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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 19th Jul 2020, 8:36 pm   #1
AdrianH
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Default Questions on simple HV PSU

As what I am doing is not vintage I put this post in here.

Longish post sorry!

I have build a simple PSU from a transformer got from Andy on here. The transformer is 250-0-250 at 100mA, to try and get the maximum voltage I could I have used a couple of silicon diodes 1KV PIV at 3 Amp rating.

Using what caps I had available means a 100uF from the diodes through a 10H RS choke, (It measured 7H on my TC1 tester) to another capacitor at 180 uF both caps are over 400 Volts rating.

I also added two 150 Ohm resistors, one is series with each diode, my initial thinking was to protect the diodes from current in-rush to the cap. I am now wondering if I need them?

The primary resistance of the transformer is 40.2 Ohm, the secondary averages 243 Ohms per leg to 0V. Looking at the spec sheet of something like the EZ81 it had a formula for the resistance of the transformer being Rt = Rs + n*n * Rp

Rs secondary resistance = 243 Ohm
Rp primary resistance = 40.2 Ohm
n turns ratio 250/240 = 1.0417

Anyway I get a value of Rt = 286 Ohm

Does this mean that the maximum current I can get out of the secondary if shorted or on initial charging up of the Cap from 0 Volts would be 240 * 1.414 / 286 = 1.19 Amps ?

If I am thinking correctly the two 150 resistors I have added would not be helping things and can be removed safely as the diodes have a forward current max of 3 Amps.

I just wish to make sure from you knowledgable people.

Adrian
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Old 19th Jul 2020, 8:42 pm   #2
AdrianH
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Default Re: Questions on simple HV PSU

Forgot to add, how do you work out if the first cap is to large a value limiting charge times at peak voltages?

Adrian
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Old 20th Jul 2020, 6:25 am   #3
Diabolical Artificer
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Default Re: Questions on simple HV PSU

Some usefull info here - http://diyaudioprojects.com/Technica...ower-Supplies/ and here - http://www.turneraudio.com.au/powersupplies.html Some usefull calculator's - https://thesubjectmatter.com/calcptcurrent.html and here - https://www.changpuak.ch/electronics...ply_design.php and here - http://www.duncanamps.com/psud2/ the latter being the best one.

Personally I never bother with calculations apart from using V x 1.141 to get a rough idea of final DC voltage, I then just add about 20v extra for supplies of 300v ish 200mA ish and 50v for 450v higher OP amps. I use my ears , IE listen to the sound the tfmr makes and load tfmr's with big resistors till V drops by 10% to figure out current capability. I also use overwound tfmr's so any error isn't so important.

Andy.
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Old 20th Jul 2020, 9:36 am   #4
AdrianH
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Default Re: Questions on simple HV PSU

I will have a better read of them thanks. The second one deals with silicon diodes so that will be a help, it also suggests the value could go a lot higher on the initial cap.

I will remove the 150 Ohm resistors and think I will add a small 1 Ohm resistor in the Center tap to 0 Volt line, then I may just scope the voltage peaks across it to get an idea nof peak current under load. It is all just playing to understand more.

Adrian
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Old 20th Jul 2020, 12:59 pm   #5
ionburn
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Default Re: Questions on simple HV PSU

Quote:
Originally Posted by Diabolical Artificer View Post

Personally I never bother with calculations apart from using V x 1.141 to get a rough idea of final DC voltage,

Andy.
A slip of the keyboard here Andy, I think. It can make a big difference at high voltage though so I will flag it up: Really 1.414
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Old 20th Jul 2020, 1:31 pm   #6
AdrianH
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Default Re: Questions on simple HV PSU

Ok had a bit of a read, but in the end decided to get down and change a few things and test it, see isf things start to make sence.

So the two 150 Ohm resistors came out and a single 0.5 Ohm resistor was installed in the centre tap line, and at 100 mA draw it made a difference of 31 Volts. Previously the output voltage was 228 Volts and now is 259 Volts. Which if I am honest is better then I thought I would get. OK so each 150 Ohm resistor would drop 15 Volts at 0.1 Amps but I did not believe it would be additive as each one was in a leg of the rectifier So that has removed quite a bit of Sag on the output.

As to the charging current pulses I scoped across the 0.5 Ohm resistor and found that the voltage peaks were 0.14 Volts and the charge happened over 5ms, so each leg of the transformer would have a 15ms rest period, and with 0.14 /0.5 = 280 mA peak current, I am happy that I am not hitting the transformer to hard and my volts are better.

The charge peaks are shown negative going in the pic.

Adrian
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