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Small HeNe brick as capacitor charger

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LAN_403

Signification

Wed Dec 09 2015, 02:05PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

Hi gang...had to have your input on something:
I have been been using a HeNe laser power supply block...INPUT: 12VDC OUTPUT: 1200VDC to 2300VDC @ a few mA (several supplies)! With the standard 5W ballast resistor. It works great for my charging purposes. Does anyone have an opinion on the effect of just removing the large, hot, big, 5W resistor? Since uncharged capacitors suck "an initial surge", why wouldn't it be just fine? A "laser guy" said the problem would be the surges killing the capacitors--I'm pretty sure he was guessing.

DerAlbi

Wed Dec 09 2015, 02:38PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

For the unknowing among us: can you tell us anything specific about the topology used?

A flyback has no problem with an output short circuit. But Flybacks provide no "initial suck", so it may not be used.
A push-pull transformer driver might die.
A self resonant tolology can display odd behavior, but might be ok

One thing is sure: Capacitors surely do not fail due to a single pulse power surge. I mean.. who knows. No idea which caps you use.
Aluminium can handle their stored energy as single pulse heat. For at least 1000 cycles.
Foils are mostly pulse capacitors anyway.

If the 5W power resistor is a pain, then use a constant current source in series to your caps instead. That limits the inrush-current (if there is any) and during normal charging operation the current source just staturates and has minimal voltage drop.

Signification

Wed Dec 09 2015, 03:10PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

@ DerAlbi, You're really on my six there!
For now, I think I would rather, the single resister than a constant current source. Some sample schematics of the PS bricks are here...

The supplies surge at startup to ignite the Ne tube, much like an NST. as you know, discharged caps pull an initial charge-surge, so it may even be THE ideal match "without" the resistor--or anything (this is what I think)--I can surely just try it!

I'll catch up and get back with more details...

DerAlbi

Wed Dec 09 2015, 03:41PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

If i understand those schematics correctly http://repairfaq.cis.upenn.edu/sam/sgsp1sch.pdf then there is a high votlage transfomer directly at the mains which supplies a DC voltage without any SMPS.
Then there is also a trigger output which is for gas ignition. Is that the output that you consider the surge-stuff...?

I currently do not unterstand how you want to charge capacitors with that (to be honest). The HV-main transformer is not designed to charge capacitors and would require a large power resistor for safe operation. (i guess thats what you have now - and thats fine, and you should not change it).
The ignition coil does not output any surge as long as you do not "trigger".

Maybe a small hint:
Capacitors do not suck an "initial charge" or anything. The amount of current that you can see is determined by the voltage differential beween your supply and the capacitor voltage and the output resistance of your supply.

A capacitor at 0V sucks initially 1A if you connect a power supply with 100V and 100 Ohm output resistance.
A capacitor initially at 100V sucks also 1A if your power supply is at 300V while having 200 Ohm output resistance.
It does not matter, if the capacitor is initially chaged or not.
Lets just call it "inrush-current"

(If you mean the part of the exponential current waveform where the current is initially really high)

The only thing that can charge capacitors efficiently are flybacks imho.

Signification

Thu Dec 10 2015, 05:17PM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

@ Albi: No, it isn't (to your 1st paragraph). Also the .pdf you point to is not what I am speaking of--as I plainly said, it was a small brick supply with about 12VDC (10-15) input. In fact, I am strictly talking about avoiding the mains (120V / 240VAC)

And, NO the trigger circuit shown in your reference is not part of the built in start-up surge circuit, this is the flashlamp (gas ionizing) EXTERNAL firing trigger--FOR SOLID STATE LASERS only--NOT gas lasers. It sounds like you don't seem to be familiar with lasers.

I think most people here know what I am speaking of when I talk about the initial surge, when charging a capacitor! Those unnecessary examples of initial current you gave serve what purpose here? Every one is simply ohm's law. i(0)=0--but then you state:
"It does not matter if the capacitor is initially charged or not" --this is why the i(0) is zero and the moment the inrush is maximum.

As for your humble opinion of the flybacks being best, do you mean as the transformer as a single part in a complete homemade PS? I am trying to avoid this effort--I can simply leave the resistor connected--I expected a reply from someone (of the many) that know much about these power blocks. I sound a bit harsh here but over many msgs from you, this is "best"!
Thanks for the reply...

Blackcurrant

Thu Dec 10 2015, 06:47PM

Registered Member #2989 Joined: Sun Jul 11 2010, 12:01AM
Location: UK
Posts: 94

Hi, I think you may need to supply a bit more information for anyone to be helpful as this is all guess work

Look up the data sheet on your HeNe power supply see if it says anything about short circuit protection, I would think most small power supplies will be current limited, however if your cap bank has bleeders etc the supply voltage may just droop. The supply itself may get a bit hot and go pop. Also I hope you are measuring the voltage across the cap bank you may find a lot more voltage after removing the 5W resistor.
I don't think the HV start up bit from your HeNe supply will do anything as it will be very low current.
Only thing I can think of is if when discharging your cap bank, if the bank goes negative would that damage the supplies some how?
Get the safety glasses on :)

johnf

Thu Dec 10 2015, 08:55PM

Registered Member #230 Joined: Tue Feb 21 2006, 08:01PM
Location: Gracefield lower Hutt
Posts: 284

I presume the resistor 5W is on the high voltage output.
Ie a ballast to limit dV/dt, dI/dt from harming the diode multiplier
Reduce the value at your own risk

DerAlbi

Thu Dec 10 2015, 09:05PM

Registered Member #2906 Joined: Sun Jun 06 2010, 02:20AM
Location: Dresden, Germany
Posts: 727

Sry for picking the wrong schematic..

But at least we know that i refered to the wrong schematic because i specified a link to it without any chance to missinterpretation.
I will learn from that. Next time i only refer to the site thats hosting the content somewhere, or ha! even better: i just refer to google

Then noone can tell me that i am wrong

Signification

Fri Dec 11 2015, 12:07AM

Registered Member #54278 Joined: Sat Jan 17 2015, 04:42AM
Location: Amite, La.
Posts: 367

OK. I researched further and found some things that I don't remember from before on Sam's--even about HeNe power supplies for capacitor charging! But not on the ballast--so far.
Also, I really don't think the ballast is going to be a problem since the PS won't be on continuously (as a capacitor charger) for days to years?!! continuously...
I also believe, after thinking further, that conversion to a cap charger will make the need for the ballast, less... maybe -not- even necessary at all! It's interesting that the initial surge *GENERATED* BY THE HENE PS JUST FOR THE HV SURGE IN THE HENE GAS TO STARTUP BY SUPPLYING THIS EXTRA ENERGY, RENDERING THE GAS CONDUCTIVE-BY-IONIZATION. IN ADDITION, THE PS-GENERATED SURGE WOULD BE TIMED JUST RIGHT (BUT NOT NECESSARY) FOR THE CAPACITOR'S INITIAL MAX CURRENT DEMAND.

Here are the links:

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