Power supply project

audioguru said:

Hi Ron,
You and your sim are correct, the project is grossly flawed. :lol:
I didn't yet but will look on the author's forum to see any discussion about it.

I figure that the circuit will work the same if a common-emitter NPN transistor was used in the LM317 circuit, and visa-versa.
When the pot is halfway, the transistor will have 1.45V across it and the output voltage from the regulator will be that 1.45V plus the 1.25V reference which equals only 2.7V.

I think this circuit will work pretty well:

Click to expand...

Pretty clever, Guru. The only thing is the extreme temperature sensitivity at the high voltage end - 50 or 60mv per degree C.
Here is a low-tc modification of your idea. It raises the minimum output voltage to about 2.5V. Note that, in either case, the main voltage reference is now the transistor Vbe or the LM385. The LM317 just adds 1.25V to whatever is generated by the reference.

Dr.EM said:

Wierd post at the same time there :lol:

I tried this with the wall wart, which actually reads 17vdc.

With the board I get from 1.90v to 2.13v, with the 2.13v being when the control is set more or less central :? past central, it drops back to 1.98 or so.
Totally useless if nothing else :lol:

Click to expand...

Are you sure you've wired the regulator the correct way round?, the article mentions that different manufacturers have different pinouts!.

Yep, Its in right. What i've done is breadboarded the below. It works fine apparently, giving 7-18v or so with linear control. I'm using an old scalectrix (sp?) power pack for testing, which is 23v, but sags very quickly and is actually rated for 12v when use with the cars. I've tried it with a fairly small load (220ohm), and only the regulator warms, but they will be on a heatsink anyhow. So, I think i'll be modifying the board to work like this, and will need to pick up a 1k dual gang.

Or, as the case may be, 2 single gang 1k's, as Maplin don't have dual 1k. At least there wouldn't be a problem with the voltages not balancing as I could adjust them independantly 8)

Hi Dr. EM,
Good, it works when you give it enough input voltage. :lol:

Your 470 ohm resistors limit its output voltage range. You can increase its max output voltage by decreasing the 100 ohms. With 68 ohms from the output to the reference, and a 1k pot to ground from the reference, the regulator's output voltage will be 19.6V, if the input voltage is 22V or more. Then the pot must dissipate 0.34W and be within a 0.5W rating. When the pot is turned down to zero ohms the output voltage will be 1.25V.
The linearity of the voltage adjustment depends on the linearity of the pot. :lol:

Yep, I prefer it with a 560k at the top of the pot and a 470k at the bottom, I want it so that the output voltage cant' exceed 18v (the maximum for most op-amps). That'll need to be tuned as the transformers voltage should be higher than 23v; 25v at least.

Alarmingly though, when I tried adding those transistors in to the breadboard version, I got different results to the stripboard one. I got the results that graph shows more or less, whereas the stripboard gives those really bizzarre results. It could be that the transistors have overheated in soldering (hopefully, as they are going anyway), that the regulators have overheated (they both behave exactly the same though) or that there is a problem with the type of pot i've used:

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22k dual linear. Thats also not a big deal, as its going too, to be replaced with the 1ks. It would be annoying if it were the overheated regulators though, as they are in quite a specific location to accomodate the heatsinks, and desoldering is tricky :x . I've checked the board carefully for solder bridges, but I don't see any and the different voltage readings for parrallel strips confirms this.

Hi Dr. EM,
Be careful with your pots. Their max is only 1/4W. :shock:

Well, i've tried it with the resistor arrangement I posted on the stripboard version, and it does indeed work . Wierd thing is that the transistors and pot i've removed are all functional too. Must have just not worked together. Think I will increase the 100ohm to 150ohm, my new 1k pots are 0.2W :shock:

Hi Dr. EM,
Increasing the 100 ohm resistor to 150 ohms will increase the max output voltage of the regulator 50%.
The pots will operate at their max power rating with only 14.1V across them.

I thought it reduced it?

"You can increase its max output voltage by decreasing the 100 ohms. With 68 ohms from the output to the reference, and a 1k pot to ground from the reference, the regulator's output voltage will be 19.6V, if the input voltage is 22V or more"

I changed it to 150ohm, and can see that the output voltage has dropped quite a bit now. The output voltage will go over 14.1v, but I doubt i'll run it at the full 16 or so it does now much, the pots can probably take a bit of abuse anyhow :lol:

Hi Dr. EM,
You are absolutely correct! I was wrong, increasing the value of the 100 ohms decreases the max regulator's output voltage.

Ok, its finished now, giving +/- 7-16v at 1.2A max (though I doubt it'll do that for long as the heatsinks arn't exactly titanic :lol: ). Anyhow, it works nicely, with no voltage sagging now, the test transformer was really bad for that. Thanks for all your help, I might try and get a photo, its very compact :shock: , glad I didn't get the smaller box.

Horray! It works well.
Don't worry about heatsinks. The ICs will simply shutdown to protect themselves. I also have an LM317 variable supply with a fairly small heatsink and it works fine. I never leave it very long with a high current, low output voltage and it hasn't shutdown.

Yep. I got some very exciting photos of it :lol:

**broken link removed**

**broken link removed**

WoW! Everything fits into the box very nicely. I like the powerful-looking torroid transformer. :lol:
What is it? :?:

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oh, this is so beautiful, you are a wonderful craftsman. everything fits so well Did you decide on the casing after the soldering work, or you forced the soldering work to fit into an already-acquired casing? just curious.

Cheers :lol: . The torriod is 60VA, 1.2A on the secondaries.

I ordered the case along with everything else, its a suprisingly tight fit and it did take some working out how to fit everything in. I'm just glad I didn't go with the smaller case now :lol: . Took a while filing off the millimetres to get that stripboard to slot in and things like that, but I didn't want anything loose inside a project that uses mains power.