small confusion with isolation transformer and hv-probe

[fezder]

Yes, i'm over my head again with pretty simple matter that i can't figure out, hopefully someone could shed some light.
situation is this, i have isolation transformer, that is grounded, i mean that it has center-grounding/tab, that is intended to be grounded, and it's grounded as intented. Now then, output voltages are alright when measured with dmm, 230v, mains. Now then, when i measure it with my DIY HV-DMM probe, which is safely insulated properly ( circuit in plastic tube, wires are silicone, banana plugs are dual-isolated) i read around 450v (450mv at dmm), so it's about twise mains voltage! but, when i measure it between hard-earth, and isolated i get mains. The guestion is, why it reads 450v when i measure it on normal isolation-output windings, as my dmm shows ok mains voltage? and here's about schematic, it's different bit, 5 x 10M and two 100k resistors in paraller, so ratio is around 1:1000. also, there are 10pf/3kv caps in paraller with 10M resistors, and 2.2nf/3kv cap in paraller with 100k's.
i would have bought this probe, but they are so expensive....of course it's not always wise to save in wrong place
thanks for help!

https://www.n2rit.com/homebrew/HVProbe/Highvoltage_Probe.jpg

 

[fezder]

small error in my previous post, it DOESNT of course read mains between isolated output and earth, sorry! and with hv probe, same as with dmm, only in 1/1000 magnitude.

 

[alec_t]

The 10meg resistance will inevitably have some leakage path in parallel with it, due to any support structure or air moisture or contaminants or capacitive reactance. So the effective resistance will be less than 10meg. That probably accounts for your measurement being higher than expected.

 

[KeepItSimpleStupid]

Are you sure your center tap is not an electrostatic shield?

 

[fezder]

thanks for replies, i wasn't thinking about those things affecting measurements, thanks alec! and as for ESD shield, i'm not sure, only i know its colored yellow/green as normal case-earth like in all metal cased things.

if it matters, i made schematic of my current circuit, it's at my previous post, attachments doesn't show pretty well, but it's there

 

[Afterburn]

There are many types of plastic and they all have different characteristics - especially heat and electrical resistance. Printing/tattooing along the length of the plastic will also effect the plastic length resistance. You bought it from a pet shop?

Different resistors i.e. carbon, metal film and ceramic resistors have different characteristics at high voltage.

Your meter is reading a higher level than you anticipated. Take this as a warning that something isn't right. Cheap is not always safe.

The purpose of an isolation transformer is to isolate from earth. The secondary is not connected to earth in any way. That is why isolation transformers are used when servicing 'live chassis' TVs.

 

[fezder]

i had a feeling this would be no-go to save in this place. this is why i asked this guestion in first place. as for tube, it's yet another cheap solution, it's from thermal meter old-fashion
**broken link removed**
perhaps i'd better off with store bought, that is designed to work, and to be safe

 

[KeepItSimpleStupid]

Fez:

These https://www.google.com/url?sa=t&rct...=nUIsdpdh8wESzYYBHEx-2Q&bvm=bv.71198958,d.cWc look like isolation transformers but aren't. I bought 3 of the series (ONEAC) for our most critical computers at work and combined them with an Isobar surge protector.

A MAC Centris computer ran for 16 years before taken out of service, No hard drive failures or other failures except mechanical. Floppy drive (3.5") and dust. I used it prior to that on a system that was getting harder to repair, and got the same deal; failures were floppies (8" this time) and fans. At the time, they ran about $1000 USD for a 1 kW unit.

 

[fezder]

hmm okay, interesting this is what i have hooked, and these were at school in some desks, before they got disassempled due lab got moved in smaller quarters...
https://www.electro-tech-online.com/attachments/muuntaja-1-jpg.74419/
i'm pretty sure this is indeed isoltaion transformer as i intentionally poked scopes ground to isoltaed live just to make sure. also i'we measured other mains powered things and probed with scope, and nothing is happened. and scope is ALWAYS grounded

dunno if this helps or only confuses this issue more

 

[KeepItSimpleStupid]

Best suggestion, I think, is the Electrostatic shield: **broken link removed**

 

[fezder]

seems so, it does have two independent windings, both carry out mains 230v.