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Old 28th February 2013, 12:00 PM   #71
MarianB is offline MarianB  Romania
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Quote:
Originally Posted by Elvee View Post
...To make it completely transparent, it should be placed upstream of the regulator...
Good ideea, in fact here is an Iadj variant, this config permits precission current adjusting, or it can be set just above 2A in witch case it will not interfeer with voltage regulation untill after the treshold has been reached, an ahmmetter wired across the shunt can show the load current.
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Old 28th February 2013, 02:32 PM   #72
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by dietnews View Post
a fuse to the transformer's secondary in case of a short circuit. i would like to have a little wiggle room to overload the LM317, for when testing homemade amplifiers, without blowing the fuse while still providing protection in the case of a short circuit. would a 2.5A fuse be a good choice?

also, i'm interested in setting up a crowbar circuit but honestly i'm not really sure what situation it applies to. i dont think i've ever experienced an "overvoltage." what causes that?
A properly dimensionned fuse in the primary is all you need, see Andrew's answer.

A crowbar is a crude overvoltage protection that prevents extensive damage when everything else has failed. Complicated to implement on a variable supply, and not useful in your case, forget it.

Quote:
Originally Posted by MarianB View Post
Good ideea, in fact here is an Iadj variant, this config permits precission current adjusting, or it can be set just above 2A in witch case it will not interfeer with voltage regulation untill after the treshold has been reached, an ahmmetter wired across the shunt can show the load current.
That variant will mostly probably have stability issues: when you throw the gain and phase shift of an opamp into the game, compensation is almost always required. Better to keeps things simple and predictable.
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Old 28th February 2013, 07:41 PM   #73
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Quote:
Originally Posted by AndrewT View Post
fit a close rated (CL) fuse in the primary circuit of the transformer.

Normal fuse for a transformer:
Fuse Rating = VA / Mains Voltage * 3

Close rated fuse for a transformer:
CL Fuse Rating = VA / Mains Voltage
VA / Mains Voltage = 0.454

is that right? i want a 0.5A fuse on the primary side? i guess it makes sense that there would be way less current on the primary..

for some reason this power supply had a 10A fuse on the primary when i bought it.
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Old 28th February 2013, 10:13 PM   #74
JMFahey is offline JMFahey  Argentina
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Quote:
is that right? i want a 0.5A fuse on the primary side? i guess it makes sense that there would be way less current on the primary..

for some reason this power supply had a 10A fuse on the primary when i bought it.
Did Mr "for some reason" look like any of these?
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Old 28th February 2013, 11:46 PM   #75
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Did Mr "for some reason" look like any of these?
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Old 1st March 2013, 09:47 AM   #76
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by dietnews View Post
a 0.5A fuse
Usually this will be marked:
T500mA 250V

If this ruptures too often at start up, then fit a soft start. DON'T fit a bigger fuse.
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Old 1st March 2013, 11:06 AM   #77
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T500mA is a Slow blow.
F500mA is a fast blow...

Choose the slow blow as Andrew suggested...
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Old 1st March 2013, 12:14 PM   #78
FoMoCo is offline FoMoCo  United States
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Quote:
Originally Posted by dietnews View Post
... i want a 0.5A fuse on the primary side? i guess it makes sense that there would be way less current on the primary..
I think you're cutting it too close. Usually a fuse should only carry 80% of it's rated current. The fuse is primarily there to prevent a fire. I'd use a 1A slow-blow. You may or may not need a soft-start. But, definitely get that 10A out of there.
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Old 3rd March 2013, 10:08 PM   #79
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thanks for all the advice. i went ahead and replaced the LM338 with a LM317K and the 18V/50VA transformer with a 24V transformer. i also replaced the fuse with a 500mA one. the new transformer doesnt have VA or current labelled but it is physically larger then the old transformer so i am assuming it is fine to use. now i have a safe to use adjustable 1.2-30V/1.5A supply.

i just need to have some small voltage and current meters to fit on the front panel.

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