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Old 31st January 2011, 03:34 PM   #1
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Default need advice on fuse value and rectifier

hi guys

i upgrade my psu cap to 50000uf.

would MUR860 do the job?
i am using 0-15v 5a transformer, would this transformer and rectifier handle the initial surge on charging the 50000uf caps?

i blow few 500ma fast blow fuse that i put on primary. whats the suggested fuse value here?

just wanna make sure i dont burn anything here

thanks in adv

erwin
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Old 31st January 2011, 03:44 PM   #2
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Put a NTC thermistor in series with the transformer, after the fuse. That way you don't have to worry about the fuse blowing anymore. The diodes you mentioned will be fine. If you can't find one in your local shop, you can find a NTC in any junked computer power supply.
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Old 31st January 2011, 03:54 PM   #3
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hi Th3 uN1Qu3

thanks for the fast reply and suggestion.

would i put the npc thermistor in series at the primary or secondary? what value? never use any thermistor before. any special note or area that i need to pay attention on using thermistor?

thanks,
erwin

Last edited by milen007; 31st January 2011 at 03:59 PM.
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Old 31st January 2011, 04:45 PM   #4
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I explained already. In series with primary after fuse. To be honest i don't know about ratings, i've always used whatever i found inside old computer power supplies. Thermistor part numbers aren't helpful in the least bit.

They are also known as "inrush current limiters", this document will help, see what you can find from your favorite distributor judging by that. 50k uF at 20 volts is 10 joules, so you need a thermistor able to handle at least double that since there is also the transformer. Another important value as far as i can see is the steady state current that the thermistor must handle - this is what your amplifier uses in normal listening after the capacitors have charged. Since you say that the fuse is 500mA, use one rated at 500mA or higher. Using one that is rated too high will result in current limiting in normal operation. But this might not be too much of an inconvenient if your amp is class A which i suspect it is.

This should be enough data to help you find a suitable one. Also check that resistance is high at room temperature but low at operating current - a thermistor that only offers 1 ohm resistance when cold won't be particularly helpful in your application.
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Last edited by Th3 uN1Qu3; 31st January 2011 at 04:53 PM.
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Old 31st January 2011, 05:11 PM   #5
fhs is offline fhs  Germany
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Quote:
Originally Posted by Th3 uN1Qu3 View Post
... Since you say that the fuse is 500mA, use one rated at 500mA or higher. Using one that is rated too high will result in current limiting in normal operation.
I beg to disagree. One reason for using an NTC thermistor is the fact that you do not want to you "too big" a fuse because that would be a potential safety hazard! "Too big" a fuse will definitely not be "current limiting".

It's fine to leave the NTC in series provided your chassis + any components in the neighborhood can handle the heat generated by the NTC thermistor. If you have any doubts, use a relay to short the NTC after it has completed it's job.
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Old 1st February 2011, 05:28 AM   #6
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hi fhs

so you are suggesting better to use soft-start circuit than the NTC thermistor? or combination of both?

this is the safety case, as i can only use 1a fuse instead of the 0.5a fuse. because the initial surge blown the 0.5a fuse while 1a fuse can take the surge.

whats the better way to do without sacrifice too much signal quality?

erwin

Last edited by milen007; 1st February 2011 at 05:32 AM.
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Old 1st February 2011, 09:54 AM   #7
AndrewT is offline AndrewT  Scotland
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You had the wrong fuse fitted.
your transformer is 15V 5A = 75VA.
The fuse for 220Vac supply is 3 * 75 / 220 ~ 1A.
Use a T1A mains fuse.

If you fit a soft start you can close rate the fuse, maybe as low as T300mA, but you will need a lot of resistance in the primary circuit to prevent the fuse wire from getting hot.
Most users of these small transformers say they don't need a soft start, but that commits you to T1A mentioned earlier, or T2A if 115Vac
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Old 1st February 2011, 10:11 AM   #8
fhs is offline fhs  Germany
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Quote:
Originally Posted by AndrewT View Post
...your transformer is 15V 5A = 75VA.
The fuse for 220Vac supply is 3 * 75 / 220 ~ 1A.
Use a T1A mains fuse.
No (not considering the <1 efficiency of your transformer): 75W at 230V = 75/230A <0.5A; 75W at 115V = 75/115A < 0.7A. OK, you have some leeway here and may use a fuse with a somewhat higher rating. For safety reasons I would not recommend going to 1A at 230V.

Quote:
Originally Posted by milen007
...so you are suggesting better to use soft-start circuit than the NTC thermistor? or combination of both?
Personally, I tend to use a relay that shorts my NTC. A soft-start circuit will also work. I believe that's a matter of personal preference.

All the best,

Fred
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Old 1st February 2011, 12:03 PM   #9
fhs is offline fhs  Germany
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Just a brief amendment to my last post: Have a look at Soft-Start Circuit For Power Amps -- the author (he may be member of this forum; I just do not know) warns against the use of NTCs and recommends a series resistor (+ a relay to short-circuit the resistor after it's job is complete) instead. I have used that combination in the past. Recently, I began to prefer NTCs, but had not been aware of "exploding NTC" issue.

BTW: As a rough rule of thumb, people often recommend a fuse that blows if the current exceeds 125%-150% of the expected current under normal conditions (no inrush).

Warm regards,

Fred
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Old 2nd February 2011, 01:57 PM   #10
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hi Fred.

thanks for the link. nice one.. ill learn from there.. and come back if i got any questions

why would you prefer NTC than the soft start circuit? the soft start degrade the sound?

erwin

Last edited by milen007; 2nd February 2011 at 02:00 PM.
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