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I've got a microcontroller taling to a DAC MAX520ACPE over i2c. This DAC gives me 0V to 5V with 8bit presision. I neev to control 0Vto 8V. I think I can use a LM317 to regulate the output voltage but I don't know how to connect the output of the MAX520ACPE to the LM317.

Output:

enter image description here

Power:

enter image description here

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  • \$\begingroup\$ How much current must the 0-8V output supply, and what will the programmable voltages be used for? \$\endgroup\$ – Bruce Abbott Dec 31 '16 at 19:21
  • \$\begingroup\$ The 0V-8V power will run some small Z scale trains. Each output would need no more than 400mA. I'll add a picture for what I think my power input is. \$\endgroup\$ – Justin808 Dec 31 '16 at 19:25
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    \$\begingroup\$ This is typically done with an H-bridge IC and pulse width modulation, not a DAC and linear (analog) power amplifier. For tiny motors in a Z scale train you probably want a PWM frequency in the KHz range (you can always filter it, too). Overall, it's a case where looking at what people in your field of use are already doing (include N gauge and even HO solution in your search) will likely be more productive than trying to invent something from first principles. \$\endgroup\$ – Chris Stratton Dec 31 '16 at 19:30
  • \$\begingroup\$ The LM317 requires about 3 volts "headroom" for operation - that is, the input voltage must be about 3 volts above the desired output voltage. \$\endgroup\$ – Peter Bennett Dec 31 '16 at 19:32
  • \$\begingroup\$ "Z scale trains" - that explains the 'railx' outputs! \$\endgroup\$ – Bruce Abbott Dec 31 '16 at 20:29
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To get 0-8V from 0-5V you must amplify the DAC output voltage by a factor of 8/5 = 1.6. However the LM317 regulates its output voltage to 1.25V above the ADJ pin, so you will also have to bias it with -1.25V. The circuit below should do the job:-

schematic

simulate this circuit – Schematic created using CircuitLab

R3 and R4 set the non-inverting voltage gain of OA1 to (R4+R3)/R3 = 1.6. R1 and R2 provide a bias voltage of 2.083V to the inverting input. The formula for Inverting gain is -(R4/R3), so the bias voltage output is 2.083*-(60k/100k) = -1.25V. V3 provides the negative supply voltage that the op amp needs to pull its output below ground.

The LM324 has 4 op amps in it so you only need one IC to do 4 channels. The negative supply voltage is not critical (just has to be at least -2.2V) and only a few milliamps is required, so you could generate it from the +5V supply using eg. an ICL7660 or LM555.

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