Use chassis-mount resistors in high voltage service?

[THD+N]

Here's a component question I have wrestled with a bit. When choosing power resistors for voltage dropping in high voltage circuits (up to, say, 500VDC), wire wound resistors have been the norm in tube gear over the years, particularly the "cement" or "glass/ceramic" types. More recently, the chassis mount types have been become available and seem like a good option, particularly their ability to dissipate heat through chassis mounting or other heat sinking, instead of dumping it into the under-chassis space and perhaps heat-stressing other components.

This particular resistor turned up in a BOM and looks to be a quality choice:

https://www.vishay.com/docs/30282/re-military.pdf

I have purchased a few of these types, but am reluctant to use them for HV because of the "maximum working voltage" spec in the datasheets, such as this one. The MWV for this one (and many of these type) are shown on the sheets as a formula: MWV = square root of (Power rating x resistance). For the above resistor rated at 5W and 1500 ohms, this works out to only 87V!

The data sheet shows a "dielectric withstand voltage" of 1000V or 4500V, depending on type for a short period of time.

So, my main question is: is the maximum working voltage for the absolute voltage in the circuit or the difference in the dropped voltage? Would this type resistor be appropriate for dropping voltage in a circuit with a 450V working voltage and startup surge voltage of 500V+?

Also worth noting is the operating temperature power dissipation limit that might knock 20-30% off the power rating at tube gear operating temperatures.

Are these resistors appropriate, then, for HV dropping service in tube gear?

Dave

These are great for high voltage. They mount elevated from the chassis so they are isolated. Easy-peasy, just not as compact. Tons of NOS out there.

 

[Dave451]

Thanks guys for all the thoughtful feedback on my question. Some reflections on the discussion:

1. Experience seems to show that the chassis-mount type resistors will work for high voltage "B+" dropping, but need careful consideration of heat sinking (incl. chassis itself) to achieve the needed thermal dissipation rating.
2. Why the manufacturers and distributors for this type show the Maximum Working Voltage as the product of the square root of power x resistance is still not clear to me, but seems extremely conservative and still worries me a bit that they don't state a useful max value. Using a fraction of the breakdown voltage shown makes sense, but what is the appropriate fraction, recognizing that the breakdown voltage spec is based on passing an HV test for a (short) period of time specified in the method?
3. The above factors still make me a bit nervous about using them in HV for tube gear (but I have a few times with no issues so far).
3. The tried-and-true choices (wire wound "cement", glass/vitreous, etc) work fine and new production are available, but are typically installed under the chassis and dump the dissipated heat there. (I've seen many a vintage Scott integrated with fried wiring, tie strips, and components from this approach).
4. Moving the "tried and trues" above the chassis to dissipate heat with the tubes requires protection against HV exposure above the chassis. I've seen a few examples of vintage amps where the manufacturer used this approach.
5. The 'chalk stick' above-chassis may be a best choice to deal with the dissipation issues, but the originals (ala Scott's approach in amps like the 299B), are no longer made in the tubular form.
6. The new production version of the 'chalk sticks' like the rectangular Ohmite versions mentioned above may be a good choice to replace the older tubular types, but require a little effort to mount them properly and securely above the chassis. I have done this a few times already by installing a tie point beneath the 'chalk stick' hole and using stiff wire to mount the new ones to the tie strip below.

Dave

 

[Selmerdave]

I think you're getting worked up over a non-issue. Kudos for checking the datasheet specs, but these types are tried and true just like any Ohmite.

It's generally a good idea to over-spec any part that is dissipating any appreciable power, and outside of volume production for profit there is little reason not to spend the extra $0.78 for a part that will be reliable in the long term.

Heat dissipation is an important consideration in layout, but again it's going to be the identical issue regardless of which resistor you use, and pretty much every tube amp has a couple of resistors that are dissipating some heat, tube amps have been doing this successfully for a century. Failures happen, but they are rare and most typically result from some other kind of failure, like a loss of bias causing power tubes to run away and excessive current draw. So start by figuring out exactly how much power you expect the resistor to dissipate, multiply that by a factor of margin (2-10x), taking into account any derating required, and heatsink it to chassis if you want (despite above comments it's a perfectly effective way to deal with it so long as you don't put it right next to some sensitive components - which is a bad idea anyway) or mount it above chassis if you want. With a properly-spec'd resistor it's a non-issue either way.

 

[gadget73]

I think that max working voltage thing is a bogey, intended for people who don't understand how to calculate power dissipation. It really doesn't have anything to do with a true max working voltage. In theory every resistor on the planet has this same "max working voltage", but its really just power dissipation limits as determined by basic Ohms Law stuff and nothing to do with insulation or anything like you'd normally think of for a true "max voltage" spec.

 

[trainbuffton]

Reading manufacturers data sheets is a great way to learn all sorts of interesting facts about the parts we use and their intended applications.

It's where to go for facts.
Audiokarma.org is great for opinions but its good to know where to find the facts when you need em.