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Old 27th July 2005, 06:28 AM   #1
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Default Reduce 24v DC to 18v DC

Hi,

I am new to electronics and I would appreciate your assistance, I have a Lambda LNS-Z-24-J power supply and I need to reduce the voltage to +/- 18-20v I called tech support at Lamdba and they suggested a voltage divider or regulator, as I have no knowledge can anyone provide me with instructions on how to build this item.

Thank you.
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Old 27th July 2005, 06:40 AM   #2
boholm is offline boholm  Denmark
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First we need to know, what the 18 volts is for.

U see, it depends on the demands of current: High, low, steady or variable?
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Old 27th July 2005, 03:24 PM   #3
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Good point...

I am building an FM Stereo Exciter which includes the Transmitter (13.3 -16Vdc 300mA max) a Limiter (15Vdc 300mA) and a Stereo Encoder (13-15Vdc 300mA)

These 3 boards are combined on one PCB and the manufacturer recommends a 18-20VDC 700mA regulated Power Supply.

Hope this helps.


Thank you

Chris
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Old 27th July 2005, 03:51 PM   #4
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hi finestocean,

These are very simple very common circuits that will achieve 18V +/- like you need.
The upper is using voltage regulators, ignore the part codes as they arent 18V refulators.
You will have to find +18v and -18V regulator I.C' part numbers yourself as i dont know any off my head. You may have to even get a variable output regulator. If you use regulators make sure you buy ones that will handle 1 amp or more.

But it is easier to use circuit 2 really, the diodes i used are 18V zeners and are very cheap, you can substitute the mfor and other 18V zener.
Make sure they can handle at least 1 amp of current through them to be safe.
This circuit will provide +and - 18V DC at up to just under 800mA per rail before the zeners start losing their regualtion, test it with a 24 ohm load to check it works for you.

Regards
Craig
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Old 27th July 2005, 04:12 PM   #5
boholm is offline boholm  Denmark
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I suggest Craigs upper suggestion. Then you do not have to think about the outer limits of components (here the zeners).

Remember to have those 100 nF close to the chip.
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Old 27th July 2005, 06:26 PM   #6
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Thank you both very much...
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Old 27th July 2005, 06:47 PM   #7
macboy is offline macboy  Canada
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Quote:
But it is easier to use circuit 2 really, the diodes i used are 18V zeners and are very cheap, you can substitute the mfor and other 18V zener.
Make sure they can handle at least 1 amp of current through them to be safe.
This circuit will provide +and - 18V DC at up to just under 800mA per rail before the zeners start losing their regualtion, test it with a 24 ohm load to check it works for you.
A zener shunt regulator is really only useful for very low current supplies. Check out what happens if the load is disconnected: .79 A of current flows through the zener, causing over 14 W of power dissipation. Most zeners are rated at 1 W, many at just 1/2 W. Those zeners will release their magic smoke in a matter of seconds. And everyone knows that once you let the magic smoke out, you can't put it back in.
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Old 28th July 2005, 07:02 PM   #8
AndrewT is offline AndrewT  Scotland
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Hi Finest,
Re post 4 schematic. The ICs 7918 & 7818 have been transposed. 7818 is positive. Note also that the order the legs come out varies from pos to neg, take care to read the datasheet.
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Old 28th July 2005, 11:06 PM   #9
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If you decide to go with the 7818 and 7918 devices, do not forget to add heat sinks. Also, once installed in the case, make sure you have top and bottom vents. Having vents only on the top does not allow for the cool air to come into the case, and the hot air ends up trapped inside.
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Old 29th July 2005, 08:46 PM   #10
cpemma is offline cpemma  United Kingdom
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Quote:
Originally posted by finestocean
I am building an FM Stereo Exciter which includes the Transmitter (13.3 -16Vdc 300mA max) a Limiter (15Vdc 300mA) and a Stereo Encoder (13-15Vdc 300mA)

These 3 boards are combined on one PCB and the manufacturer recommends a 18-20VDC 700mA regulated Power Supply.
I'm slightly confused here as your original request was for a dual-rail -18V_0V_+18V supply which has been answered.

From the look of the amendment quoted, a single-rail regulated 15V supply is more in line with what you need, unless the pcbs have components to reduce the 18-20V recommended to the quoted levels.
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