diyAudio

diyAudio (http://www.diyaudio.com/forums/)
-   Power Supplies (http://www.diyaudio.com/forums/power-supplies/)
-   -   10a Voltage Regulator Circuit - Resistor Question (http://www.diyaudio.com/forums/power-supplies/245286-10a-voltage-regulator-circuit-resistor-question.html)

kballgame 4th November 2013 05:03 PM

10a Voltage Regulator Circuit - Resistor Question
 
Hello and thanks in advance for your help. I am fairly new to circuits, so please bear with me.

I am building this circuit (subst. the lm318 for the obsolete lm107)
https://lh4.googleusercontent.com/-8...adjvoltreg.jpg

Questions:
1) while .1 Ohm resistors exist, they seem rare (not available at Jameco or Mouser); do they really mean 0.1 ohm resistors for R1 and R2? Which leads me to...
2) what, exactly, is the purpose of these resistors in this circuit. Can I change the value and, if so, what would you suggest?

Mooly 4th November 2013 05:07 PM

0.1 ohm is actually a common value :) Many amplifiers use those as emitter resistors in outputs stages.

What do they do... well it looks like they allow the opamp to respond to differences in current between the two regs and thus be able to alter the drive to one to allow perfect current sharing.

The resistors have to be low value to minimise power losses. 10amps would drop 1 volt across 0.1 ohms.

kballgame 4th November 2013 05:29 PM

Mooly - thank you, that makes sense.

If I were to use the .1 resistors at full draw that means they are dissipating 10 watts together and, therefore, need a 5W rating p/each. I am calculating this right, correct?

I have found a .1 Ohm 5 watt resistor at Parts Express but I find it strange they aren't carried at major suppliers like Mouser and Jameco? Perhaps I am just looking incorrectly...

tauro0221 4th November 2013 06:11 PM

Hi,
Yes, Mouser have it. Check it out 71-LVR05R1000FE12. 0.1 ohm 5 watts

Mooly 4th November 2013 06:19 PM

Quote:

Originally Posted by kballgame (Post 3691182)
Mooly - thank you, that makes sense.

If I were to use the .1 resistors at full draw that means they are dissipating 10 watts together and, therefore, need a 5W rating p/each. I am calculating this right, correct?

I have found a .1 Ohm 5 watt resistor at Parts Express but I find it strange they aren't carried at major suppliers like Mouser and Jameco? Perhaps I am just looking incorrectly...

5 amps in each. Power dissipated is I squared *R so that's 25*0.1 which is 2.5 watts. So even a 3 watt resistor should be OK if used correctly but a 5 is a safer bet.

kballgame 4th November 2013 07:40 PM

Thanks Tauro and Mooly.

Elvee 5th November 2013 06:41 AM

Quote:

Originally Posted by kballgame (Post 3691158)
I am building this circuit (subst. the lm318 for the obsolete lm107)

The 318 will probably not work: you need an amplifier that includes Vcc in its common mode range, like a rail to rail opamp for example, or a TLO81 that also does, but unofficially (it works though)

KatieandDad 5th November 2013 06:56 AM

You obviously didn't search with the correct criteria.

0.1 resistor Resistors | Mouser

Arty 5th November 2013 11:03 AM

or You can paralell some resistors to achieve 0.1 ohms.
neat thing to do in situations where dissipation would be high.
as the current would split between them.

DUG 5th November 2013 11:31 AM

I would connect the op-amp +power (pin 7) to Vin.

Not required with the TL081 but I just would.

:)


All times are GMT. The time now is 11:19 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio


Content Relevant URLs by vBSEO 3.3.2