Need help choosing mains fuse value

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#23
#24
* Normal operating current (The current rating of a fuse is typically derated 25% for operation at 25C to avoid nuisance blowing. For example, a fuse with a current rating of 10A is not usually recommended for operation at more than 7.5A in a 25C ambient.)
* Overload current and time interval in which the fuse must open.
* Application voltage (AC or DC Voltage).
* Inrush currents, surge currents, pulses, start-up currents characteristics.
* Ambient temperature.
* Applicable standards agency required, such as UL, CSA, VDE.
* Considerations: Reduce installation cost, ease of removal, mounting type/form factor, etc.
Click to expand...
This passage from Discrete's link show a number of factors that influence fuse value selection.
Inrush currents, surge currents, pulses, start-up currents characteristics.
Click to expand...
transformers and motors require a starting current that far exceeds the normal running current.
This requirement overrides all other if nuisance blowing at start up is to be avoided.

Andy5's
A 225VA transformer at 115V should draw about 2A. It is usual to specify a fuse at 3 X operating current.
Click to expand...
is particularly aimed at transformers and is applicable to Audio equipment.

If a lower value fuse is required, then some form of soft start must be used.

I repeatedly advise soft start circuit and close rated fusing.
 
#26
An externally hosted image should be here but it was not working when we last tested it.



A couple of things to change. Change the 7.5ohm 10 watt to a CL-80 or CL-90. And replace the electrolytic across the coil with a standard protection diode. It only needs to be active for like a half second or less. Caps are 80% charged in a half second and full charged in 2 sec, and thats if you have a 10 ohm resistor.

This is the simplest and full proof. The only hard part is selecting the relay voltage and the resistor in series with the coil. Id pick a relay coil voltage that is close to my B+, and then tune it with the resistor so that the relay on sees the needed voltage to flip. I just did this with my t-amp. I had a 28v rail, and a 24v relay, then i just put a 500 ohm resistor in series with the relay coil, and it dropped the relay voltage down to 20v, which in my case was perfect for the coil voltage.
 
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#27
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Are the thermistors some kind of inrush limiter across hot and cold secondary of transformer?
And the psu ground is connected to chassis?
 
#28
Um...

A thermistor (NTC) is ran in series with the power switch as a inrush limiter on the primary side.

Unlike a 10 watt resistor, a NTC can handle the full amplifier load if the inrush limiter circuit fails. Every pc power supply has a ntc, and lot of amps use them with no bypass. So, cold the ntc is 10-100ohms, and then drop to below a half an ohm in the second it takes to heat up.
 
#30
spst. Open till engaged.

File:Relay symbols.svg - Wikipedia, the free encyclopedia

it bypasses the inrush limiter when engaged. Id run the coil from the +- rails so you equal the load. Use the highest coil voltage you can get thats slightly below your rails. So if your rails are +-25vdc, then get a 48v coil and tune it with a series resistor. So you have 50v across your 48v relay, so you put a series resistor in it to eat say, 10v, so the coil sees 40v. a 48v relay doesnt need 48v to trigger, odds are it needs less than 40v. Like i said my 24v relay worked great at 20v. And that keeps the coil heat down and lowers the current that the relay uses.

http://www.components.omron.com/components/web/PDFLIB.nsf/0/4F7D9D9CA026C97E862573930070AC4F/$file/G5LA_E_0609.pdf

G5LA-1A4-DC48

This guy want 75% of coil voltage so 36 volts.
 
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