### Network

#### Tezza1

Dec 9, 2011
18
Hi guy's
I am planning to rectify my 12000v 30ma Neon Sign Transformer from Ac to DC & could do with a little help & advice please.
I have 4 high frequency (Tesla Ham 2cl - 30KV - 30ma) silicon rectifier diodes which i intend to use for the bridge rectifier! could someone please confirm that these diodes are suitable for the job & would i need to put a resistor before the rectifier? if so what resistor would you advise.
Thanks
Tezza

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,461
The diodes should work (according to the data you gave).
Why would you put a resistor in series?

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
I guess he wants to put a resistor in series with them to limit the inrush current to avoid damaging the diodes. If that's the case, the total resistance needs to drop the peak secondary voltage (12 kV * sqrt(2) which is 17 kV) at 30 mA, so from Ohm's Law, R = V / I = 17000 / 0.03 = 570 kilohms.

You could split that into two resistor strings of 285k each, one in each line from the transformer. Peak voltage across each string would be 8500V so you would want 34 resistors in each string, unless you use high-voltage resistors.

This would of course slow down the charging of the output capacitance of the bridge rectifier, compromise regulation, and reduce the maximum current available. So whether it's workable depends on what you're connecting to the DC output of the bridge.

#### Tezza1

Dec 9, 2011
18
Hi Kris

Thanks heaps for your detailed advice Kris you are also spot on with why I was thinking about putting a resistor in series with the bridge rectifier!.

If the bridge diodes were 60ma or more then I wouldn't really worry about adding a resistor because the diodes in the bridge would be of a higher value than the transformer which is 30ma so they would hopefully handle any extra current.
But on saying that the transformer is current limited to 30ma the same as the diodes in the bridge so maybe the transformer wouldn't draw anymore than the 30ma specified which would save me a lot of messing around with a resistor' do you think that makes sense?.

I haven't actually decided what I'm going to do with the transformer yet my main concern at the moment is to find out how to rectifiy it safely.

Cheers Kris I'm also in New Zealand I'm a Jaffa!
Happy new year to you.

#### davenn

Moderator
Sep 5, 2009
14,214
Jaffa!

But spelt with only 1 f

Dave
Ex Dunedin

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
But on saying that the transformer is current limited to 30ma the same as the diodes in the bridge so maybe the transformer wouldn't draw anymore than the 30ma specified which would save me a lot of messing around with a resistor' do you think that makes sense?
I don't know. It depends how the transformer is current-limited. I have no experience with very-high-voltage transformers so I can't answer, I'm afraid. But I can say that the fact that the transformer is rated for 30 mA doesn't necessarily mean that it won't deliver more than that.
I haven't actually decided what I'm going to do with the transformer yet my main concern at the moment is to find out how to rectifiy it safely.
Right. I wonder what construction method you're going to use. Dealing with those voltages requires special experience, which I hope you have, because I don't. Actually I probably should have left your question for someone else here who has done it before.
Cheers Kris I'm also in New Zealand I'm a Jaffa! Happy new year to you.
And to you Good luck with this project!

BTW for those who don't know, JAFA stands for Just Another ****ing Aucklander. The greater Auckland region has about 1/3 of the total population of New Zealand and Aucklanders are often accused of forgetting that the rest of the country even exists.

#### davenn

Moderator
Sep 5, 2009
14,214
Aucklanders are often accused of forgetting that the rest of the country even exists.

specifically anything south of the Bombay Hills hahaha

cheers
Dave

#### Tezza1

Dec 9, 2011
18
I'm actually from the Bombay hills Dave I use a nice pub just down the road in Mercer (The Muddy Waters) LOL. Thanks for all your replies & help guy's It's been a great fun post.
Thanks

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