Correct. But not for an LM317. 240 ohms can be used for the more expensive LM117.The datasheet says that R2 can be 240R.
Yes but the thermal stress of heating and cooling over and over will shorten its life.If the regulator overheats, will it start again as soon as the excess load has been removed?
If your circuit is connected correctly then the current in the pot is only 5mA which will not burn it.I did find that if I turned the voltage up, R2 would burn out. It's power rating must be too low.
You are talking about the load. It has up to 27V at up to 1A which is 27W.If P=IV, and the circuit can provide up to 1A at a max of 27V, does that mean the resistor must be 27W? That sounds really huge... Have I got that wrong?
Using the LM317 I found that it was hard to get an accurate output.
Would it be good to use multiple LMxx rectifiers with a switch?
Or one 'large' rectifier like the LM7818 and have voltage dividers running off it? Would the voltage dividers cause the voltage to be inaccurate too?
I have included a conceptual schematic of the multiple rectifier design...
as you have a potentiometer in the circuit, and you did not intimate the output voltage, it is difficult to answer. now you say that you tested a motor with 9V, should we take it as 9V DC output ?This is the other thread:
https://www.electro-tech-online.com/threads/power-supply.91484/
The datasheet says that R2 can be 240R. I thought this could be changed (to change the ratio of R1 and VR1). I'll change it back to 240R
I'll change the resistors and caps at some point and see how that goes.
If the regulator overheats, will it start again as soon as the excess load has been removed? I'll put on a good heatsink and see how that goes too.
I'm not sure what the current rating of the motor is, but it's only small, so it wouldn't be 1A. It does run from a 9V battery, so it can't be too high.
I did find that if I turned the voltage up, R2 would burn out. It's power rating must be too low.
If P=IV, and the circuit can provide up to 1A at a max of 27V, does that mean the resistor must be 27W? That sounds really huge... Have I got that wrong?
Perhaps you did not think over the issue that 317 can also be configured with a set of pre selected Resistors and switchable so that errors cant creep in, instead of a potentiometer.Yes, you're right, Multiple regulator design.
I only want to have one voltage output at a time, but I want it to be selectable. So I can turn it to 9v for one project, 5v for another, etc...
The inaccurate output is like this:
I was using the LM317 with the voltage divider (one regular resistor and one pot) on the common pin as shown in the data sheet.
I wired up a simple 555 LED flasher as a 'load' to test the circuit. With no load, I turned the pot up to 9v.
The volt meter needle was not staying steady at 9v, but was jumping around a bit. When I use the LM7809 with the common pin grounded, the needle on the volt meter stays steady on 9v, while the needle on the amp meter jumps with every flash.
That's why I was thinking of using multiple regulators.
Yes, heat will be a big problem. I will of course need to have big heatsinks on each regulator. Perhaps I could use a multitap transformer so the 'smaller' regulators won't have to deal with as much voltage?
The other reason that I don't want to go with the LM317 is because it would be more open to user error. I could accidentally turn it to 10v instead of 9v by mistake, whereas a rotary switch would 'click' into place.
The thought did cross my mind...
But how accurate would it be? Would I need low tolerance resistors for it to be accurate enough?
Would it still get really hot if it's turned down (eg, 3v or 5v)?
Would a heatsink like the one in the top right of this PDF be good enough? Would I need a small fan?
https://www.electro-tech-online.com/custompdfs/2009/04/heatsink-1.pdf
Better concept than searching for independent resistors of selected special values.hi,
One reason the voltage on the output dips is due to the output capacitor being to low a value, use a 470uF or 1000uF for motors.
The LM317 can be set to switched outputs by using small pots so set each voltage on the switch.
With that high voltage input from the rectifier the medium and low voltage settings will make the reg hot on low/medium currents.
EDIT: another point you should consider is the resistance of the ammeter, its in series with load.
No.hi,
One reason the voltage on the output dips is due to the output capacitor being to low a value, use a 470uF or 1000uF for motors.
It is wrong as you never got the DC bridge through properly and these switching transistors are shown connected at AC level. It should be like the attached modified schematic.I have included a new conceptual idea for a power supply. Unfortunately it is a little crude, so I will explain it here.
The idea of this one is to have the LM317 with trimpots to adjust the voltage.
A rotary switch is used to select the voltage.
There is a transistor (used as a switch) with the base connected to the rotary switch. The emitter goes to the bridge rectifier, but the collector goes to a multitap transformer.
Although not shown very well on the schematic, the collector of each transistor has a different voltage from the transformer. The reason for this is so there is less voltage going into the voltage regulator when it has a low output.
The less voltage in should mean that it does not have to dissipate as much heat, and the risk of burning out is much lower. It will still have a heatsink.
I haven't worked out the details on the transistors yet.
Will this work?
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