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Old 29th February 2004, 12:50 PM   #1
w00t is offline w00t  Singapore
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Default LM3x7 HV regulating.

Hello,

I would like to know how to regulate a HV/HT line with the LM3x7 series. I konw I can do so by raising the ground with the use of a zener and have seen a few schematics employing this method, but I'm still not certain what the true generic schematic is like. Those I have seen seems to be 'upgraded' versions.

All versions would be much appreciated. Thanks!

Cheers,
w00t
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Old 29th February 2004, 06:19 PM   #2
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It's a bad idea because they only have a range of maybe 40V input. Thus your power supply needs good regulation anyway, because at no-load you have an absolute maximum of say 290V versus a minimum of 250V. A very tight range for anything. Not to mention a 10% variation in line voltage (which is within spec) will take up most of that range.

Better to use a purpose-made HV regulator, either discrete or integrated.

Tim
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Old 29th February 2004, 07:11 PM   #3
andy_c is offline andy_c  United States
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Default Re: LM3x7 HV regulating.

Quote:
Originally posted by w00t
I would like to know how to regulate a HV/HT line with the LM3x7 series.(...)
Hi w00t,

See the Linear Technology app note 2 http://www.linear.com/pdf/an2.pdf. Figure 11 and the text above it shows how to use the LT317 in a high-voltage configuration.
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Old 29th February 2004, 09:42 PM   #4
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Just to give an example :

The transistor takes care of the input voltage which otherwise is out of scale, read destroy the 317.
The voltage across the 317 is determined by the zener minus the Vbe of the transistor = approx. 6V.

I have used this many times and it works very well as long as the output is not shorted to ground.

Dick.
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