Re: High Voltage but Low Current fuses...

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <teslalist-at-qwest-dot-net>

Hi Dan,

At 11:12 PM 7/17/2003 -0400, you wrote:
>I'm still very confused to why you are worrying about providing high voltage
>fuses for the differential probes.  The first stage of your differential
>probe should be
>a very high impedance divider on the order of 1Meg up to 100Meg (assuming
>pure resistance now) depending on the voltage being used.  You should not be
>providing
>any type of fusing or transient suppression until after this divider.

Unlike "normal" uses for diff probes, users "here" might connect them to 
NSTs, pole pigs, MOT circuits held together with duct tape...  We may be 
measuring a 20 volt gate signal in a 20kV IGBT array...  Or, we may be 
monitoring IGBT gate voltage while running the IGBT at 1000% of it's Ic 
rating ;-))  In other words, We ain't normal folks :o))

If I hook my Tek diff probe to a MOT, it will make a very pretty and "very" 
expensive fire.  The sudden application of a 20 kV impulse cap to the input 
will tear right though the divider crap of a normal diff probe.  All I am 
trying to do is harden the probe.  To increase the chance of survival (of 
the scope) if instead of it seeing a 20V gate signal riding on 500V, 
suppose it has to suck down a 20000V 20 Joule hit!!  Or eat the output of a 
60,000V 10,000A hit from my Lichtenberg project!!

>Therefore, there is no need for a high voltage fuse etc...  If you look at
>any run-of-the-mill differential probes made
>by Tektronix or similar, they are all built this way.  Remember, you could
>have a 1000:1 divider, so a 10,000V transient spike would only look like 10V
>at the output of this stage.

No, it will arc right through the dividers, fry them like ants...  Tek 
probes would then require a "replacement module"...  I have seen a fair 
number of very nice Tek, HP, and Agilent devices fried like bacon...  In 
one case, the only recognizable part the was left just showed the number of 
the lab we borrowed it from ;-))  I have seen HP34401 multimeters "float" 
around the lab fueled by flame spewing out the bottom...  Fluke data 
loggers give off a very distinctive odor when fried...  I once handed an 
intern an HP 34970a data logger exposed to 1200 volts and asked him to 
salvage the usable parts.  He returned the line cord, handle, and the two 
rubber parts on the ends.  He reported that the insides were "cremated to 
the max!!"...   Many times I have loaned things out to people only to have 
them return saying "Terry!  %^#%, I sure am sorry!!  I am filling out a PO 
for a new one for you right now..."  I too have gone to the lab with PO in 
hand just writing the part numbers down after an "event"...

So I am trying to "increase" the chance of scope survival (mine cost "me" 
$3800) in the case of "the probe is in a ball of plasma" event...

Our little probe idea costs 1/40 of two Tek probes that does the same 
thing.  They may be put together by "anyone" without and CE certified 
testing....  We just have to try and make them much more bullet proof than 
the average probe...

>If say 5V was
>the limit going into your differential amplifiers etc..., then a clamp /
>suppression network at 5V would suffice here.

Got any 5 volt clamps that can take the 20000V,  25amp shorted pole pig 
"input" ;o))


We can't protect against "everything", but we can "try" to protect against 
the "expected"...

Cheers,

         Terry