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I am trying to use a Fairchild LM350 to get 8 volt output. I am using basic circuit (page 5 of datasheet) to do that. I get the voltage I want but whenever I try to measure the current (by putting the meter in series with a resistor) it gives me 0 A. I am giving it 8 volt and the resistor is 650kΩ. I tried smaller resistor also but still nothing. Why is this happening?

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  • \$\begingroup\$ Check the fuse of you multimeter. \$\endgroup\$ – Martin Petrei Oct 2 '14 at 16:13
  • \$\begingroup\$ A quick test would be to put an LED and something like a 600 Ohm resistor in series. If the LED lights up, you're getting current and your multimeter is bad (probably a blown fuse, as Martin Petrei said). \$\endgroup\$ – Dan Laks Oct 2 '14 at 16:27
  • \$\begingroup\$ To add to Dan Laks comment, put an LED with a series resistor in. If it lights, put your meter in series too. If the LED stays on, and the meter reads 0, then tell us, and post a link to your meter. \$\endgroup\$ – gbulmer Oct 2 '14 at 16:36
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There are a few possibilities:

  • The resistor is not pulling enough current for your meter to measure. 8V across 650kΩ is 12.3μA, a fairly small amount. Some meters may not be able to measure this small current.

  • You may have forgotten to move the meter's test probes to the appropriate ports on your meter. Some meters require you to move one probe to another port to measure current, and often there may be more than one port: one for higher currents and one for lower currents.

  • You might be using a range that is too large, for example a 10A range, when a 200mA range would be more appropriate. (Varies by meter.)

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A voltage of 8V tied to a load of 650kΩ means you're trying to measure 12uA. It's entirely likely that your meter can't measure less than 1mA accurately. Which multimeter are you using? Things to try in the meantime:

  1. Try 800Ω instead of 650kΩ for load and see if your meter picks up 10mA.
  2. With the meter in series with the 650kΩ load use a 2nd multimeter if available to ohm out the first meter. If it reads 0Ω the fuse is not blown
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Probably because you're connecting 8 volts to the input through a 650k resistor and expecting something to happen.

In the first place, if you expect to get 8 volts out of the output you'll need to connect greater than 8 volts to the input, with no series resistor between the source and the regulator. How much greater depends on your load current; read the data sheet.

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So the problem was what Martin Petrei, Dan Laks and gbulmer suggested. My fuse was bad. I tried the LED and it was working. Changed fuse and now I can read current just fine :)

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