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2 watt resistor sizes?

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klugesmith

Tue Jan 26 2016, 01:23AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

I want to make a high-voltage passively-cooled dummy load from many small axial-leaded resistors not scrounged from surplus or ebay stores. Should be in the general neighborhood of 2.5 megohms and 250 watts, good for 25 kV x 10 mA DC.

The motive is partly to explore and demonstrate cost reduction for hobbyists.
The purchase cost per watt seems to have a sweet spot below $0.02, if one buys a whole reel of 2W axial R's.

I have narrowed the choices down to two candidates, shown in attached comparison from an online store.

The Stackpole RSF is more expensive per unit, but we only need to buy 1,000 (US $36). The physical size is what I expect for 2 watts, but rated working voltage is only 350 V. 47K is close to the highest resistance where we can use the full power rating.

The Yageo FMP is substantially less expensive per watt, but comes in a reel of 2,500 (US $57). Don't need 5 kW of dummy loads, nor do I want to connect that many wires, but if money were tight I could re-sell the surplus. These resistors are rated for 500 volts, so we can go up to 100K per resistor at full power. In a modular design, the FMP's can meet a given total voltage with fewer resistors per string, but then need more strings in parallel to meet a resistance or current target.

What concerns me is that the FMP's are physically so much smaller, in spite of higher voltage rating. We know they'll have a much higher temperature rise than the Stackpole resistors. Is there any reason to think they are made of better stuff? Anybody think the voltage difference is just a specmanship thing?

[edit] The FMP series also lacks a specified temperature coefficient, at least in this vendor catalog.

Sulaiman

Tue Jan 26 2016, 06:18AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

a few thoughts
1
2.5 MOhm is a task specific load, not very versatile
four 'sticks' of 10 MOhm in parallel would be more versatile,
e.g. 2.5, 3.333, 5, 10, 20, 30, 40 MOhm available plus other combinations.

2
I would prefer low dR/dT resistors so that by measuring the current I know the voltage, or vie versa
e.g. +/- 200 ppm/C x 100C = 2% which I could accept, but would prefer better

3
Running resistors at their rated power dissipation for extended periods is likely to cause failures

4
A small fan would make a large difference to power handling

5
Have you considered surface mount resistors ?
Cheaper
can glue onto fiberglass (or a ceramic tile etc. which conducts heat better) in a line then solder together
or solder together in a line then glue to the tile, I've not tried.

6
I have had excellent deals buying surplus resistors via eBay
look for partly used reels or boxes, and where seller indicates the source.

Dr. Slack

Wed Jan 27 2016, 02:02PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

Ge the detailed data sheets for both type of resistor before you make any buying decisions. In that power rating and size, I would bet that there is an element of 'lead cooling', and there may be some specification about how long the free leads should be, or what soldered to, for the rating.

I notice one type is metal film, the other metal oxide. Do they have different maximum operating temperatures?

I will second the need for derating from the maximum stated power for long term reliability.

A fan will improve the rating significantly, though you did specify a passive load. Depending on your target volume, a chimney to make best use of convection may improve temperatures.

Sulaiman

Wed Jan 27 2016, 06:55PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

metal oxide resistors can operate at very high temperature
so for economy they are designed to be small (cheaper) so run very hot

(temperature rise in Celcius)^1.2 = (power in mW) / (area in cm2)

this approximation for convection cooling seems to be fit for purpose from my limited experience,
so for a given dissipation and temperature, larger area resistors run cooler,
translated ... a resistor operating at half rated power would have a temperature rise 56% of full power
(approximately, if convection is the main cooling mechanism)
full rated temperature can be 235 C (one example, others may vary)
so at 25C ambient, half and full power, 157C and 260C, ouch and OUCH !
also, lead-tin solder melts at 188C
I can't count the number of failures caused by 'dry' solder joints between resistors and pcb, that we see at work.

The cost of a fan (via eBay etc.) can be cheaper than the extra resistors, assembly, size ...
250W is a lot of heat, two fans may be better, three on reduced voltage quieter ...
you could use a.c. mains, 12 V battery, 5V USB adapter or power pack .....

As you are dumping 250 W, I guess that mains power is available, so ac fans would be logical.
OR
using the formula above, calculate the total area exposed to free air required for convection cooling ;)

johnf

Thu Jan 28 2016, 09:03PM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

Rick
try something like digikey part# VR68J10MTB-ND
I use these all the time @ work. We buy boxes of 500 pieces
cost around US$180 for 500

Sulaiman

Thu Jan 28 2016, 10:08PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I have not used the VR68 series but I have 100's of VR37s (cheap surplus from eBay)
I do not run them at full power but at <= 1/2 power, zero failures, so far.
e.g. I have a few 'sticks' of 2M2 VR37s in series for 99 or 100 MOhm each stick,
(tolerance of resistors means sometimes one more or less resistor required than calculated)
I use them as loads or 'voltmeter resistors', usually only up to 10 kV per stick for a gentle 1W per stick.
I tested one stick with a 35 kV flyback ... no problem.

klugesmith

Fri Feb 26 2016, 12:37AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Thanks for suggestions! One goal of this project is to practice/demonstrate low cost methods.
After more study of sizes and ratings,

I bought, from regular stock, one full reel of 47KÎ© 2W Stackpole resistors. 1000 for $36.

I know that hot spot temp of 235 Â°C can melt even unleaded solder. Discovered that traditional carbon comp R's are no longer made in 2W size, and old stock can cost several $ per resistor. The Vishay VR68's voltage spec of 10kV is a pulse rating by definition, because the largest R value in the series is lower than the critical R where sqrt(P*R) begins to exceed MWV.

Going to experiment with series "sticks" for up to 5 mA AC (peak voltage approaching limit) or 6 mA DC (power approaching limit for cooling by natural convection). 22 resistors per megohm. First test will be 3 megohms on a 15 kV NST.

There's a lot of connecting to do, so I want to minimize tasks like lead cutting and forming. None required for configurations A and B.

Sulaiman

Fri Feb 26 2016, 07:46AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Before you go too far with that design,
consider eliminating sharp points that will spray corona at >=3kV

klugesmith

Fri Feb 26 2016, 07:38PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714

Understood. When I get to that point (pun intended), possible remedies include:
- turning back the wire ends
- putting resistor sticks into tubes with graded potential rings of copper tape outside (and maybe a fan on one end).

Having relatively little experience with five-digit voltages, I am looking forward to listening for corona, and darkening the room to look for it. Now that FLIR cams (thermal infrared imagers) are sold as iphone extensions, when will we see inexpensive ultraviolet coronacams?

Would someone like to explain the basis of the term "partial discharge"?

radiotech

Sat Mar 05 2016, 02:04PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Resistors have voltage coefficients. When I test resistors in the megohm range with a 250-500-1000 volt Megger,
each voltage returns a different value.

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