I need an adjustable power supply

[Tipsy]

Hi Mr RB, thank you for responding again. Yes, the resistor reads 118Ω or 116Ω depending on which cheap digimeter I use and still stuck with the same annoying 5K pot.

At a measured 10V output with no load;
Vin to Ground = ~18.2V
Vout to Vadj = ~0.9V

At 10V Preset output (actual reading dropped to 8.5V) with fan loading (12v @0.17A);
Vin to Ground = ~17.5V
Vout to Vadj = ~0.7V

Unfortunately, switch cleaner has no effect on the (assumed) POT going high resistance intermittently so I'll be replacing the POT asap but I did manage to keep it stable enough to double check the readings are repeatable.

Hi ercigibbs, I'm in the next County, close to your border. (Hint: I live in the "OAP capital" of the UK, I believe).

 

[ericgibbs]

Hi ercigibbs, I'm in the next County, close to your border. (Hint: I live in the "OAP capital" of the UK, I believe).

Tipsy,
What is the source of the 18.2Volts.
If its a simple mains plug type power supply [wall wart] then it should have a label on it stating its specification.

Lets know what you find.

I'm guessing but I would say OAP = Bourn..............???

 

[Mr RB]

Tipsy, (thanks for doing that test!) that Vout should have stayed at 10v when you connected the 12v 170mA fan!

I have a feeling your wall wart is the cause of all these weird readings. If it has a very high ripple (like if it is unfiltered DC) I think you would get readings like that.

Putting some 470uF caps from Vin->gnd and Vout->gnd near the LM317 would prove if that is the problem.

 

[Tipsy]

Hi Eric, the mains adapter I'm using is actually a charger if that makes any difference? Hopefully I have other donors to use if this one is unsuitable. It was supplied to recharge a 14v cordless drill battery and reads;
MODEL SH-14.4V400 INPUT: 230-240 a.c. 50Hz 15W, OUTPUT: 18V d.c. 400mA.

I'm in Christchurch. And you?

Hi Mr RB, I'd read in the datasheet that insufficient load will see voltage rise so assumed the opposite would be true (which tied in with seeing the 10V output drop) though it wouldn't surprise me if my power source is the cause of confusion as I do have a knack of doing things the hard way whenever possible and sincerely apologise for this time-wasting

Rather than "band-aid" the adapters shortcomings I think I'll have another look through my box of alternatives first as it's always been in the back of my mind I'll need more than 400mA sooner than later, especially if I am to run dual adjustable regulators from it as originally planned.

I'll be back later with either a new (stable) source or the results of adding your advised 470µf caps to the current one.

EDIT: That was easy, grabbed a 12v supply, set output to 10v, got a smack on 1.25v differential on the LM. Loaded it with fan, still 10v showing and 1.25v so I think Mr RB and Eric are bang on the money, well done lads and again sorry for throwing you all a "googly" (a cricket term for an unpredicted event).

So here's my dilemma. All other adapters I have are rated 12v maximum @1A but with this circuit there is voltage drop and I need at least 12v outputs, possibly as much as 14v. Is there an alternative or do I go back to my unstable low current adapter and add the caps as advised?

2nd EDIT: Stop Press, I've just found another candidate I put away for possible fix or break for parts. A laptop power adapter with an output rated at 15W and a whopping 6A.

The only problem is it's dangerous, I've got a couple of shocks using it and brush metal parts on the laptop. Also, whenever it's powering the laptop, which it does just fine, it will trip the main ciruit breaker whenever another powered device is attached. ie A projector linked to the laptop via a VGA and USB cable, plug it in and wham! The mains goes off. Same for my hi-fi amplifier.

Can this be easily fixed? It would be ideal. I've broken the box open for a looksee but nothing obvious to my novice eyes.

 

[ericgibbs]

hi t,
You can get fairly cheap 15Vdc at say 1A or 1.5A mains power adaptors for about £7GBP.

This is an example of a SMPS a little more expensive.
**broken link removed**

BTW: its Pompey.!

 

[Tipsy]

Putting the choice of power source aside, I wanted to make some progress but I still don't know how to chose the capacitor values (or diodes for that matter) - the example circuits I reference use differing values. The cap voltage is obvious but any simple rule of thumb I can follow? Hints and tips are most welcome.