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Forum Index : Electronics : resistor vallue

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irishron40
Senior Member

Joined: 22/09/2014
Location: Ireland
Posts: 254
Posted: 04:10pm 19 May 2015
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Hello.

in this power supply it suggests R1 to be 2k2 2,5 Watt

http://electronics-diy.com/power-supply-lm317-2n3055-0-28v-6-8a.php

I dont have it, but I do have a 2k 5 watt cement resistor

would that be ok to use? or does it need to be 2k2

thank you

ron
 
BobD

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Joined: 07/12/2011
Location: Australia
Posts: 935
Posted: 04:16pm 19 May 2015
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Ron
That looks OK. It looks like it is there to drain the capacitor C2 when the power is shut off.
Bob
Edited by BobD 2015-05-21
 
Pete Locke
Senior Member

Joined: 26/06/2013
Location: New Zealand
Posts: 182
Posted: 11:04pm 19 May 2015
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Bob is right. But given it's the low voltage side of the power supply you can leave it out. Just a discharge resistor. You could put anything above that ohm value to do the job. The wattage needed will decrease as the ohm value goes up. Simple equation is 'Watts (of resistor) = v squared over resistor value.
All good fun :-)
 
Pete Locke
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Joined: 26/06/2013
Location: New Zealand
Posts: 182
Posted: 11:13pm 19 May 2015
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This is a very good link to basic Ohms law...
http://commons.wikimedia.org/wiki/File:Ohm's_Law_Pie_chart.svg
 
irishron40
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Joined: 22/09/2014
Location: Ireland
Posts: 254
Posted: 11:13pm 19 May 2015
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so i could even put in a 20k 1/4 watt?


thanks

ron
 
BobD

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Joined: 07/12/2011
Location: Australia
Posts: 935
Posted: 11:35pm 19 May 2015
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Ron
Yes, you could. It all depends on how quick you want the stored electricity in the capacitors to bleed away.
Pete
I loved that diagram so much that here it is. I know the basics but to get the other I usually manipulate formulae. No need now.
Bob
2015-05-20_093343_OHMS_LAW.pdf
 
irishron40
Senior Member

Joined: 22/09/2014
Location: Ireland
Posts: 254
Posted: 12:12am 20 May 2015
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thank you both.

ron
 
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