making bootleg gigaohm resistors - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: July 23, 2022, 11:44:16 pm
content: I'm messing around with building condenser mic's, and wondered what descent ways there are to make high resistance resistors?anything wet ofc is a no since it'll dry out over time. I liked the idea of coating a wooden dowel with graphite, saw that on a video about building a tube radio authentically to 1920.maybe instead of wood which absorbs water, I could use some foam... maybe graphite foam made from bread? hmmm...what other ways can you suggest?and of course I know you canbuygigaohm resistors. and also that you can use a CF to bias the capsule.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 24, 2022, 01:47:36 am
content: A neon bulb could be used since there's a trace amount of radioactive material inside to encourage starting.http://techlib.com/science/ion.html#Neon%20as%20Resistor

author: moffy
author_rank: Super Contributor
timestamp: July 24, 2022, 02:13:41 am
content: Quote from: ELS122 on July 23, 2022, 11:44:16 pmI do that when I make toast, never knew I could turn it into a resistor.

author: TimFox
author_rank: Super Contributor
timestamp: July 24, 2022, 03:32:08 am
content: Quote from: NiHaoMike on July 24, 2022, 01:47:36 amThe I-V curve of a neon bulb, even with a bit of radioactive material, is nothing like a resistor, and unsuitable for biasing the capsule.OP: have you tried "Aquadag"?https://www.abrimagery.com/store/product_info.php/aquadag-colloidal-graphite-coating-p-96035That page is for 2 lb av of material, and for $85 you could buy lots of real resistors.Another source:https://www.laddresearch.com/index.php/lanotattachments/download/file/id/40/store/1/

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 24, 2022, 04:23:29 am
content: I've made resistive paint using ground up pencil lead (HB is very conductive) and epoxy or other binders. I suspect the problem with high values will be control and noise. You can probably cut it with a handpiece to trim the value but I've no idea how to make a low noise high value resistor other than metal film.

author: Gyro
author_rank: Super Contributor
timestamp: July 24, 2022, 09:36:41 am
content: How noisy do you want your resistor?

author: Ian.M
author_rank: Super Contributor
timestamp: July 24, 2022, 10:15:22 am
content: Quote from: Gyro on July 24, 2022, 09:36:41 amVery?I don't know how viable this would be up in the Gigaohm range, but I'd start with a variation of the hand-drawn pencil or India ink on card grid leak resistor. (See:https://antiqueradios.com/forums//viewtopic.php?f=6&t=325886)Here's what I'd do if I had to develop a DIY process for making extremely high value resistors, without access to vacuum deposition equipment:Start by metalizing two cross-ways strips near opposite ends of a sheet of a suitable engineering ceramic insulator e.g. alumina, or acid-etched glass. The well-known mirror silvering process is probably the easiest. Then draw your resistor element with a sharp hard pencil or a fine tip traditional drafting pen filled with thinned India ink, as a long serpentine trace starting on one silver strip and ending on the other. Thicken it up a bit where it crosses onto the silvering. If using India ink, dry thoroughly then bake gently with hot air. Check the resistance - if its where you want it, carefully melt a small bead of pure paraffin wax onto the surface and apply a thin glass cover slip to exclude moisture and air and prevent anything rubbing the element, leaving part of the silvering each end exposed. If not, you can decrease the resistance by thickening parts of the trace, or shorting out loops. Increasing it is more problematic. Finally, use high silver content soft solder to solder on leadout wires, taking great care not to get any flux over the edge of the substrate or coverslip, as you can't afford the resulting leakage path.

author: Someone
author_rank: Super Contributor
timestamp: July 24, 2022, 10:44:30 am
content: Quote from: Ian.M on July 24, 2022, 10:15:22 amEven for the "poor" conductors giga-ohms is needing sub micron dimensioning, not many hobby/DIY processes working in that size.I wonder if the solution is to go the other direction? a large area glass dielectric capacitor? 50um glass for 10's to 100's of gigaohms "leakage". Any poorer yet stable insulators sold as very thin sheet?

author: TimFox
author_rank: Super Contributor
timestamp: July 24, 2022, 02:18:12 pm
content: 1 Gohm resistors are not hard to find, nor expensive. Example:https://www.mouser.com/ProductDetail/TE-Connectivity-Holsworthy/RGP0207CHK1G0?qs=sGAEpiMZZMtlubZbdhIBIFho3SHfDXSt5nRQ4pdsqnc%3DThese are in stock at Mouser for $1.00 in onesies.AquadagTMis a colloidal suspension of graphite in water. It can be diluted to reduce the carbon density and increase the resistivity.In grad school, we obtained it in small bottles, but for some reason it seems to be hard to find now.

author: NiHaoMike
author_rank: Super Contributor
timestamp: July 24, 2022, 02:59:31 pm
content: Quote from: TimFox on July 24, 2022, 03:32:08 amIf you stay well below the ionization voltage, the radioactive material makes a tiny concentration of free ions that acts as a very high value resistor.

author: ConKbot
author_rank: Super Contributor
timestamp: July 24, 2022, 04:05:37 pm
content: Have a PCB or other substrate with your contacts, then use a capacitor bank to pop short segments of copper wire in proximity to the substrate repeatedly till you get your desired resistance?I had a "small" photoflash capacitor bank, (10,000uf-ish worth of photoflash caps from disposable cameras) with a large SCR on the output to some binding posts to power a disc-launcher. For giggles, put a 30 awg wire link across the output and popped it. Surprisingly, it didn't pop the SCR, and even more surprisingly, the terminals were left with a coppery sheen. Not heavy enough to be under 20Mohm for my DMM to measure it, but definitely shiny like it was still metal, and not just copper oxides.I suppose there was enough copper present to deplete the atmosphere locally of oxygen so metallic copper vapor made it a few mm to the plastic terminal body and condensed. I'm more sure if doing it repeatedly would build a conductive coating, or if intermediate layers of copper oxide would ruin it. Or maybe it was never even metal, and just a form of copper oxide that looked metallic.The best or safest? Definitely not. Bootleg? Definitely.

author: BrokenYugo
author_rank: Super Contributor
timestamp: July 24, 2022, 05:02:41 pm
content: Silicon diode reverse leakage is up in that ballpark.

author: Gyro
author_rank: Super Contributor
timestamp: July 24, 2022, 05:59:55 pm
content: Again, maybe not the quietest form of resistor.

author: jonpaul
author_rank: Super Contributor
timestamp: July 25, 2022, 01:58:00 am
content: Vishay, Craddock and others make stable gigohm resistors 10m...100m...1 GOhm and up.10..22m are stock metal film,allez the other DIY will be noisyj

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: August 04, 2022, 01:20:26 pm
content: Quote from: Gyro on July 24, 2022, 05:59:55 pmI'm a bit slow and rusty today but as I remember it, the noise of the biasing resistor is not an issue in most condenser mikes, the high resistance and the capsule capacity make a very good low pass filter.

author: AnalogueLove1867
author_rank: Regular Contributor
timestamp: August 06, 2022, 09:26:01 am
content: This isn't hard at all. But it does require patience and some equipment.I improvised my multi-gigaohm resistors by -1) cutting a long thin piece of paper ( 2mm X 50mm ). cover the tips with a heavy layer of pencil graphite2) Slightly wet it .3) let it sit in a test-tube in a "U" shape. Held in place with the plastic lid.The sealed tube prevents evaporation and excess water pools in the bottom.The graphite on the paper ends that stick out the top of the test-tube can dry out and still conduct to thewet portions in the test tube. You can optionally use aluminium foil on the ends.You really need a picture to get this. But I have not made one in years. They seem to last indefinitely.Used one for half a year before taking it apart again.Other people have used water filled plastic tubes sealed at each end with screws and silicone-sealer. ( high voltage applications ).Another one is powdered carbon mixed with super-Glue and applied to paper.For a few ohm resistors with a high power dissipation ability try 0.9mm 2B graphite pencil leads.They easily withstand direct shorts of battery packs and can be connected in series / parallel to obtain your required characteristics.

author: richnormand
author_rank: Unknown
timestamp: August 06, 2022, 06:01:32 pm
content: You might try Ted Pella PELCO colloidal graphite:https://www.tedpella.com/SEMmisc_html/SEMpaint.aspx#_16053It is easy to dilute in IPA and is only $8 a bottle.I use it in my SEM for sample mounting but I also repaired remote control buttons with it.Since the solvent is isopropyl alcohol I would not be surprised you could dilute it to be a very high resistance.Also on the inexpensive side Electronics Goldmine (surplus stuff) in the US has various very high resistances in stock.

author: TimFox
author_rank: Super Contributor
timestamp: August 07, 2022, 01:58:32 am
content: Quote from: pardo-bsso on August 04, 2022, 01:20:26 pm“Excess noise” in resistors (often called 1/f noise) occurs when there is a DC voltage across a “noisy” resistor.

author: AnalogueLove1867
author_rank: Regular Contributor
timestamp: August 07, 2022, 07:43:38 am
content: Quote from: richnormand on August 06, 2022, 06:01:32 pmHey! talking about Electronic Goldmine ( It literally is a goldmine! ) Have you or anyone else had success in ordering and shipping to Australia?I tried once ages ago and it didn't work out. They don't accept An Australian Paypal account or credit card.Emailed an enquiry and the response was that it is too much hassle. Understandable. They are running a tight margin in a niche market.Everyone has a nightmare story about trying to get a product to an overseas purchaser.