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Old 6th November 2012, 11:51 AM   #11
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Are you sure it's not a thermal issue?

You are feeding the regulator with 12.73VDC (9* 2^0.5) so a voltage drop of 6.23V. For a current of 2A you are dissipating 12.46W

If we assume a room temp of 25C and a Rth of 5C/W we get a max temp of 25 + (12.46*5) = 88C. The shutdown limit is 125C.

I guess it's not a thermal issue


Still quite toasty so I would add a small heatsink.
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Old 6th November 2012, 02:15 PM   #12
DF96 is offline DF96  England
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Cold heater current could easily be 3 or 4 times normal. That might push it over the top. Once the 317 shuts down the heater stays cold. Could you 'slug' the 317 with a fat capacitor on the adjust pin so the output voltage rises only slowly? Or add a resistor in series with the output and increase the 317 output to compensate.
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Old 6th November 2012, 02:40 PM   #13
CSlee is offline CSlee  United Kingdom
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There is definitely something wrong, the good reg will not work at all if the bad reg is connected. I guess I should of not shared the recitifier etc at all.

The markings on the bad 317 have vanished so that might suggest over heating, so I will replace it and test again. It may be as DF96 suggests as the regs are pushed to well over 1.5A on start up.

Currently I have a 10uF on the Adj to ground, is that big enough? I have some voltage to play with so can also try a resistor on the output.
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Old 6th November 2012, 03:20 PM   #14
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In my experience LM317s tend to be fairly rugged things. I've abused a fair few leaving some shorted for extended periods of time, with them getting ridiculously hot and then working just fine with the short removed. I've also used a few in situations with less then adequate heatsinking (fail the finger test but not beyond spec) and I've never had a problem. I've also never blown one either despite the abuse.

I'd be looking for an overlooked implementation error, like a dead resistor or a small solder bridge or something. Either that or one of the regs is faulty. If connecting the unloaded, second regulator, up to the shared power supply causes the functioning regulator to stop working, then this implies that the second regulator is drawing way too much current and is causing the unregulated voltage to drop significantly.
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Old 6th November 2012, 03:55 PM   #15
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Hi,
Why you do not try the LM338 5 amps linear voltage regulator? Also it may be that the initial current it is too high since the filament resistance it is low when cold.
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Old 6th November 2012, 04:05 PM   #16
jitter is offline jitter  Netherlands
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Quote:
Originally Posted by CSlee View Post
There is a total of 6 valves split into 1 reg feeding 4 and the other feeding 2. I tested the working reg with the other section of valves and it works fine, so I am assuming that there is an issue with the reg itself.
OK, if the good reg powers either circuit just fine, the fault must ly with the bad regulator.
Even though these are very rugged devices that are protected against thermal and electric overload, they can fail.

Quote:
Originally Posted by CSlee View Post
There is definitely something wrong, the good reg will not work at all if the bad reg is connected. I guess I should of not shared the recitifier etc at all.
If the outputs of the regs are not interconnected (which I assume based on your answer of separated loads), then it must be the voltage on the pre reg side that gets pulled down, perhaps by an overload. Measure the voltage on the 10,000 uF cap in the setup quoted above and compare it to the voltage when only the good reg is connected (and powering its load), there shouldn't be too much difference. If there is, it looks like the faulty reg draws much more current than it should.

Quote:
The markings on the bad 317 have vanished so that might suggest over heating, so I will replace it and test again.
That's a sign. Another distinct sign of overheating is when the metal tab has gotton a blueish discouloration.

Quote:
It may be as DF96 suggests as the regs are pushed to well over 1.5A on start up.
That may be the case and we're indeed dealing with tolerance issues between the regulators. Replace it first and see what happens.

Quote:
Currently I have a 10uF on the Adj to ground, is that big enough? I have some voltage to play with so can also try a resistor on the output.
The cap between ADJ and GND mainly improves ripple rejection, but in the datasheet of the LM117/317 there's a "slow turn on" circuit in the application notes (page 16). This might be interesting for preventing those high current peaks at power on by (relatively) slowly letting the voltage on the output rise to desired voltage. This would "preheat" the heaters and lower the initial current draw.

Last edited by jitter; 6th November 2012 at 04:13 PM.
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Old 6th November 2012, 05:31 PM   #17
AndrewT is offline AndrewT  Scotland
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CCS drive for the heaters does not suffer from current surge at start up.
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Old 6th November 2012, 05:33 PM   #18
CSlee is offline CSlee  United Kingdom
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Thanks for all the comments so far, all very useful.

I tried a few different setups and seperated both supplies completely, and there was still instability now in both regs. So after some more trial and error I found that actually the common mode chokes were causing the issue. I wired out both chokes and now both regs are working fine, and have been stable for the past hour. They are running at about 60 degrees C so within their spec.

I will keep trying to get back to the original schematic as there must be a reason for the instability.

Can anyone offer suggestions on this?

Cheers

Charlie
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Old 6th November 2012, 05:35 PM   #19
CSlee is offline CSlee  United Kingdom
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Originally Posted by AndrewT View Post
CCS drive for the heaters does not suffer from current surge at start up.
Thanks Andrew, I had not even concidered this option but will look into it.
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Old 6th November 2012, 05:40 PM   #20
DF96 is offline DF96  England
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A regulator is essentially an amplifier with heavy feedback. Fiddle with gain or phase margin by adding reactance and you can create instability. Did you follow any advice in the datasheet about local decoupling etc.? Where were the chokes? Why use common-mode chokes; they seem very popular nowadays but very little PSU noise really is mainly common-mode.
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