Electronics > Beginners
Diode in GND pin of voltage regulator
Zero999:
--- Quote from: KL27x on July 04, 2018, 09:51:19 am ---
--- Quote --- VF (forward voltage) changes with temperature and current. so keep that in mind if you're looking for any kind of stability and accuracy.
--- End quote ---
There's no need to dig around for the right combinations of diodes to get approximately the voltage you want and the resulting instability. You can dial in w/e output voltage you want, very precisely. Put one feedback resistor between the OUT pin and the GND pin. Let's call it R1. And you put one resistor between the GND pin and ground. Call that R2. Your output voltage is now Vref * (1+R2/R1). Vref is the nominal output voltage of the regulator, so for an LM7805 it is 5V. If that equation looks familiar, it's because an LM317 operates pretty much exactly the same way as an LM78XX, except the GND pin is called an ADJ pin and its Vref (nominal output voltage) is 1.25V. (And on the LM317, the heatsink goes on the output pin rather than the GND/ADJ pin).
This is just Kirchoff's Law. If the voltage between OUT pin and GND pin is 5V, you can look at the resistor ladder to figure out the voltage between GND pin and the actual ground. And add those 2 numbers together to get the voltage bewteen OUT pin and ground.
Using this "trick", you have to consider that using resistors of too high value will cause instability/malfunction. But it doesn't take much. R1 1K or lower for 7805 will probably work in most applications to get output voltage up to 20V.
--- End quote ---
That will work, but you need to consider the bias current, i.e. the current through the LM7805's ground pin, even for modest value resistors, so the calculation is a little bit more involved than that.
VOUT = VREF(1 + R2 / R1) + (IBIAS× R2)
wraper:
--- Quote from: Hero999 on July 04, 2018, 10:12:30 am ---
--- Quote from: KL27x on July 04, 2018, 09:51:19 am ---
--- Quote --- VF (forward voltage) changes with temperature and current. so keep that in mind if you're looking for any kind of stability and accuracy.
--- End quote ---
There's no need to dig around for the right combinations of diodes to get approximately the voltage you want and the resulting instability. You can dial in w/e output voltage you want, very precisely. Put one feedback resistor between the OUT pin and the GND pin. Let's call it R1. And you put one resistor between the GND pin and ground. Call that R2. Your output voltage is now Vref * (1+R2/R1). Vref is the nominal output voltage of the regulator, so for an LM7805 it is 5V. If that equation looks familiar, it's because an LM317 operates pretty much exactly the same way as an LM78XX, except the GND pin is called an ADJ pin and its Vref (nominal output voltage) is 1.25V. (And on the LM317, the heatsink goes on the output pin rather than the GND/ADJ pin).
This is just Kirchoff's Law. If the voltage between OUT pin and GND pin is 5V, you can look at the resistor ladder to figure out the voltage between GND pin and the actual ground. And add those 2 numbers together to get the voltage bewteen OUT pin and ground.
Using this "trick", you have to consider that using resistors of too high value will cause instability/malfunction. But it doesn't take much. R1 1K or lower for 7805 will probably work in most applications to get output voltage up to 20V.
--- End quote ---
That will work, but you need to consider the bias current, i.e. the current through the LM7805's ground pin, even for modest value resistors, so the calculation is a little bit more involved than that.
VOUT = VREF(1 + R2 / R1) + (IBIAS× R2)
--- End quote ---
Bias current will also very significantly depend on particular IC, temperature, input voltage and output current. So using 78xx like this should be avoided.
KL27x:
You can choose your own compromise with the feedback resistor method. You can squash the effect and offset of the bias current by choosing resistors that are low enough in value, at the cost of higher quiescent current. W/e your application, you can figure out for yourself if the regulation and the quiescent current are acceptable.
In practice, this works perfectly fine. Of course neither of these things would ever find their way into a PCB unless you happen to have a ton of 78XX in your inventory which you want to get rid of. For prototyping and learning, they are both quite useful.
The bias current is included in the equation in datasheet for virtually all LM317. And 99% of EE will ignore it for 99% of their applications. Just because you can doesn't mean have to. The end result of your circuit is going to be what it is, and if precision and temp stability is that important, tweaking is going to be virtually inevitable. And you are probably not going to be using an LM78XX to begin with.
Zero999:
--- Quote from: KL27x on July 04, 2018, 11:08:32 am ---You can easily squash the effect and offset of the bias current by choosing resistors that are low enough in value, as I suggested. This is another benefit of this method vs the diode trick. At the cost of slightly higher quiescent current, of course. W/e your application, you can figure out for yourself if the regulation and the quiescent current are acceptable.
In practice, this works perfectly fine. Of course neither of these things would ever find their way into a PCB unless you happen to have a ton of 78XX in your inventory which you want to get rid of. For prototyping and learning, they are both quite useful.
--- End quote ---
But you have to go very low, to make the bias current negligible. For example, you want 10V, so use 470R resistors with the LM7805, for R1 & R2, the output voltage will be nearly 12V, an error of 20%, because the typical bias current is 4.3mA, according to the data sheet, but it could be as high as 8mA, giving just over 13V out.
The bias current needs to be factored in, when calculating the resistor values and even then there's the change due to the input voltage, output current and temperature, which will ruin the performance slightly.
--- Quote ---The bias current is included in the equation in datasheet for virtually all LM317. And 99% of EE will ignore it for 99% of their applications. Just because you can doesn't mean have to. The end result of your circuit is going to be what it is, and if precision and temp stability is that important, tweaking is going to be virtually inevitable. And you are probably not going to be using an LM78XX to begin with.
--- End quote ---
Because with the LM317, it's typically 50μA, nearly two orders of a magnitude lower than the LM7805 and the percentage variation due to temperture, input voltage and load current, is much less.
In short, using the LM7805 as a variable voltage regulator is a hack. Use the LM317 in the first place.
KL27x:
--- Quote ---But you have to go very low, to make the bias current negligible. For example, you want 10V, so use 470R resistors with the LM7805, for R1 & R2, the output voltage will be nearly 12V, an error of 20%, because the typical bias current is 4.3mA, according to the data sheet, but it could be as high as 8mA, giving just over 13V out.
--- End quote ---
This equation might work for LM317, but I don't think the bias current of 8mA will make that effect on the LM7805. It's been awhile since I did this, but I have a feeling even 5% error offset (which is fixable) with 470R R1 at double Vref is wayyyy too much. I think this is case of applying wrong equation correctly? Using values like this, I seem to recall ignoring bias current and getting output I wanted. Maybe I am wrong.
anyhow, I agree fundamentally with what you said Hero, I just don't think it's significant as you think.
I shall have to break out the breadboard and try it. If I have any 78xx.
* edit: Well, color me wrong. Your application of the equation seems to be spot on. Using 500R resistors for R1 and R2, I got output voltage of 13V, lol. With this large of an offset contributed by the bias current, I suspect the diode/zener trick could be more temp stable when increasing output voltage by more than a couple volts.
For small bump in voltage within a few diode drops, any effect would be proportionally less, of course. So I'm still conflicted if this isn't still better than diode trick for small increase in output voltage.
I understand what Wraper is saying, now. Yeah, 78XX are technically not as good as LM317 in any way, except when they're good enough. 2 extra resistors is more cost of manufacture. Or more complex BOM and inventory if you are the manufacturer.
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