Author Topic: LM317 digital control  (Read 26112 times)

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Offline spec

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Re: LM317 digital control
« Reply #50 on: February 04, 2019, 12:58:15 pm »
Below is yet another schematic for a DAC controlled LM317 PSU which is scaled for 0V to 3V input to 0V to 30V output. Although the output goes down to 0V, a separate negative power supply is not required.

The scaling can easily be changed according to the formula Vout = Vin * (1+ R4/R8)

The three diodes must be 1N540x types and should be mounted in cool free air for heat dissipation.

The schematic is an outline, so decoupling compactors, frequency compensation (if required), etc are not shown.

Also there is no optimization, or fault analysis, and there is probably an issue if the LM317 goes into constant current, but nothing that can't be sorted.
« Last Edit: February 04, 2019, 01:15:06 pm by spec »
 

Offline SiliconWizard

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Re: LM317 digital control
« Reply #51 on: February 04, 2019, 06:04:29 pm »
Technically, you're still generating a negative supply.
(It's going to make a nice heater...  ;D )

You can probably get away with a higher value for R2, so the opamp will have to sink a bit less current.

As a remark, we can see that whereas the opamp sinks ~7mA when the LM317 is not limiting current, when it starts limiting current, depending on your input voltage, the opamp could start sourcing current instead of sinking, and depending on the input/output voltage/and how much current the opamp is able to source, it could get pretty hot in this situation.


 

Offline spec

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Re: LM317 digital control
« Reply #52 on: February 04, 2019, 10:22:50 pm »
Technically, you're still generating a negative supply.
(It's going to make a nice heater...  ;D )
Very profound ^-^ The heat generated by the diodes, will be insignificant compared to the heat generated by the LM317 itself, not to mention the heat generated by the other four diodes in the bridge rectifier and the load itself.

You can probably get away with a higher value for R2, so the opamp will have to sink a bit less current.
The opposite is true. From the LM317 data sheet (link below), the minimum output current for an LM317 is 10mA, so R2 should be 125R maximum rather than 180R. A practical worst-case opamp dissipation would then be 36V x 10mA = 360mW, which is hardly a problem, especially if you use a decent opamp. Anyway, there are probably simple methods of reducing the opamp dissipation if absolutely necessary.

As a remark, we can see that whereas the opamp sinks ~7mA when the LM317 is not limiting current, when it starts limiting current, depending on your input voltage, the opamp could start sourcing current instead of sinking, and depending on the input/output voltage/and how much current the opamp is able to source, it could get pretty hot in this situation.
I already said it was an outline schematic and there could be an issue if the LM317 went into constant current. It is not exactly a difficult problem to sort though: a Schottky diode may do the job. The purpose of the outline schematic is illustrate the principle of the design: it is the architecture that counts.

http://www.ti.com/lit/ds/slvs044x/slvs044x.pdf
« Last Edit: February 04, 2019, 10:51:46 pm by spec »
 

Offline iMo

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Re: LM317 digital control
« Reply #53 on: February 04, 2019, 10:53:59 pm »
You can probably get away with a higher value for R2, so the opamp will have to sink a bit less current.
The opposite is true. From the LM317 data sheet (link below), the minimum output current for an LM317 is 10mA, so R2 should be 125R maximum rather than 180R. A practical worst-case opamp dissipation would be 36V x 10mA =360mW, which is hardly a problem. Anyway, there are probably simple methods of reducing the opamp dissipation if required.
You may have 317' 10mA min current and 50uA opamp current (the ADJ current).
Readers discretion is advised..
 

Offline spec

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Re: LM317 digital control
« Reply #54 on: February 04, 2019, 10:54:40 pm »
Technically, you're still generating a negative supply.
(It's going to make a nice heater...  ;D )
Very profound ^-^ The heat generated by the diodes, will be insignificant compared to the heat generated by the LM317 itself, not to mention the heat generated by the other four diodes in the bridge rectifier and the load itself. And then there would be the heat generated by the user: 100W. :-DD

You can probably get away with a higher value for R2, so the opamp will have to sink a bit less current.
The opposite is true. From the LM317 data sheet (link below), the minimum output current for an LM317 is 10mA, so R2 should be 125R maximum rather than 180R. A practical worst-case opamp dissipation would then be 36V x 10mA = 360mW, which is hardly a problem, especially if you use a decent opamp. Anyway, there are probably simple methods of reducing the opamp dissipation if absolutely necessary.

Or are you suggesting that the LM317 output has current drawn off it by another means, so that R2 can be an open circuit, when the tail current would be only 50uA or so.

As a remark, we can see that whereas the opamp sinks ~7mA when the LM317 is not limiting current, when it starts limiting current, depending on your input voltage, the opamp could start sourcing current instead of sinking, and depending on the input/output voltage/and how much current the opamp is able to source, it could get pretty hot in this situation.
I already said it was an outline schematic and there could be an issue if the LM317 went into constant current. It is not exactly a difficult problem to sort though: a Schottky diode may do the job. The purpose of the outline schematic is to illustrate the principle of the design: it is the architecture that counts.

http://www.ti.com/lit/ds/slvs044x/slvs044x.pdf
« Last Edit: February 04, 2019, 11:19:08 pm by spec »
 

Offline spec

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Re: LM317 digital control
« Reply #55 on: February 04, 2019, 11:05:14 pm »
You can probably get away with a higher value for R2, so the opamp will have to sink a bit less current.
The opposite is true. From the LM317 data sheet (link below), the minimum output current for an LM317 is 10mA, so R2 should be 125R maximum rather than 180R. A practical worst-case opamp dissipation would be 36V x 10mA =360mW, which is hardly a problem. Anyway, there are probably simple methods of reducing the opamp dissipation if required.
You may have 317' 10mA min current and 50uA opamp current (the ADJ current).
Our posts crossed. :)

Yes you are quite right: 10mA is the minimum current for an LM317, not 7mA as I indicated on the schematic of reply #52. You can reduce the LM317 tail current  by connecting a current mirror of 10mA to the output of the LM317, or you may get away with just a resistor, depending on the output voltage range in use.
« Last Edit: February 04, 2019, 11:16:10 pm by spec »
 

Offline not1xor1

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Re: LM317 digital control
« Reply #56 on: February 05, 2019, 08:55:39 am »
You can probably get away with a higher value for R2, so the opamp will have to sink a bit less current.
The opposite is true. From the LM317 data sheet (link below), the minimum output current for an LM317 is 10mA, so R2 should be 125R maximum rather than 180R. A practical worst-case opamp dissipation would then be 36V x 10mA = 360mW, which is hardly a problem, especially if you use a decent opamp. Anyway, there are probably simple methods of reducing the opamp dissipation if absolutely necessary.

Or are you suggesting that the LM317 output has current drawn off it by another means, so that R2 can be an open circuit, when the tail current would be only 50uA or so.

Most LM317 work flawlessly with just 3-4mA of load. If one just needs to build 1-2 PSUs can test the ICs for stability on a breadboard.
A different but still valid approach is to provide a 10mA constant current sink for the LM317. Then the value of the resistor between output and adjust is no longer critical.

BTW a relative high power dissipation in opamps affects their performance.
 

Online Zero999

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Re: LM317 digital control
« Reply #57 on: February 05, 2019, 09:27:26 am »
not1xor1,
The LM317 definitely is specified for a minimum load of 10mA. Most devices will work with much lower minimum load currents, but relying on 4mA is marginal design.

Technically, you're still generating a negative supply.
(It's going to make a nice heater...  ;D )
Very profound ^-^ The heat generated by the diodes, will be insignificant compared to the heat generated by the LM317 itself, not to mention the heat generated by the other four diodes in the bridge rectifier and the load itself.
P = IV so irrespective of where the voltage is dropped, the same amount of heat, in terms of Watts, will be dissipated. I know what you mean though: it won't  be as hot as dissipating all of the power in the LM317, as it's more spread out.

Quote
Or are you suggesting that the LM317 output has current drawn off it by another means, so that R2 can be an open circuit, when the tail current would be only 50uA or so.

Yes, a current sink could be added, but in this case it's not needed: the resistor is fine since the op-amp effectively has a negative supply.

The main issue with this circuit isn't the power dissipation or LM317's minimum load current, but the voltage ratings of the components. What transformer voltage do you recommend? The original schematic showed a 22-0-22V primary, but no secondary voltage. Presumably it was inadvertently drawn mirrored and you meant 22V-0-22V secondary? That would give a DC bus voltage of nearly 62V, which would be far too high, so that doesn't seem right either.

To get 30V out, the LM317 needs at least 33V in, plus five diode drops, nearly 1V each at full load, giving a minimum DC bus voltage of around 38V. At first glance, a 30V transformer looks like a good fit, as it would give just over 42V out. The trouble is it's a marginal design. When unloaded, the transformer voltage will be around 10% higher than its fully loaded rating, the diodes will drop less voltage and the mains voltage could also be on the high end of the tolerance band, another 10% in the UK, causing the DC bus voltage to be much higher. VDC = 30*√2*1.1*1.1* - 1.2 = 50V. The LM317HV could be used, which is rated to 60V, but finding op-amps, which can do this is more challenging.

Another problem with the LM317 is its safe operating area protection kicks in when the voltage difference between its input and output exceeds around 15V. Using a centre tapped transformer and a relay or transistor to select between the centre tap, depending on the output voltage will alleviate this to some degree.
 


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