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attune

Mon Sept 04 2006, 02:04AM

Registered Member #259 Joined: Sun Feb 26 2006, 05:57PM
Location: Friday Harbor, Wa
Posts: 18

I refuse to spend hundreds on high voltage, high power resistors to limit the current on my 100kv c-w multiplier. There must be a better way! The three options I have found are:

hundreds of smaller resistors in series

salt water resistor

rubber vacuum tubing (high carbon content)

I want to keep the current down to a few mA. This still means a few hundred watts. I find that distilled water resistors (lower resistance than salt water) would have to be impractically long to get the 100mOhms. I have only found one mention of using black tubing, does anyone know if this is practical?

It seems my best bet is a few hundred metal film resistors. The 3W ones are rated at 750 volts. I have read that (under oil) they can exceed this? At only 3W, it seems inevitable a 100w design would destroy them, but under oil could they dissipate heat better, or perhaps the could withstand higher wattage anyway? Thanks for any help!

Part Scavenger

Mon Sept 04 2006, 03:20AM

Registered Member #79 Joined: Thu Feb 09 2006, 11:35AM
Location: Arkansas
Posts: 673

You can get 100 10Mohm or 1Mohm (or heck 1, 10, 1k, 10k, etc.) 1/2W resistors from mouser for $1.40. IIRC, it's under 10 bucks for 1000. I can dig up a part# if you want. They're rated about 700V-1kV I think, haven't looked in a while.

Electroholic

Mon Sept 04 2006, 03:38AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

teh 10w or 20w ceramic/cement ones are not that expensive, juat have like 10 of them in series on heatsink.

HV Enthusiast

Mon Sept 04 2006, 03:42AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

wrote ...

I want to keep the current down to a few mA. This still means a few hundred watts.

Not really. The voltage drop vs. load current of a CW multiplier drops quite significantly with increased load, and also increases as the number of stages increases.

What are the specifications of your CW multiplier?

Frequency of Input Voltage
Number of Stages
Voltage Input
Capacitance per Stage
etc...

ragnar

Mon Sept 04 2006, 06:05AM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

If the only reason you're investigating these is cost... see if someone's flogging exactly what you want on eBay... chances they are! And at a price that's worth your while, too.

attune

Mon Sept 04 2006, 02:54PM

Registered Member #259 Joined: Sun Feb 26 2006, 05:57PM
Location: Friday Harbor, Wa
Posts: 18

I thought of large ceramic resistors, but I couldn't find any with high enough resistance. Those mouser ones look great! Specs:

100 W flyback. 1cm arc = 10kv? Hopefully increase to 300 W and 15kv

5 stages @ 30kv per stage

100 pf per stage

HV Enthusiast

Mon Sept 04 2006, 03:56PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Firstly, with your set-up, you will not need to limit current for continuous duty. So, no need whatsoever for big 100W resistors. You only need to limit for the peak discharge when you initially discharge the CW stack, and the power dissipation when this occurs is practically zero.

Even if you got the large power resistors sized to limit current at 3mA at 100kV and put them on the output of your CW multiplier, EVEN with an infinite power flyback supply, those resistors will dissipate ZERO watts as the output voltage will be ZERO!!!

Secondarly, the CW multiplier, as is, will limit current itself for continuous operation. In fact, with your set-up (5 stages, 100pF per stage, assuming 10kHz flyback frequency) at 3mA output current, you would have a voltage drop across the multiplier of 143kV which basically means if you try to drop 3mA of load across the output, it will quickly go to zero and stay there until the load is removed. Even at 3mA!

In summary, if you want the several mA at 100kV, you are going to need to invest in larger capacitors, and actually do some math to determine the best set-up (number stages, etc...) to achieve 100kV at 2-3mA. And with your current set-up, as far as current limiting goes, you only need to limit for the peak discharge which dissipates virtually nothing - the CW multiplier in itself will provide its own current limiting action.

Roger

Mon Sept 04 2006, 10:28PM

Registered Member #221 Joined: Mon Feb 20 2006, 05:36PM
Location: Chillicothe Ohio
Posts: 12

I just bought some nice power resistors off eBay a few weeks ago for my DC Tesla coil project. Here is a list of what I got.

One 15,000 ohms at 200 Watts
One 10,000 ohms at 200 Watts
ten 1000 ohms at 200 Watts
ten 5000 ohms at 50 Watts

I cant remember the exact prices but they where all very reasonable. I also saw some resistors that where in the megaohm range but I didn't need any thing like that.

attune

Tue Sept 05 2006, 04:32PM

Registered Member #259 Joined: Sun Feb 26 2006, 05:57PM
Location: Friday Harbor, Wa
Posts: 18

I see. Thanks for setting me straight. Could you explain what you mean by limiting only the peak voltage (me, the uneducated high school student). I used a voltage multiplier calculator http://www.blazelabs.com/cw-brm-java.asp to design it. The peak voltage is calculated at 150kv. Slightly confused.

Steve Ward

Tue Sept 05 2006, 06:09PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

He said limit the peak output current to a safe level for your diodes. What happens when you short circuit a CW is that the capacitors can dump their charge (very quickly, meaning high current) through the charging diodes, this can kill them. You only need a few k ohms, and an appropriate stand off voltage of 150kV in your case. Why 150kV? well, when the supply sits open circuit it has 150kv, then suppose you ground the other end of your resistor chain, you instantly have 150kv across it (but only until you discharge the CW stack, so not for long). You do *not* want the resistor to flash over.

I bought a bunch of 200W ceramic resistors on ebay for about $1 each. 10k-100k ohms should be plenty of limiting resistance for your project, and the 200W size is physically long enough to stand off at least 80kV (not sure about 150kV), so you may need 2 in series. You want your ballast resistance to be much less resistance than your load impedance, or else you will burn up a very significant amount of power in the resistors alone. A 150kV 3mA supply has an impedance of 50Mohms, so something in the 10k-100k range will have very little adverse effect, and will dissipate very little power.

So, figure out what peak current your diodes can handle. Say its 15A, then at 150kV, you need 10k ohms to limit the peak discharge current to 15A.

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