How best to drop ~180V?

"rjtucke"

** 6 x 5 watt zeners in series - 28 volts each.

Mounted apart on a tag strip for heatsinking and ventilation.

BTW 520 - 350 = 170

....... Phil

Is the 520 volts from a transformer that also powers filaments?

Does the transformer that produces the 520 volts have any primary taps?

Does the filter include a choke?

A switching regulator (buck converter) would do the job without heating your house in the process.

Jon

Phil suggested a string of zener diodes in series. But dropping a fixed voltage exacerbates poor voltage regulation by keeping all of your source variations, compared to your lower output voltage.

John asked if the transformer had taps, or other secondaries that you could use to modify the ac before rectification. That's good.

I'll suggest you use a HV regulator with a feedback loop. It has to be mentioned you could make it using a large vacuum tube. :-)

I'll be conventional (I know you asked for a creative way, but...) and suggest a power MOSFET pass element. The FET should be rated for 600V and be able to safely handle 180V * 80mA = 15W with a hot heatsink, and much more power under short-circuit conditions.

Checking the DigiKey catalog for an inexpensive part, I spied the IRFPC40 at $3.30 (it's a lead-free version, but you can solder it).

(Yes, there are much more capable MOSFETs, but they cost a lot more too.)

The irfpc40 is rated for 150W max with a 25C heatsink, but lowering the maximum junction temperature to 125C, and assuming a 60C max heatsink temperature, the dissipation capability is reduced to (125-60)/(0.83+0.24+0.33) = 46 watts. The last two terms come from adding an insulator with heat-sink grease (the FET's case will be at +520V, whereas you should have the heatsink bolted to the chassis).

To insure operation at 520V 80mA, we'll pick a 150mA current limit. Passing 150mA while dropping 180V creates 27 watts of dissipation, nicely under our 46 watt limit. But during a short-circuit your regulator will have to limit the current to under 46/520 = 88mA. We'll choose a nice cool 60mA, which means we'll need a "foldback" current limiting circuit with a 2.5 ratio, which isn't very hard. We discuss foldback current-limit designs in AoE pages 316-317.

There are multiple safety margins in all the specs above, so your regulator should work well under abnormal conditions, like high ac-line voltage, etc., or other mistreatment. Because you're regulating down from a voltage considerably higher than the output, your design can benefit from two simplifications: You can use an N-type FET source follower, for easy feedback-loop compensation, and you can use a simple resistor pullup to drive the FET.

Over the years I've posted several suitable ASCII circuits here on s.e.d., but Googling for a few minutes this morning didn't reveal one, and I have to leave for work now. But if you're interested in this approach I'll look further later on, or type up a new one.

--
 Thanks,
    - Win

An SCR based phase controlled switch (light dimmer) in place of the existing rectifier could do it.

The power you need is within the practial range for using a linear regulator. You will need a good heat sink on the pass element. Chances are, a simple zener diode + power MOSFET design would be good enough. The

350V zener would be a string of lower voltage ones. You will likely want to make it have foldback current limiting.

If you are making a bucking switcher, consider adding a small power supply that flys up and down on the MOSFETs source. This way, you can run all the electronics of the switcher on that. If you do this with a small mains transformer and bridge rectifier, remember that there will be lots of capacitance to couple RF noise through to the mains input.

--
--
kensmith@rahul.net   forging knowledge

I have a tube power supply that is putting out 520VDC. I need 350VDC (at ~80mA) maximum. I am trying to think of a creative way to drop the voltage.

(snip massive overkill solution)

** You haven't got a f****ng clue what the app is have you - Win ?

Never stops you from make a whole string a unjustified assumptions.

Ever heard of "Occam's Razor" ??

........ Phil

Who hasn't? Its just too bad that it doesn't have a sharp enough edge for you to slit your sorry throat and spare the world all your foul mouthed rantings.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.

"Michael A. Terrell"

** This asshole is a criminal psychopath.

........ Phil

As usual, you have absolutely nothing worthwhile to add, so you throw in one of your tired old cliché replies.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.

"Michael A. Terrell"

** This stinking asshole is a criminal psychopath.

An autistic and a schizophrenic.

........ Phil

The most efficient way would probably be a PWM circuit, but it might be too complex for a simple application. An SCR or triac phase firing circuit on the transformer would also be efficient.

A resistor and zener is the simplest, but 80 mA x 180 volts is 14 watts in the series resistor. The zeners will dissipate about twice that with open circuit, until the load draws power. OK if you have enough power and space and no heat problems.

A creative low power solution might be to use a voltage doubler using a capacitor (C1) on the transformer output, two diodes, and a final filter capacitor (C2), shunted by a 350 volt zener stack. You can choose the capacitor C1 so it can supply just about 80 mA into the zeners, and when the load is connected, it will just barely drop the zeners out of regulation, and very little power will be dissipated in the regulator. However, the transformer RMS current may be greater than 80 mA, so it may get hot.

Paul E. Schoen

Yes, Phillys, we've all seen your medical records, so you don't have to remind us every day. We know that you're the descendant of the early Australian criminals, and the results of so much inbreeding that you can't function in any normal social setting. Now be a good little girl and turn the damn computer off before your boyfriend comes home drunk and beats the living shit out of you, again.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.

"Michael Asshole Terrell"

( snip psycho abuse)

** This Terrell asshole is a criminal psychopath.

An autistic cretin & a paranoid aggressive.

Paranoid schizophrenia is incurable.

........ Phil

SNIP

Give up, you useless little used vibrator tester, you are the dumbest piece of garbage on usenet, but the frog is gaining on you. He'll soon outrank up on the Usenet dipshit scale. I'm amazed that you can cut and paste your crap while testing a couple vibrators at the same time.

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.

I did this once for 24V 0-40A; it was a battery charger. I used a 68HC11 to monitor the output voltage and current and time the SCRs firing. The SCRs were on the secondary, of course. But I don't think I personally would try it with a 520V transformer, but that doesn't mean it isn't doable - I'm just chicken. ;-)

Good Luck! Rich

" Michael Asshole Terrell"

( snip more psycho abuse)

** This Terrell asshole is a criminal psychopath.

An autistic cretin & a paranoid aggressive.

Paranoid schizophrenics are incurable.

...... Phil

--
Service to my country? Been there, Done that, and I\'ve got my DD214 to
prove it.

On Thu, 13 Apr 2006 10:07:23 +1000, "Phil Allison" Gave us:

Why do you insist on posting stalker like, off topic, retarded baby bullshit to this technical group, you stupid fucktard?

"Roy L. Fuckwit"

** A criminal psychopath called Terrell is stalking me

- you FUCKING MORON.

........ Phil