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Tool Talk Discussion Forum

what size resistor

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dieseldaawg

06-26-2006 21:08:41




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what size resistor do i need to drop 12-13vdc down to 9vdc with 210ma draw. Trying to setup a cordless phone in a cabin with no constant 110 ac power available. It does have 12 vdc solar and batterys.




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souNdguy

06-28-2006 11:24:19




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 Re: what size resistor in reply to dieseldaawg, 06-26-2006 21:08:41  
I'd look for a 78XX series regulator.. Not sure what sizes ratschack still has.. I've seen them with 7805 and 7812.. etc. ( if they have a 7810 or 7809 you are set. i don't have my xref book with me.. nor do i htink they still carry half the discreet components they used to ).

Easy wire up.. power in.. ground.. power out.

You can bias the ground if you need a slightly higher output voltage than the regulator is calibrated for.

You can also get a variable vreg in a dip package that will require a few extra support components.. and then you can custom tailor your circuit voltage.

Also not to hard to make a low-I series pass regulator with a generic npn device.

Could also make a single oscilator PWM using a mosfet.. I think rs carries the irf 511 or a z40 mosfet.. Filter the output of the pwm with a suitable electrolytic, and the load should see steady state votlage... Calibrate for desired voltage.. ( this circuit makes a great low cost motor controller... )...

Soundguy

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john *.?-!.* cub owner

06-27-2006 19:50:04




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 Re: what size resistor in reply to dieseldaawg, 06-26-2006 21:08:41  
Radio Shack has what you need.



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jdemaris

06-27-2006 09:36:34




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 Re: what size resistor in reply to dieseldaawg, 06-26-2006 21:08:41  
I know of many people using solar-electric via a 12VDC battery-bank system to directly run their cordless phones - that were originally intended to run at 9 to 14 VDC. They all have worked fine at 12VDC. The slight difference in voltage has not been a issue, but conflicting grounds has. Depending on your ground system - you might get static on the phone and need a DC ground-isolator made for this situation. Cost around $30. If your ground system is somehow tied or close to the the phone company ground - with perhaps a twin-rod system near the phone companie's ground rod - then all might be fine as is.

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RandyBee

06-27-2006 06:15:33




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 Re: what size resistor in reply to dieseldaawg, 06-26-2006 21:08:41  
The information given is correct. If you do not want to assemble your own regulator, then look for one of the commercial regulators as shown in the website below.



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MarkB_MI

06-27-2006 03:48:30




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 Re: what size resistor in reply to dieseldaawg, 06-26-2006 21:08:41  
Guaranteed that your current draw is not a constant 210 mA. Trying to do this with a resistor is likely to fry your phone. As Mr. B&D sez, you need a voltage regulator. Radio Shack (last time I checked) sells these components, probably with a schematic to wire them up.



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buickanddeere

06-26-2006 22:00:31




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 Re: what size resistor in reply to dieseldaawg, 06-26-2006 21:08:41  
15 ohm. Rather than a resisitor you will be happer with a simple three pin regulator. No matter if the input voltage is 10V or 18V the batteries will only see 9V. How many cells in the phone are being charged. The 9V wallwart DC power supply may supplying an internal regulator cicuit to provide 4.2V to a three cell nicad battery pack.



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