Look who's all grown up! Finally able to pull the trigger and get an oscilloscope

[SoulFetish]

Here is my version. Ideas shamelessly stolen from Juan


[ATTACH=CONFIG]43841[/ATTACH]

Oh..., the 1pf is just a guess. It's there to maintain the HF response and can usually be left out. The actual value depends on the zeners and chosen to get a flat top on a square wave. Resistors do have a voltage limitation so a string of six is used to get to 3kV

Okay, i dig that. I'm pretty much looking at ceramics for those 3KV caps, yeah?
Should I include a capacitive divider for the attenuator? I guess you have the 10X probe function as well. May be overkill.

[nickb]

Okay, i dig that. I'm pretty much looking at ceramics for those 3KV caps, yeah?
Should I include a capacitive divider for the attenuator? I guess you have the 10X probe function as well. May be overkill.

The ratio is 100:1, not 10:1. The capacitive divider is already there and consists of the zeners in parallel with the scope input in the tail and the nominal 1pF in the head of the divider.

[SoulFetish]

The ratio is 100:1, not 10:1. The capacitive divider is already there and consists of the zeners in parallel with the scope input in the tail and the nominal 1pF in the head of the divider.

Oh dude,my bad. I had a momentary lapse and was thinking about capacitor dividers backwards instead of how the ratio
Works with the reactance. Im sorry. The example was clear, my mind was not

[Chuck H]

FWIW Enzo's capacitor in series with the scope' thing is great for monitoring PI AC circumstances. PI measurements are sometimes problematic because of the high input impedance. A series cap solves for this WRT AC measurements.

[nickb]

Oh dude,my bad. I had a momentary lapse and was thinking about capacitor dividers backwards instead of how the ratio
Works with the reactance. Im sorry. The example was clear, my mind was not

Mea Cupla too: I swapped the labels for AC and DC on the switch

[SoulFetish]

Nick, should I use a short lenght of RG58 (or some other 50 ohm cable) to couple the attenuator to the scope?

[nickb]

Nick, should I use a short lenght of RG58 (or some other 50 ohm cable) to couple the attenuator to the scope?

Something screened, flexible and rated for these voltages is good. The impedance is not an issue.

[SoulFetish]

Yeah, the probe I have now isnt rated for the high voltage side of this circuit. Im going to need something suitable.

[nickb]

Yeah, the probe I have now isnt rated for the high voltage side of this circuit. Im going to need something suitable.

Afterthought: Whatever you use keep it a short as possible. A foot of twisted HV pair might be good. The objectives are insulation and low capacitance. If you can build the megohm resistors right at the tip that would be ideal - just like a real probe. Then longer screened cable would be fine as it's capacitance appears across the output where the impedance is lower and that is a good thing. You seem to have excellent build & mechanical skills (jealous) so I'm sure you'll come up with something

[Tom Phillips]

Nick, should I use a short lenght of RG58 (or some other 50 ohm cable) to couple the attenuator to the scope?

Something screened, flexible and rated for these voltages is good. The impedance is not an issue.

I suggest that you just use a X1 scope probe. Referring to the diagram in post #30 just connect the probe to the nodes after the attenuator labeled “To scope.”

[nickb]

I suggest that you just use a X1 scope probe. Referring to the diagram in post #30 just connect the probe to the nodes after the attenuator labels “To scope.”

I had thought about that dismissed it on grounds of not specified for 3KV.

[Tom Phillips]

I had thought about that dismissed it on grounds of not specified for 3KV.

I meant to use the 1X probe to connect between the attenuator output and the scope. If the voltage at the output of the atternuator is 3kV then somethings really amiss and you can't hook it to the scope input with a wire anyway.

[Chuck H]

For a 3KV rating you'll need to put a bunch of resistors in series anyway. This almost calls for it's own housing. Something like a Bic pen case with a probe at the tip end then a shielded cable coming out of the other. Now the cable only needs to handle 300V for a 3KV input at the Bic probe tip. Then some way to couple that cable to the 1X probe.

[SoulFetish]

For a 3KV rating you'll need to put a bunch of resistors in series anyway. This almost calls for it's own housing. Something like a Bic pen case with a probe at the tip end then a shielded cable coming out of the other. Now the cable only needs to handle 300V for a 3KV input at the Bic probe tip. Then some way to couple that cable to the 1X probe.

This is what i was thinking. I was thinking about finding or fashioning something ceramic i cound use as a probe head/tip and building in in there.

[Tom Phillips]

This is what i was thinking. I was thinking about finding or fashioning something ceramic i cound use as a probe head/tip and building in in there.

Or you could just buy a used Tektronix Model P6007 100X Probe. If you are patient, you can get one for a really reasonable price. Here is a link to a completed eBay sale for $20 including shipping. ( Tektronix Probe 2883619. P6007, BNC, 6, 100X~ 2.2pf. 10M? | eBay )

The P6007 is the model I posted about in the thread at http://music-electronics-forum.com/t31492/. It's design is such that it attenuates both the AC and the DC voltage. Therefore, the P6007 does not exhibit the problem that allows the capacitor inside the scope to charge to the full un-attenuated DC voltage being probed. The MEF thread I referenced already explains this and contains the Tek Probe circuit so I won't repeat the info here. The designs and performance of commercial 100X probes vary. One needs to check the specifications and read the operating manuals to determine if the probe will work for your application.

HTH,
Tom