popin' caps

[Learner]

hey guys,
had bit of a drama today with my testing of the high voltage regulating circuit, it is based on the TL783 data sheet

http://www-s.ti.com/sc/ds/tl783.pdf

I couldn't get any TIPL762 since Farnell was out of stock, so they recommanded the philips BU508AW instead and that's what I got in the circuit above.

http://www.philipsmcu.com/acrobat/datasheets/BU508AW_2.pdf

Without thinking thoroughly I just went ahead and used a 450V 50uF electrolytic cap at the output as shown in the diagram above, and used a 1 meg pot for R2.

Being all excited ready to test the circuit, as soon as I pluged it in I saw very tiny amout of smoke begin to come out of the cap as I proceed to turn the pot more smoke came out. I was stunned wondering what the hell the problem was, so I unplugged the socket but it was too late the cap blew smoke and fumes just gushed out of the cap :shock:

I am just glad it didn't blow my head off or set the house on fire or something, went back and checked on the G9 schematic and I see 100u at the regulator output, in the LA2a it has 47uF in parallel at the DC output.

On the diagram above it only has 50u and the input is 125V, for the G9 and the la2a it is 100uF with output of around 250V, so it seems to me that I would require a 250uF cap (or 3 x 100u cap in parallel, which is what I will use) to be able to cover 500VDC output?

Also, if the value of the output cap at the regulator is bigger then it should be eg. 300uF would that actually adds more ripple to the DC output and therefore defeat the purpose of using the regulator?

I would also like to change R1 to 2.5k in order get 500VDC output

Vout = 1.25V(1 + R2/R1)
Vout = 1.25V(1 + 1.0025M/2.5k)= 502.5V

and placing a 2k resistor after the pot before the ground, any thoughts on that? PRR? Jakob?

I have used 1 W resistor for the whole circuit, does it make any difference if I use 1/2 W instead?

Also, do I need to change the values of the resistors that bias the BU508AW transistors?:?

I am definitely buying an industrial face mask before I will do any more circuit testing, I feel dizzy.......

 

[gyraf]

I don't see how capacitor value should influence on smoke emission?

I don't think the TL783 is any good for a 500V supply. Remember that it has a max. in/out voltage difference of 120V. The above circuit is running to it's max - any higher input voltage will probably destroy the regulator.

Also observe that you're shorting the output voltage with the r1/r2 divider. At 82R and 2k5 you dissipate some 75W here..

 

[NewYorkDave]

I'm not following everything you say in your message, but I have a feeling you either over-voltaged your cap, put it in backwards, or fed AC into the circuit instead of DC.

Wearing safety glasses when firing up a new circuit is always a smart idea. I've been doing this for years, and I even managed to burn up a 1/2W resistor the other day from making a wrong connection.

 

[Learner]

[quote author="gyraf"]I don't see how capacitor value should influence on smoke emission?

I don't think the TL783 is any good for a 500V supply. Remember that it has a max. in/out voltage difference of 120V. The above circuit is running to it's max - any higher input voltage will probably destroy the regulator. [/quote]

Right, looks like I would need to start again and may be look for transistor circuit regulation instead of the ic regulators? I just cant find ANY IC regulators that can output that kind of voltage, any suggestions?:sad:

Also observe that you're shorting the output voltage with the r1/r2 divider. At 82R and 2k5 you dissipate some 75W here..

you lost me there, 75W? :?

I find it hard to put what's learnt from books into real life application :sad:

 

[nwsoundman]

Wearing safety glasses when firing up a new circuit is always a smart idea.

That's for sure!!! I gave myself a black eye with a small electrolytic. It launched off the PCB hit my eye and landed on another techs bench across from me. The cap was dented by my face. Way too close that time.

NWSM

 

[Learner]

[quote author="NewYorkDave"]I'm not following everything you say in your message, but I have a feeling you either over-voltaged your cap, put it in backwards, or fed AC into the circuit instead of DC.
[/quote]

could very much well be the first one, since I double and triple checked the placement of the cap and the voltage comes after the diode bridge......

but I am now scared to mess with it now till may be when I find a new solution, since I don't think there is any point to pursuit this circuit as a 500V bench PSU.....

And I have to wait for my safety mask to arrive!!!!

 

[toby]

don't be a girl :grin: , tiny amount of smoke?

i once wired in a big cap the wrong way round after a long day of point to point stuff, like a disco smoke machine when i turned it on, or there was the time i accidently didn't check the filament contiuaty on a tube circuit and turned on a 75watt psu..........................

Toby

 

[gyraf]

For a 500V bench supply, I wouldn't use any regulation, just some good reservoir caps. You'll probably only use 500V for power output stages anyway.

For normal preamp HT voltages around 250V, use the G9 PSU - that works very well

For safety, be sure to put in meters for voltage read-out as well as a "discharge" switch.

 

[Steve Jones]

I put a new element in my $800.00 Denon de-soldering gun today, not realizing that there was a shorted bridge rectifier in there as well. I joked to the other tech that when I threw the switch that either it would work, or the shop would go dark. Well it went POW! and scared the s..t out of me, covered my hand in soot that spurted out the ventilation ports, and threw the breakers. Oddly enough this was about the time today that I spoke to Learner on the phone and he told me about his Electrolytic Adventure.

 

[Learner]

[quote author="toby"]don't be a girl :grin:
[/quote]

Oh, its not that. I just need to take care of my valueble assests for all my ladies :shock:

special shout out to Hoochi, Booby and Looney! :green:

 

[Learner]

[quote author="gyraf"]For a 500V bench supply, I wouldn't use any regulation, just some good reservoir caps. You'll probably only use 500V for power output stages anyway.[/quote]

Thanks for the advise Jakob, you da man!!!!:thumb:

I have been looking at the cockcroft-walton voltage multiplier but I can't seem to get it to work as expected in Pspice, anyway the thought of finding an oscillator circuit that will sink 500mA of current and calculating to replace the components for higher frequency just causes more trouble than I need at the moment. :roll:

so I guess the easiest way is like you've mentioned above, rectify the AC with no regulation then use voltage divider to drop the voltage with a rotary switch and may be add a potentiometer for fine trim or something.........

Or find something cheap on ebay again! risking the fact that it might not be work by the time I get it, after paying big $$$ for the heavy shipping.... :roll:

For normal preamp HT voltages around 250V, use the G9 PSU - that works very well

Yea, that's what I am going to do with the lala2.
:green: :thumb: :guinness:

 

[Learner]

[quote author="Steve Jones"]I put a new element in my $800.00 Denon de-soldering gun today, not realizing that there was a shorted bridge rectifier in there as well. I joked to the other tech that when I threw the switch that either it would work, or the shop would go dark. Well it went POW! and scared the s..t out of me, covered my hand in soot that spurted out the ventilation ports, and threw the breakers. Oddly enough this was about the time today that I spoke to Learner on the phone and he told me about his Electrolytic Adventure.[/quote]

Yea, must be fireworx week @ down under........... :shock:

BTW steve, any chance of sharing some pics of your custom mic pres and may be what's under the hood? :green: