Current Flowing Indicator.

[Joe]

That is what I said in the first place. I can't remember exactly which book it was that I read this in and it looks like I no longer own it.

Thankfully it isn't something I'm likely to come up against.

To get this thread back On Topic, I 'scoped my PSU earlier and there's about 40mV of noise (a saw-tooth pattern) on it which dropped to an immeasurable level with an extra smoother across it. Certainly room for improvement when I get the bits.

- Joe

[Skywave]

Yup! For their positions in the cct., 1 meg-ohm and 250 k-ohm would be about right √ . So we have small omega meaning 'ohms' and large omega meaning 'meg-ohms': goodness me! I've never come across that before: I feel just like a 13-year old, starting out all over again.
"Pass me the 'Practical Wireless Encyclopaedia', please mum. I promise that this time I won't read it at the dinner table!

Al.

[pwdrive]

That is what I said in the first place. I can't remember exactly which book it was that I read this in and it looks like I no longer own it.

Thankfully it isn't something I'm likely to come up against.

To get this thread back On Topic, I 'scoped my PSU earlier and there's about 40mV of noise (a saw-tooth pattern) on it which dropped to an immeasurable level with an extra smoother across it. Certainly room for improvement when I get the bits.

- Joe

Hi Joe, yes that ohms thing is a relic of the past and well spotted, I for one have not come across that before.

That waveform is the ripple voltage, more C cetrainly will reduce it, the ripple voltage will increase with increased load. Sounds as though you are nearly there with it now.

Lawrence

[Skywave]

Joe - when you had that 40 mV of noise on the PSU rail, how much current were you drawing from that rail, please? (N.B. There's a logical chain of thought behind that Q.: I'm not asking for the sake of making small talk. )

Al.

[Joe]

That was off-load. I haven't put the scope away yet so will do some better tests when I've had my tea.

- Joe

[Joe]

I used a 180Ω resistor as a load and found that the noise level was about 4mV irrespective of the voltage across, and therefore the current through, it.

Off-load was more interesting. The saw-tooth thing started at 10mV and increased with voltage until by the time this was at 15V it was 100mV. It then started to rapidly fall and by the time it was up to 18V the noise was down to 4mV.

I wouldn't swear to the accuracy of these figures as with the 'scope on 10mV/Div some of the 'noise' could be radiated stuff being picked up by the wires rather than coming direct from the PSU.

- Joe

[Skywave]

I can think of several reasons that might explain that behavior.

1. Each LM317 needs a 0.1 µF cap. from its input terminal to 0v.
2. The 2nd. LM317: the junction of the 390 Ω and the 4k7 variable resistor needs a 10 µF capacitor to 0v. (Incidentally, the manufacturer's datasheet recommends a 240 Ω instead of that 390 Ω when an LM317 is used in this configuration).
3. The arrangement of the earthing between all the components may not be optimal. Wiring from rectifier -ve to reservoir cap. -ve needs to be short and fairly thick. Then all other 0v. returns, plus the wiring to '0v. out', need to be taken only to the -ve of that cap. This ensures single-point earthing and thus eliminates 'hum loops'.
4. Your 'scope earth may be picking up noise and hum on account of the changes that may need to be implemented as in 4 above.
5. Connect a 0.1 µF cap. in parallel with the main reservoir cap. This reduces the high-frequency impedance of the main reservoir cap., leading to reduced noise on the unreg. HT line.
6. The unusual rectification scheme you're using could be making a significant contribution to the noise, in particular the additional half-wave rectifier diode. If it is temporarily removed, does the noise performance substantially improve? A bleeder resistance across the final reservoir capacitor might help.

Not related to noise performance, but to add some reliability to the PSU, I would add two protection diodes: one from input to output of the 2nd. LM317 (wired with cathode towards the reservoir capacitor) and one in parallel with that aforementioned 240 Ω: cathode to the LM317 O/P pin. 1N4004 / 5 / 6 / 7 are just fine for those diodes.

Plenty of ideas to experiment with in that lot!
Best of luck -

Al.

[Joe]

Protection diodes are definitely on the 'To do' list. When I'm charging batteries with it there's a diode built into the connection plug which gives protection in the even of power failure.

The 390Ω resistor is so I could get the desired output range with a preferred valve pot. This example from the National Semiconductor data-sheet uses a 1.2kΩ one so I should be all right.

   

Thanks for the suggestions. When I get the other bits I need I'll return to it.

- Joe

[Skywave]

But that example from Nat. Semi. uses a 20 kΩ variable resistor: your design uses 4.7 kΩ. The Nat. Semi. values are approx. in proportion to a 240 Ω fixed and a 4.7 kΩ variable, respectively. I'm not trying to split hairs, Joe: you presented the Nat. Semi. design: I looked at it and I suddenly thought "Hang on a tick . . . " However, at the end of the day, if what you have gives you the required voltage range and the current into the LM317 common pin is within the spec. for the device, it doesn't really matter.

Good luck with your cct. when you return to it - then, if you want, we can talk about it again.

Al.

[Yorkie]

Just a small point: I appreciate that what Joe wants to do is to have an LED indicator - not an in-line Ammeter, but analogue 0-5A panel meters cost just £2.50 post free from Asia Engineer, or digital backlit blue panel meters cost just £4.00 post free. True, the digital ammeters do need a separate power supply and must not share the same ground as the circuit being measured, but their voltmeters don't - nor do they need a shunt. If the object of the exercise is to explore the technicalities of using an LED to give an indication of current flowing, rather than a meter, that's one thing, but if the aim is to provide a cheap alternative to using a meter, given that meters are now so cheap, there's really no economic argument to support the use of an LED

http://www.ebay.co.uk/itm/New-Analog-AMP...ltDomain_0&hash=item56427c509f

Asia Engineer have about 600 different meters, both analoge and digital for just about every requirement. I've used several for small projects and found them to be excellent. True, they take about 3 weeks to arrive on a slow boat from China, but at such low prices and free P&P, who's going to complain about that? Not me, that's for sure!!

Well worth browsing their 'e-bay shop'.

Have fun!

Looking forward to the Swapmeet!