Simple voltage regulator get crazy... and so am I

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Hi folks,
Maybe I deserve to be treated as an idiot but I can't figure it out.

Take a TX, some schottkys, a 2200/35V Rubycon ZL to smooth that.
Then I have 17V.
Well that is too much for the numerous LM317@5V after (heat!).

So i fitted, just after the cap, a µA7810 from TI, followed by a 2200/16 ZL.
With output leg in air it's 10V, when soldered with the cap... madness, my DMM can't auto-scale, it happened once that I saw 10V for few seconds but also that the TX started to humm...

I just don't understand, did I over rated the later cap? I think it is better with, else there will be nothing between my 7810 and many 7805 at 10/20 cm away... help!!!! When I see a 4700µF after a 7805 in my CDP I don't understand why that stupid thing goes wrong...

Matthieu wich is used to have troubles only with simple things...
 
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Malefoda said:
Hi folks,
Maybe I deserve to be treated as an idiot but I can't figure it out.

Take a TX, some schottkys, a 2200/35V Rubycon ZL to smooth that.
Then I have 17V.
Well that is too much for the numerous LM317@5V after (heat!).

So i fitted, just after the cap, a µA7810 from TI, followed by a 2200/16 ZL.
With output leg in air it's 10V, when soldered with the cap... madness, my DMM can't auto-scale, it happened once that I saw 10V for few seconds but also that the TX started to humm...

I just don't understand, did I over rated the later cap? I think it is better with, else there will be nothing between my 7810 and many 7805 at 10/20 cm away... help!!!! When I see a 4700µF after a 7805 in my CDP I don't understand why that stupid thing goes wrong...

Matthieu wich is used to have troubles only with simple things...


Two possibilities I can think of: the 7810 is oscillating, or it is connected the wrong way around. Did you connect the ground pin of the 7810 to the samr ground as the caps? How long is the wire between caps and 7810?
But if you could post a circuit diagram it may jump out at you/us.
I know, the circuit should be easy to visualize, but then if it was so easy it would work, right? ;)

Edit: any chance that the last cap is the wrong way around? That would explain the tx hum and the 10V when unloaded. It would also explain the multimeter going crazy: the cyclic current limiting of the 7810 causes the Vout to fly all over the place.

jd
 
Thanks gentlemen,

the cap is on a test board with the 7810, maybe 3mm after ;) And yes I've checked many times and can say it's the right wiring.
I will try the same but a LM317 and/or a 220/50 ZL. In datasheets they say a 0.1 or 1µF is recommended in most cases before the filter cap if it's far away (what does "far-away" stands for in mm???) and after (I have loads of Wima's MKS if needed...). But what in the case of "bewteen" pre-reg 7810 and regs LM317?

Matthieu
 
Unfortunately my testboard will not survive that I guess...(bended cap legs to make links...) anyway it's faulty.
Until this answer I read on Internet many opinions... like input//filter cap (same in my case) smaller or as big as the output (2200//2200µF), lack of tiny caps (1/0.1) around or such...
In fact I'm also concerned by the best solution, AFAIK LM317/Low ESR in audio (CDP) do better than 7810/Non-Low ESR? If the 7810 is a part of the problem I may remove it :)
I'll tell you what I've found tomorrow, no light at the work table right now.
Matthieu
 
I have had a few 7810s oscillate like mad before. Is it an old one? I find the older 7810 and 7809 to be particularly troublesome. I guess they just fool a standard 5V one by floating the ground pin with a few PN junctions, a sure way to make them unstable. The newer ones seem fine however (better, more accurate trimming of an actual 10V reg).

The 78/79 series of regs are very variable from manufacturer/manufacturer and batch to batch. So try another if you have one and or lose/lower the 2200uF cap. Or if you want to keep it, first try the circuit without it, then add some R in series with it untill it is OK.

Cheers Matt.
 
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Probably a stupid question but what are you using for a heat sink on the regulator? Also most LM317 are not happy if the load current is significantly less than 10mA which you can usually cater to by setting the reference current to 10mA. (121 ohm resistor)

In terms of mm if the supply cap is within 50mm of the regulator there generally should not be a problem.

The 317 generally works fine with 30V supplies and can be operated at much higher voltages provided that the differential across it does not exceed about 35V. (I've used them in 300V supplies - similar to the Maida without problems, I believe something SY does to this day.)
 
I would second the comment on heat. Is the regulator a TO-220 package? If it is, these are suitable for current up to 1A, which should be fine for your application. You could push it even higher with suitable heat sinking.

If it is in a TO-92 package, it will be unsuitable for your application, as the heat generated will cause it to become unstable after a few seconds of operation.
 
Hi guys,

they are brand new µA7810 from TI, TO220 package and right now with no load at all. (I want to make it work before plug anything!). And I've tried two, no heatsink here it's ice cold ;)
As this 15/16V will feed LM317 wich can stand more dropout voltage (up to 40V in the datasheet!) it may be easier for me to give them lovely heatsink.
I still had no time to check as yesterday night I had to try to catch a bat wich came to check my bedroom and this morning a magpie try to steel a withe handkerchief, jump everywhere including my CDP pcb and parts etc...
http://img37.imageshack.us/img37/8323/dsc00052rki.jpg

What do really matter is sound, do some of you have experienced some sound behaviour difference with or without 78xx pre-regulator?
 
Nice magpie:D

It must be something with the Loire Valley. We used to go there every couple of years, beautiful. We had a bullfinch type bird in the bedroom one night and another night some kind of small lizzard fell on the bed. Needless to say I thought it was hillarious, the better half was mortified ,which I guess is why I was so amused;-)

Anyway, regulators. Why do you think the 7810 is current limiting?

Having no load could cause a problem with the 7810. It will definately with a 317. Also check your rectifiers. If you have a bridge and any are short/open circuit (usually fail short circuit) this will give weird symptoms.

Some form of heatsink is a good idea. Charging that big cap. could cause it to overheat and go into thermal limiting, the cap discharges into the following regs then the 7810 recovers only to overheat again. I would think this would happen rather slowly though.

Cheers Matt.
 
Hi Matt,

well I must confess that when my wife was shouting to death for a lovely "couleuvre" in the living room (my translator says "grass snake") It was very funny :D
The big black spider on the wall is another story wich is not fun at all for me, I'll have to check under the bed several times and still feel bad... should have kept the bat!
Fellow DIYers are welcome when the come around :) Always something to fix/try or just listen to music!

As for the regulator I'll get rid of it and will go for direct feed from the main big cap. If and only if some regs get hot I'll may use the 7810 with no output cap but a small film one.
And yes 7805 were running fine there with the 15/16V... why bother?

With the 2200/25 ZL it was oscilliating seems as without it was working nice.

Thanks,
Matthieu
 
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