Voltage rating of 1W 10M carbon film resistors ? - Page 1
Forum section: electronics / beginners

author: lordvader88
author_rank: Frequent Contributor
timestamp: August 25, 2018, 04:03:21 am
content: I've been looking around today and so far I only really found 1 datasheet, for 1/4W and 1/2W CF- resistors, and they are listing 250V, 300V for max continuous operation.I have 100 of the really light blue ceramic coated type, and want to make some high voltage probes for DMM use as a start. Later I could buy proper HV rated resistors, at the time I thought that these were maybe 3.2kV just from sqrt (1W* 10M), but highly doubt thats right.

author: Tomorokoshi
author_rank: Super Contributor
timestamp: August 25, 2018, 04:32:46 am
content: Resistor ratings are more than just the active electrical element.Power dissipation is based on the package independent of resistance in some specified mounting condition with specified lead lengths, air flow, track sizes, etc.Voltage rating is often listed as the minimum of what it takes to achieve either the derated power or the dielectric strength of the package. The dielectric strength can breakdown either across the body of the resistor, from lead to lead, or through the package of the resistor to a track from a different circuit. Therefore, in your case, the 250V or 300V rating applies.Watch out for pulse ratings as well. A big ceramic might have a 5x pulse power rating, while a 1206 might have a 2.5x pulse power rating.Take into account safety margins knowing that your installation and use conditions may not be the ideal used to characterize the parts. Perhaps 50%.

author: lordvader88
author_rank: Frequent Contributor
timestamp: August 25, 2018, 04:41:12 am
content: Yup, For now I'm just like to measure my little plasma ball, not TV CRTs or anything.

author: JS
author_rank: Frequent Contributor
timestamp: August 25, 2018, 05:15:02 am
content: As you already considered, high value resistors are usually not rated for it's full power. Also true for some low value resistors.Usually resistor ratings show power, voltage and some current ratings, mainly packaged limited. A certain package has some power dissipation capability considering it's size, which limits the power, also a certain distance between the pins, limiting the voltage and a certain lead thickness limiting the current to maximum rates.After that, to stay within specs you want a maximum temperature rise, limiting the power even further, a certain linearity, limiting voltage as well and the internal elements have a current rating that could also be a limiting factor.Current is the one that could not be present as the range of resistors goes so much low and power might hit the limit first for the lowest value of the product. Power and voltage are usually there, certainly for a 10M resistor voltage is the limiting factor so don't trust the power to have any significance there.JS

author: Wolfgang
author_rank: Super Contributor
timestamp: August 25, 2018, 01:02:06 pm
content: Hi,you could consult H&H Art of electronics for the "Voltage Effect" in resistors.For normal high-value resistors, the voltage limit from the datasheets is normally a lot lower that sqrt(P*R).Play safe !

author: David Hess
author_rank: Super Contributor
timestamp: August 25, 2018, 03:01:33 pm
content: They are unlikely to be anything special. Mouser shows two different 1 megohm 1 watt carbon film resistors and they both have a 500 volt maximum working voltage and a 1000 volt maximum overload voltage which is consistent with common carbon film resistors.Higher voltage parts will be thick film or thin metal film.

author: Jwillis
author_rank: Super Contributor
timestamp: August 26, 2018, 03:01:00 am
content: There are high voltage 1/2 watt carbon film resistors.Here's a few datasheets to look at.https://www.vishay.com/docs/31036/hvwmvw.pdf,https://www.vishay.com/docs/31039/bt.pdf,https://www.vishay.com/docs/30260/hvr2537.pdf. Or try a thick film planarhttps://www.mouser.ca/datasheet/2/427/fhvr-230664.pdf. Orhttps://www.vishay.com/docs/28907/vr25vr37vr68.pdfIf you need higher voltage ratings,you'll probably need to go with a higher wattage.

author: David Hess
author_rank: Super Contributor
timestamp: August 26, 2018, 04:42:19 am
content: Ah, so they do make high voltage carbon film parts but with a very limited selection of values. I am not sure why they would be used instead of metal film or thick film high voltage parts.

author: Wolfgang
author_rank: Super Contributor
timestamp: August 26, 2018, 08:58:25 am
content: Hi,I think you mean the "tiger style" resistors with a carbon film helix used in old TV sets. They have an awesome Tc and should not beused for measurent or precision purposes anyway.

author: PartialDischarge
author_rank: Super Contributor
timestamp: August 26, 2018, 09:27:24 am
content: Quote from: Jwillis on August 26, 2018, 03:01:00 amThe only true carbon resistor there is the Vishay series T and B, easily recognizable for its size, 44.5mm for a measly 7.5kV max, but useful for their power handling capability. The MHV, HVW series is really misleading saying "carbon film" at first but specifying "Element: metal alloy" later, it is pictured below. The HVR is also metal film.One important caveat. Max working voltage does not consider in these resistors creepage distances, or any ambiental factors like humidity or pressure. So although it is not stated in the datasheets most of this resistors are for use in oil or potted assemblies if those voltages are to be applied. This is specially true for the VR series (very small for the rated voltage) and the MHV, MVW, HVX that are uncoated. The FHV series however is fine for air use. Again this depends on particular conditions, like impedance of the source voltage and max voltage applied.Here is a bunch of VR68 and MHV resistors,

author: lordvader88
author_rank: Frequent Contributor
timestamp: August 26, 2018, 09:56:11 am
content: I figured I'd use the max continuous voltages, to be safer. I've seen 300-500V for 10MI'll try 9x10M and just to be extra careful. I don't have to hold the probe, or the meter or anything, just flip a switch and see.I have a few TV flyback, but I'm not messing with them until I have a real HV probe, or real HV resistors. But soon I want to experiment, I've never tried anything with HF/HV except plugging in a plasma ball.

author: JS
author_rank: Frequent Contributor
timestamp: August 27, 2018, 03:38:48 am
content: Quote from: MasterTech on August 26, 2018, 09:27:24 amI wouldn't use any kind of HV circuit without some kind of conformal coating, good clean and a few layers of spray insulator applied should give a better rigidity over plain air on a PCB. I see how in some low leakage applications that could be a problem but at that point the voltage rating of the resistors is the least of your problems, and usually end with much bigger resistors anyway just to be safe. If you feel lucky, I've heard about people melting old acrylic parts into thinner to use later as HV insulation coating.JS