10a Voltage Regulator Circuit - Resistor Question - diyAudio
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Old 4th November 2013, 05:03 PM   #1
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Default 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)
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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?
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Old 4th November 2013, 05:07 PM   #2
Mooly is offline Mooly  United Kingdom
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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.
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Old 4th November 2013, 05:29 PM   #3
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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...
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Old 4th November 2013, 06:11 PM   #4
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Hi,
Yes, Mouser have it. Check it out 71-LVR05R1000FE12. 0.1 ohm 5 watts
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Old 4th November 2013, 06:19 PM   #5
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by kballgame View Post
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.
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Old 4th November 2013, 07:40 PM   #6
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Thanks Tauro and Mooly.
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Old 5th November 2013, 06:41 AM   #7
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by kballgame View Post
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)
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Old 5th November 2013, 06:56 AM   #8
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You obviously didn't search with the correct criteria.

0.1 resistor Resistors | Mouser
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Old 5th November 2013, 11:03 AM   #9
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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.
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Old 5th November 2013, 11:31 AM   #10
DUG is offline DUG  Canada
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I would connect the op-amp +power (pin 7) to Vin.

Not required with the TL081 but I just would.

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