EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Mat923 on April 05, 2024, 03:29:37 am
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I am designing a (0-15 V) (0-1A ) variable linear CV CC power supply. The schematic is attached for reference. 0.5 ohm resistance is used for current sensing.
1)But LM317 is creating a constant 1.25V drop which is not allowing regulation below 1.25V.
2)Also short circuit current limiting is not happening due to the 1.25V drop.
Could you help me in modifying the above circuit to acheive the above tasks
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I cannot devote the time to re-design your power supply. But:
I have great fondness for the LM317 (seriously). But it is not a good fit for what you are doing. One other problem is you can get very high power dissipation in your regulator if you have 18V input and 1V 1A output (for example). My bench linear power supply (an inexpensive Chinese made unit) produces up to 50V at 3A but it switches taps on the 60 Hz transformer in order to control power dissipation. This seems to work nicely but it is not a good approach for a 1-off home project.
Instead of using an LM317, I would use a voltage reference (something like an LM4040?) and adjust the voltage by use of an op amp section as a summing junction. If you have the + input of the op amp connected to +5V Reference, and also use the reference and a potentiometer to vary the input through resistor "B" to the - input. Now your Vout feedback goes through resistor "A" to the same op amp - input. As your voltage applied to B goes up, your feedback from A must go down in order to maintain the - input at the same potential as the + input.
The current limiting would probably need another input to the summing junction, but needs to be switched off via a diode or transistor collector. The output of this power supply would probably use discrete power transistors. If you do not need very fast response, you might do well with one or more darlington PNP transistors such as maybe TIP125. (Oh, my Darlington!!) :D
I did something like this with a Maxim dual 256 step digipot (but this was a SEPIC switcher and not a linear power supply.) The output voltage from this puppy is amazingly stable (at least by casual observation on my bench.) One digipot output controls coarse voltage and the other controls fine voltage. Input power comes from a surplus laptop power supply. But it does not have adjustable current limiting; that would require more parts. I sorta wish I had included that.
I cannot give you a design but I can feed you ideas..... I have been designing power supplies since 1981. Good luck.
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Could a pmos connected between unregulated input and Vin of LM317 with the gate terminal connected to R10 left end be of help?. Acting like a variable resistor reducing the input to LM317 during a short circuit
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The circuit has several problems:
1) the worst case power dissipation is boarderline high for the LM317. It would need quite some cooling.
2) The shunt on the output side is not compensated. So the voltage is not very stable under load with the 0.5 Ohm still in series.
3) with the heat and ref. in the same chip, there can be quite some thermal drift, as the LM317 will get quite hot under load.
4) The LM317 is a voltage regulator and as this not made for difficult loads. Here the shunt in series help, but it gets tricky if one tries to avoid that problem.
5) The current regulation with the differential amplifier and shunt at the output has the tendency to get unstable and oacillate with some loads. It is quite some task to get this stable and this usually means a slow response. Because of the other weaknesses it is likely not worth the effort to get it stable.
It would likely be better to use the more classic circuit with separate reference chip, 2 op-amps (e.g. LM358), power transistor and the shunt at the low side.
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Yeah like other's said, you would need a huge heatsink to keep 1 TO-220 package from cooking, once you get over a few watts across it.
My 1st DIY PSU was some LM317 one, I need to make a another multi-rail, adjustable version, just for small breadboard circuits. But yeah, I'll have to read up on heatsinks and all that again, to make it safe.
One thing about the reference with 500R and a zener in parallel with 500R, why have the lower 500R, IDK if it's good or bad, but I just don't remember seeing people have resistors in para. with zener's as a reference, and not for clamping reasons.
And you really want a capacitor on your reference voltage too.
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Thanks #Kleinstein and #Mathwizard for your inputs on the topic. #Kleinstein could you elaborate more on the classic circuit with lm358 you were mentioning. Circuit schematic will be highly appreciated. Thank you in advance
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The adjust pin of the LM317 needs to be pulled down to -1.25V, in order for its output to go down to 0V. In this case, you could connect Q1's emitter to Vss.
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#Zero999 as instructed by you I connected Vss to emitter but in simulation at short circuit load it showing heavy currents of 30A range.Also on variation of the voltage adjusting potentiometer at some value of load the voltage across it is showing constant at 15V without changing in simulation. What maybe the reason?
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30A through where? Also note that's highly unlikely to happen in real life. A simulator is just performing calculations, based on idealised models.
I've also realised, that it will only solve the short circuit issue. It won't allow regulation down to 0V.
This thread shows some circuits with the LM317 at 0V.
https://www.eevblog.com/forum/beginners/zero-volts-from-a-lm-317/msg110760/#msg110760 (https://www.eevblog.com/forum/beginners/zero-volts-from-a-lm-317/msg110760/#msg110760)
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#Zero999 i did a parameter sweep for output load from 0 to 150ohm.the current across it is shown in green trace and the light blue is the voltage.Ignore the bottom blue trace. Could you verify why green trace on top came out like this at 30A at short circuit?
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A relatively simple OP based lab supply circuit is shown in this thread:
https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/msg5431856/#msg5431856 (https://www.eevblog.com/forum/beginners/lm324-power-supply-with-variable-voltage-and-current/msg5431856/#msg5431856)
Much of the later dicussion is about details / performance tweaking, way beyond what can be expected from a normal lab supply. So the interesting part is more the start of that thread.
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#Zero999 i did a parameter sweep for output load from 0 to 150ohm.the current across it is shown in green trace and the light blue is the voltage.Ignore the bottom blue trace. Could you verify why green trace on top came out like this at 30A at short circuit?
Your simulator is lying to you. There must be a problem with one of the models or calculations. Irrespective of the rest of the circuit, the LM317 will limit the current to 3.4A absolute maximum, more likely 2.2A or even less, as there's over 15V between the input and output pins.
Refer to page 5 of the data sheet: current limit.
https://www.jameco.com/Jameco/Products/ProdDS/898800.pdf (https://www.jameco.com/Jameco/Products/ProdDS/898800.pdf)
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Thank #Kleinstein and #Zero999 for the clarification. #Kleinstein i had referred the initial threads but the circuit schematic looks really daunting for a CV CC power supply. #Kleinstein and #Zero999 could you suggest a simpler power supply design for the above specifications without much focus on intricate details since my focus is more on achieving short circuit current limiting and a voltage output levels starting from zero. I hope a very simple design exists with less focus on thermal aspects which i could manage with proper heat sink design. Thanks in advance
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Your simulator is lying to you. There must be a problem with one of the models or calculations. Irrespective of the rest of the circuit, the LM317 will limit the current to 3.4A absolute maximum, more likely 2.2A or even less, as there's over 15V between the input and output pins.
FWIW, TI's spice model for LM317-N does implement the output current limit of ~2.2A. It might make sense to use it, if possible, instead of whatever is currently used.
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Thank #Kleinstein and #Zero999 for the clarification. #Kleinstein i had referred the initial threads but the circuit schematic looks really daunting for a CV CC power supply. #Kleinstein and #Zero999 could you suggest a simpler power supply design for the above specifications without much focus on intricate details since my focus is more on achieving short circuit current limiting and a voltage output levels starting from zero. I hope a very simple design exists with less focus on thermal aspects which i could manage with proper heat sink design. Thanks in advance
This is simple if you want to experiment with it.
Also: It will regulate current down to zero but not voltage.
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A simple way to get it to output 0V is to add a 1.25V voltage reference. It's also possible to use a differential amplifier to generate to -1.25V reference from the LM317's reference.