAC.

 

Off the top of my head, a 5 MEG Ω resistor makes a good safety device for a personal ESD work station. Extremely low shock hazard should something go wrong with the grounding system. I think the one in my wrist strap is 1 MEG, but I'm not certain. The high resistance affords protection while also providing sufficient pathway to ground for stray static electricity you might generate by wiggling around in your seat.

The wife put one of those plastic protectors under her work desk (at home). Sitting and typing on her computer, I sat there in socks. Sliding my feet across the plastic I got a good snapping from touching her keyboard. Fortunately it was a better keyboard than not and the static charge was directed safely to ground. Socks on plastic is a HUGE generator of static electricity. So are those plastic lawn chairs and sweat pants and shirts.

I once did a demonstration using two of those chairs and two people dressed in sweats. One stood up. Using a copper rod (6 inches) he grounded himself to a grounded outlet. Everybody heard the snap. And they saw him wince with the snap. Then he sat down and the other guy stood up. They held the copper rods out toward each other - and BANG! They both jumped. After demonstrating the obvious amount of power that can be generated from the wrong clothing and furniture I then put wrist straps on both of them and had them repeat the experiment. There was no snapping, no wincing, no nothing. The 1 MEG ohm was doing its job.

 

Socks on plastic is a HUGE generator of static electricity. So are those plastic lawn chairs and sweat pants and shirts.

So are slippers, especially if they have rubber or polyurethane soles. Cotton socks aren't much of a problem, I've found, but anything else spells trouble.

 

especially if they have rubber or polyurethane soles.

Many "Man Made" products are good generators of static. Animal fir is another one. Just ask my cat on a dry winter day when you want to pet her. She lets you know in an absolutely clear clawed way not to be petting her. Or try to touch her nose after a pet-fest.

Funny how many people think "Pink Polly" protects against static charge. It doesn't. It doesn't generate static but it does nothing to shield electronics from it. Worked one place where they had wire shelves on rollers with a chain dragging on a conductive floor. Then they put pink polly on the shelving before placing boards on it. They swore it was protection. No - it was a pathway to ground for anyone not already grounded. Through the board to the shelves to the chain to the floor.

Velostat bag - a bag that has a conductive coating forming a Faraday Cage. Any charge is conducted around the board to the shelf. Polly or no polly. But polly alone does not protect.

 

5MΩ is nothing. You will find 10MΩ and 22MΩ across the quartz crystal of oscillator circuits.
There is a 100MΩ resistor in the HV inverter of a Panasonic microwave oven.
I have worked radiation detectors that have 1GΩ resistors on the gate of the FET preamp.