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Old 26th January 2011, 09:12 PM   #1
superR is offline superR  Netherlands
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Default Bringing down the voltage by 10 Volts

I have a problem. I can get cheap 2*30V torroids, but the output voltage is too high for my purpose, supplying a LM4780. I would like to get the voltage within the specs of the LM4780, say, between 30 en 35 Volts. What is the best way to do this and what is the easiest/cheapest way to do this?

LM317HV with transistors
5reg.gif (image)
LM338HV
http://freecircuitdiagram.net/wp-con...sing-LM338.gif
Switching bulk converters
LM2576HV - SIMPLE SWITCHER 3A Step-Down Voltage Regulator

Or, the trick I have yet to discover. What is my best option?
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Old 26th January 2011, 09:24 PM   #2
Boofers is offline Boofers  Canada
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The easiest way to drop 10V is probably just to put 16 diodes in series
Check the datasheet for the LM4780 - looks like you would be fine with +/-40V rails.
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Old 26th January 2011, 11:19 PM   #3
Minion is offline Minion  Canada
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I have heard that you can take a few turns off of the transformer to lower the voltage but I haven"t ever done it myself .....
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Old 27th January 2011, 06:15 AM   #4
superR is offline superR  Netherlands
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The transformer has a solid plastic core so I can't unwind the secondaries. Furthermore, I have too much pride for using a diode-stack to lower the voltage. I am interested in using a switching step-down solution, but I am a bit scared of the low switching frequency of the chip. (50kHz). I have found a lower ampere alternative with a 150kHz switching frequency. I might try that one, unless someone has a better idea
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Old 27th January 2011, 07:09 AM   #5
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Quote:
Originally Posted by superR View Post
I have a problem. I can get cheap 2*30V torroids, but the output voltage is too high for my purpose, supplying a LM4780. I would like to get the voltage within the specs of the LM4780, say, between 30 en 35 Volts. What is the best way to do this and what is the easiest/cheapest way to do this?

LM317HV with transistors
5reg.gif (image)
LM338HV
http://freecircuitdiagram.net/wp-con...sing-LM338.gif
Switching bulk converters
LM2576HV - SIMPLE SWITCHER 3A Step-Down Voltage Regulator

Or, the trick I have yet to discover. What is my best option?
The easiest and best way is to buy the correct transformer in the first place. It will probably be cheaper than building a SMPS and will give better results.
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Old 27th January 2011, 07:11 AM   #6
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Or just build Symasyms instead of the LM4780s.
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Old 27th January 2011, 11:41 AM   #7
superR is offline superR  Netherlands
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You know I am the guy who just built your LM4780 SS pcb right? I want to make a small amp with the lm4780. The reason I would like to use a 2*30V torroid is because of the fact they only cost 10 euro's / 13 dollars for 2*30V 120VA. A 2*18, 2*20 or 2*25V torroid with similar rating costs around 3 or 4 times as much I am interested in using the cheap one and altering the voltage.

4 pieces of lm317 and 4 pieces of lm337 only cost 3,5 euro's, 4,5 dollars in total
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Old 27th January 2011, 12:54 PM   #8
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If you can live with 35V a pair of mosfet cap multipliers might be the best way.

30V X 1.41 = 42V but you lose ~2v across the bridge rectifier so you have 40V
and a pair of cap multipliers using mosfets will lose about 5V giving you 35V
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Old 27th January 2011, 01:37 PM   #9
superR is offline superR  Netherlands
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I have been googling, but I can't seem to find a good example of a symmetric "cap multiplier using mosfet"-based psu. Can you tell me where to look for more information?
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Old 27th January 2011, 01:50 PM   #10
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2 caps, 2 resistors, 2-30V zeners, 2 power transistors, one heatsink - simple regulator.
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