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Old 26th February 2013, 08:42 PM   #51
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Theoretically, the transformer secondary should be about 1.6x the DC output. Here's a concise reference for power supply types.
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Old 27th February 2013, 04:38 AM   #52
MarianB is offline MarianB  Romania
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Quote:
Originally Posted by Elvee View Post
...when cold, it will initially limit at 3A, but as soon as it heats up, the limit will drop: to 1.5A..
Where do you get of with this ridicoulus so called advices? and better yet, what gives you the right to consider yoursel better then those that wrote a component's pdf? cus that is just what you have implyed when you last quoted me.

Now LM317 limmits the current accordingly with the Vin-Vout differential, don't believe me, just take a look at the attachement... oh i forgot, you are way better than the ones that produced the component, ofcourse you know better then them... Anyway, at a shortcuircuit on the output things will be just about the same for the transformer, the regulator will be protected but it will put great stress on the power transformer just like the othe regulator, nothing new here. Now don't missunderstand me, i like LM317, it is simple and cheap, most of the time an easy fix for small jobs, but let's not make it more than it is.
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Old 27th February 2013, 08:31 AM   #53
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by MarianB View Post
Now LM317 limmits the current accordingly with the Vin-Vout differential, don't believe me, just take a look at the attachement... .
That is something else: it is the SOAR protection. It reacts instantly and depends little on temperature.
I am talking about the thermal limitation: when the regulator dissipates ~25W or more, the heat flow through the 4°C/W of the regulator itself plus the interface to the heatsink will already be sufficient to bring the die to the temperature of thermal safety, even on a cold, large heatsink. The current will be reduced to stay below the limit, and when the heatsink begins to heat up, the current will be reduced further.
That is the reason why I said the heatsink must not be too large, so that it heats up faster than the transformer.
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Old 27th February 2013, 09:08 AM   #54
JMFahey is offline JMFahey  Argentina
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Quote:
That is something else: it is the SOAR protection.
You say it as if it were a different thing.
SOAR protection is a type of current limiting.
Which is exactly shown in the graph.

Just in case the type is too small to read, in the bottom of the graph it says:
Quote:
Figure 6 . Current Limit
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Old 27th February 2013, 09:10 AM   #55
MarianB is offline MarianB  Romania
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The maximum current obtainable from LM317 is at a range of about 4 to ~11V Vin-Vout difference, the graphic shows that in that range at 25deg junction temp the max current is just over 2A ( maybe 2,2 or somthing like that ), at 150deg that current drops to just below 2A ( maybe 1,8-1,9A ), that is all you need to know cus i doubt verry much you will allow the junction temp to rise above 150 no matter what.
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Old 27th February 2013, 10:26 AM   #56
MarianB is offline MarianB  Romania
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Sorry for the double posting but i cannot edit the last one. I was playing around with Multisim and tested a bit LM317, the immages speak for them selves:
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Old 27th February 2013, 10:53 AM   #57
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What are you trying to say?

That the simulation shows 6.5 A with shorted output contradicts your previous graph from the datasheet which says the current limit is 1 A at a 25 V input-output differential.

If some simulation model of unknown origin contradicts the manufacturer's datasheet it's probably the datasheet that should be trusted!
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Old 27th February 2013, 11:17 AM   #58
Elvee is online now Elvee  Belgium
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Quote:
Originally Posted by JMFahey View Post
You say it as if it were a different thing.
SOAR protection is a type of current limiting.
Which is exactly shown in the graph.

Just in case the type is too small to read, in the bottom of the graph it says:
The LM317 and similar regulators include three different types of protection: plain current limiting at low I/O voltages (up to ~12V, flat part of the graph), SOAR for I/O >12V and overtemperature protection, when the die T° exceeds ~160°C.
All of these mechanisms result in current reduction, because that is the only parameter the chip can control, but they are completely distinct.
They all operate concurrently: the first limit that is exceeded dictates the output current

Quote:
Quote:
Originally Posted by MarianB View Post
Sorry for the double posting but i cannot edit the last one. I was playing around with Multisim and tested a bit LM317, the immages speak for them selves:
Your multisim model is obviously wrong: it contradicts the datasheet and the actual behavior of a 317.
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Old 27th February 2013, 11:26 AM   #59
MarianB is offline MarianB  Romania
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Quote:
Originally Posted by megajocke View Post
What are you trying to say?..
Transistor helps, that is true no matter what some people say.
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Old 27th February 2013, 11:35 AM   #60
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The expected behaviour for a 1.25 V three terminal regulator with a high internal current limit is shown in the attached graphs.

The X-axis is the output voltage and the green graph shows the output current as a function of the output voltage. As can be seen, the current limit doubles at low output voltage.

The reason can be seen in the red graph which shows Q1 Vce. The transistor saturates below 0.6 V output voltage. When the output is shorted (0 V) the full 1.25 V reference voltage will fall over the sense resistor, giving an ouput current of twice the nominal value.
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