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Patrick

Mon Jul 13 2015, 10:02PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Im looking for a high voltage resistor with some requirements to follow.

5 to 10kv @ 0.004 A
1.25 Mohms
20 watts.

i effectivly want a load resistor to keep the "real HV present" and the bogus short duration peaks out.

is there anyway to make our own ? does nichrome wire still work if the voltage gradient is low enough ?

Antonio

Mon Jul 13 2015, 10:56PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

Many resistors in series. But I would prefer redesigning the system to not waste all this power.

Patrick

Mon Jul 13 2015, 10:57PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Antonio wrote ...

Many resistors in series. But I would prefer redesigning the system to not waste all this power.

I need to temporarily waste this power as heat, for measurement purposes. the final device wont waste much power at all.

edit: thats alot in series and paralellel.

Signification

Tue Jul 14 2015, 12:00AM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

Patrick wrote ...

5 to 10kv @ 0.004 A
1.25 Mohms
20 watts.

Maybe five 250Kohm in series (4W ea. for "watts only")...but larger (25W ea.?) for 1kV ea. Perhaps in oil or Si for 2kV ea?

I DON'T think secondary's of:
NST's
Ignition coils
Flybacks, etc.
makes the "R",
But,
ebay sells very cheap (few dollars) potted HV modules (IIRC around 700,000V out!). I do not know the secondary characteristics.

klugesmith

Tue Jul 14 2015, 12:01AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Yes, what Antonio said.
Set up a spreadsheet calculator program and start shopping, perhaps on ebay.
To dissipate 20 watts with passive cooling & no heatsink attachment, the dummy load has to be spread out a bit.
Even more so if you need an 80 watt load (10 kV, 1.25 mÎ©).

Here is one example on ebay now:
51000 ohm / 51K - 1 watt 1W - 5% - Carbon Film Resistors (8 Pieces) $3.10
24 of those in series would make a nominal 1.22 MÎ© 24 W resistor bank.
5000 volts would give you 4.08 mA, 208 volts per resistor, 0.85 watts per resistor.
Plenty of margin on V rating for this resistor package style & size.
Slim margin on power rating, but the R's can be mounted close together if there's free air circulation between them.

To save soldering, you could look for high voltage (and higher power) resistors.
10 Ohmite Slim-Mox 108 2M 2.5W 10% TF Planar Resistors US $12.99

Ash Small

Tue Jul 14 2015, 12:03AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Suppressed auto ignition cable (the cheap stuff) will take the voltage. I'm guessing four or five feet might get you 1.25Mohms, from past experience. That's not a lot of amps, so it might be worth a go, especially if you can get it cheap, or free.

Electra

Tue Jul 14 2015, 12:12AM

Registered Member #816 Joined: Sun Jun 03 2007, 07:29PM
Location:
Posts: 156

Well you can't always get it spot on, but 32 x 39k = 1.248 M ohm. For 10kv that's about 321.5v each. You can get 350v resistors in 2 w . So they shouldn't run really hot.

Of course you can get high voltage resistors, but they tend to be expensive and don't come in a huge range of values.

(Edit)
Just comparing results with Klugesmith's I didn't check 10kv is yes 80w so you may need more than 2w resistors if operating constantly

Patrick

Tue Jul 14 2015, 12:55AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

it only need to operate at 5 intervals. and itll be under oil.

klugesmith

Tue Jul 14 2015, 02:56AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

You might get lucky at a swap meet. These guys are 100K (total 1 Meg) and the whole lot was $10.

I just learned something from Electra. Had thought that 350 V spec was usual in 1/2 watt axial-leaded resistors, and that 1 watt size were normally 500 V. Here's a counterexample. Panasonic ERX series 1/2 watters are 300 V. 1, 2, and 3 W sizes are good for 350 V. Not until 5 watts does the continuous "limiting element voltage" reach 500 V. See page 2 of:

One way or another, your load gets 20 watts (or 80 watts) and its internal temperature is what counts.
Rated power can be exceeded substantially (see datasheets) for a few seconds, if the R's are initially not very hot.
It can be exceeded for longer in oil, until the oil itself gets too hot.

IMHO, neither short duty cycle nor oil will protect your R's from morbidly excessive _voltage_ per resistor.

Patrick

Tue Jul 14 2015, 03:06AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

yes the more i calculate, the more heat-per-resistor seems to be the greater limiting factor than the voltage-per-resistor.

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