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Shunt Resistors

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angelo:
Hello all,

I'm the poster who started the DIY power supply topic and I figured that this was different enough to warrant its own topic.

It was suggested that I use a current shunt to alleviate some of the power that would otherwise fry the pot connected to the LM317 in current mode.

I understand that if I put say a 1ohm resistor in parallel with the pot then the 1ohm will take most of the current, but how then, can I adjust the current between 1-5000 ohms? The total resistance would be 1/Rt = 1/R1+...+1/Rn  and will only ever approach the lowest valued resistor (hence the 1 ohm) and not the 5000 needed for the 317's range to be met.

how can I use a pot to control the 317's output current without running the full current through the pot ?

given that I'd run say 20v at max 1.5A.

Simon:
I think you "shunt" is the LM317 isn't it ?

alm:
I believe I posted this in the other topic, but I'll repeat it here again. One of the sample application circuits in either the National or OnSemi LM317 datasheet does exactly what you want. I just double checked it, and they even have a complete lab supply example which regulates all the way to 0V. It doesn't get any easier than this.

angelo:
I must have missed your earlier post alm, sorry, however this design will require an LM301 op-amp, a high power transistor, a few tantalum caps and means I'll have to supply a negative voltage to the op-amp.

I'm looking more-so a the "precision current limiter" but am worried about blowing something by drawing too much current.

what setup can I use to ensure that I'm not going to draw more than 1.5 A, but have it adjustable from a few mA to 1A or so

alm:
What are you talking about? I think it contained an LM317, a low-resistance shunt resistor, a pot and a low-power MOSFET (plus the standard caps and resistors)? You need the negative voltage to get all the way to 0A, just like you need a negative voltage for the voltage regulator to regulate all the way to 0V, it could be skipped if you can live with 1.25V over the sense resistance as minimum current.