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LR8 High Voltage Regulator from alternator source
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Author:  b0ba [ 20 Nov 2016, 12:39 ]
Post subject:  LR8 High Voltage Regulator from alternator source

Hi Everyone!

Sorry for the offtopic here, but I have very typical ( from my understanding ) schematic with LR8 regulator. I burned 3 of them without any reason. I am using this design as PSU for PIC microcontroller, I need to regulate voltage came from moped alternator ( 0-200 Acv ) to 5v. Expected max current is 250ma. Could you please advise what can be the reason, why LR8 is not working properly. For the tests, instead of alternator, I was using UPS with AC regulator. Input voltage was 50v Ac.

Could you please advise what can be the reason, why LR8 is not working properly ? Thanks in advance for any suggestions.

Regards,
Vlad.

Image

Author:  Geek [ 21 Nov 2016, 00:39 ]
Post subject:  Re: LR8 High Voltage Regulator

Hi,

The mods may split this into it's own topic. Who knows?

Why is your alternator kicking out 200V? Is the field regulator shorted?

Your circuit is failing due to SOA (Safe Operating Area) oopsies.

With such a wide input voltage, if you want to stay linear instead of switching, a pre-regulator is the way most all commercial builds do it with good reason - less stress on the regulator.

An alternator isn't 60Hz. It's frequency varies with the speed of your moped. A "1N" series of diode is a poor choice. The UF series is a better one because it's fast recovery, inexpensive and easily obtained.

The MOSFET is doing double duty as a pre-regulator and a power filter. Because of the large frequency variance and the inefficient half-wave rectification, you need a larger input capacitor and one with low ESR. Given the automotive application, I'd use a Nichicon LGR or LGX, long-life or the like.

The MOSFET shown has a Vgs of 4V. So for an 13V zener, you'll get 9V out. That's a 4V difference across the 7805 at 1/4 amp = 1 watt. Just a little set of "wings" on the 7805 will do, while the MOSFET will need more.

200V - 9V = 191V * 0.25A = just under 48 watts!

So, unless you're thinking of a large box, I'd think more of switchmode.

Author:  b0ba [ 21 Nov 2016, 05:34 ]
Post subject:  Re: LR8 High Voltage Regulator

Geek wrote:

Why is your alternator kicking out 200V? Is the field regulator shorted?
An alternator isn't 60Hz. It's frequency varies with the speed of your moped. A "1N" series of diode is a poor choice. The UF series is a better one because it's fast recovery, inexpensive and easily obtained.


Thanks a lot for the answer.

Alternator voltage is 0 - 200 Vac and 20-150 Hz frequency varies with the speed of engine. Main purpose of voltage from Alternator is to power AC-CDI ignition. I just use PIC to time delay. Oldtimer mopeds has no 12V battery and I have no other way how to get power for it from the same coil. I see right now, big mistake in this design, if LR8 is fail, PIC controller will burn too. How I wrote, for the test purpose I am using stable source of AC 50V, with load 1K resistor. Expected current is very low. Why LR8 was burned in such kind of conditions, that is riddle for me. Do you think absolutely impossible to use LR8 for such kind of purpose with some mods of presented schematic ?

Author:  Geek [ 21 Nov 2016, 06:10 ]
Post subject:  Re: LR8 High Voltage Regulator

Hi,

If you insist on an LR8, this is what I use successfully in tube circuits. But really, an LR8 is *not* a LV-output performer.

A little TO-92 sink on the LR8 keeps the voltage stable once warm - it's a pretty good "thermistor" and needs the bulk of the sink not for dissipation, but thermal inertia.

The PNP wraparound keeps the LR8's error amplifier in the loop - an NPN as you have it makes it clueless as to what's going on the load side.

Small cap on the output. Your large cap is most likely what cooked it (the pass transistor would have demanded base current to charge the cap that probably exceeded the LR8's ratings. They aren't short-circuit proof).

Cheers!

Author:  b0ba [ 21 Nov 2016, 07:03 ]
Post subject:  Re: LR8 High Voltage Regulator

Hi,

Thanks a lot for your help, I will reduce cap to 1uF and let you know about result. About NPN. For me the same if not very clear how this typical LR8 design with NPN key is working and what's going on the load side. I have found other example, where author was using the same, but just with two NPNs. Post from stefa38 :
http://www.costruirehifi.net/forum/view ... 9&start=75
Internet full of such kind of solutions, based on LR8.

Author:  b0ba [ 13 Jan 2017, 05:15 ]
Post subject:  Re: LR8 High Voltage Regulator from alternator source

Big cap was a reason of issue. Reduced to 1uF solved my problem. Thanks a lot for your help !

Author:  jigneshfc [ 08 Jul 2018, 13:47 ]
Post subject:  Re: LR8 High Voltage Regulator from alternator source

hi,
is your 230V Ac to 5V Dc power supply circuit using LR8 is working ok ?
i am asking because i am as well looking for 230v ac to 5V dc voltage converter for micro controller project .
any suggestion then please suggest.

Author:  gofar99 [ 09 Jul 2018, 18:22 ]
Post subject:  Re: LR8 High Voltage Regulator from alternator source

Hi, I use LR8 regulators all the time...great when used properly. My sense regarding the last post 230 ac to 5 vdc is that using an LR8 is the hard and inefficient way to do it. use a small filament transformer and then something like a LM7805. A simple bridge rectifier and filter caps on both sides of the LM7805 and you have clean power. A whole lot less dangerous if there is a failure. Stuffing 230 vac into something that uses 5 vdc will not be pretty.

In the first postings I might consider an audio transformer as a step down device and use it in a similar manner. Also many filament transformers will work up to 400HZ and would be suitable. Why do things the hard and IMO less reliable way when simple works great?

Good listening
Bruce

Author:  jigneshfc [ 10 Jul 2018, 02:58 ]
Post subject:  Re: LR8 High Voltage Regulator from alternator source

yes with transformer it is great but i have space issue i mean there is no space in enclosure. so looking for compact without transformer circuit solution.

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