To fuse or not to fuse (on the secondary side of the transformer)?

[moamps]

I have one anecdote about fuses and NTC resistor in power supply.
I received an huge audiophile amplifier from a friend at a service with a fault that the mains fuse burns out when it is turned on. I checked everything and set the rated fuse and everything worked ok. What actually happened? The amplifier has a soft start and a NTC resistor connected in series with the mains transformer to avoid large inrush currents. When the amplifier is turned on, the NTC limits the current with its high resistance but it starts to heat up and its resistance decreases so that the transformer gets full voltage in a short time. The NTC remains active in the circuit but is hot and its resistance is low. What was the friend actually doing?
He tested the audibility of silver and gold fuses (expensive) by turning on an amplifier with a standard fuse, the amplifier would work for a while, then he would turn off the amplifier, replace the ordinary fuse with an audiophile one, and turn on the amplifier again. And then the audiophile fuse would burn out, simply because the NTC resistor failed to cool down for that short time, increase the resistance, and decrease the inrush current.

It was a rather expensive experiment.

From my experience the worst problem in general with power supplies is the short-term loss of mains voltage, which can cause a lot of damage especially on computers power supplies where ordinary fuses are not too helpful.

 

[trobbins]

I'm not across what silver/gold fuses are available, but I am aware of the performance of the most common 5x20mm 250VAC rated fuses that are compliant to IEC 60127-2 and UL 248-14 and used by most manufacturers and sold by most main stream suppliers - with the IEC type common in Europe and many other countries, and the UL type in USA. The IEC spec fuses have much better specified performance limits than the UL, and therefore by design allow a much better confidence in knowing what fault conditions they must blow for. Having specified performance limits, those fuses can be selected to not blow under given power-up and continuous current conditions, and to blow for given levels of fault current.

Aiming to use a fuse should be based on much more than having a box of available fuses, especially for secondary side fusing where the type and rating of fuse should imho be much more aligned and designed for the circuitry being protected.

Even mains side fuses can often be optimised for protection of secondary side faults, especially when the value/type of a mains side fuse has been chosen to meet product safety requirements for a range of countries (eg. using a 2A fuse when a 1A fuse may well be adequate for local conditions and the setup of equipment).

 

[deveng]

Old thread but content is somewhat relavent for me this week! I have 2 old Peavey CEQ 280a equalizers that I repaired a long time ago (havent' used in a long time). They were in a rack and powered on when I used other units in the rack. Recently one of them did not power up. After investigating I found not only a burned LM340-T5 regulator but a open secondary and blown transformer primary fuse. I didn't remember this but there is no inline fuse or power switch for this unit. I don't know which came first the transformer fail or regulator fail but either way its seems dumb to not have an inline fuse. I don't have an issue with no power switch. So now I can't simply replace the failed components but have to replace the transformer as well. Since these are old, not needed and worth almost nothing I will most likely junk it. But, still I think its silly to design a unit this way.

 

[CJ]

Cheap insurance ,

Weird that the fuse is blown and the open secondary, both would have to occur at the exact same time, possibly more problems? Call Peavey, they might have a pwr xfmr or buy a junk one on ebilvay,

 

[deveng]

Cheap insurance ,

Weird that the fuse is blown and the open secondary, both would have to occur at the exact same time, possibly more problems? Call Peavey, they might have a pwr xfmr or buy a junk one on ebilvay,

Cheap for builder, not so cheap for user/repair. My mistake, I meant to write the transformer primary was open and blown primary fuse.

 

[abbey road d enfer]

Cheap for builder, not so cheap for user/repair. My mistake, I meant to write the transformer primary was open and blown primary fuse.

Looks like the xfmr went too hot, shorting out turns, which increases the secondary voltage, for a time, until the primary draws too much and blows the fuse.
A fuse on the secondary may have saved the regulator, but as is often the case, silicon protected fuse.

 

[JohnRoberts]

I don't recall details inside the CEQ but if it had an internal transformer there was probably a thermal fuse inside the transformer that opened when it overheated.

Did you try asking Peavey Service...?

JR

 

[Johnny1234]

I like to use fuses if possible. Much cheaper than transformers.

 

[deveng]

I don't recall details inside the CEQ but if it had an internal transformer there was probably a thermal fuse inside the transformer that opened when it overheated.

Did you try asking Peavey Service...?

JR

Yes a fuse in primary opened as well as the primary winding. Didn't ask Peavery service as I might just dump this thing.

 

[deveng]

I like to use fuses if possible. Much cheaper than transformers.

If I would have noticed this when I was originally repairing years ago, I probably would have added a fuse. Oh well, too late. I think I will do it for the working unit though.

 

[moamps]

Looks like the xfmr went too hot, shorting out turns, which increases the secondary voltage, for a time, until the primary draws too much and blows the fuse.

This is not possible, IMO. A shorted section of the primary will prevent the ratio from increasing and in fact the secondary voltage will drop.

What probably happened here is a standard scenario where the load on the regulator has increased excessively (shorted capacitor or some active element), leading to regulator failure, secondary short circuit, primary overheating, loss of insulation of the primary copper windings and ultimately blowing the fuse on the primary side.

 

[vintageampfixer]

One has to question whether the fuse fitted to the primary side was the correct value, per the maker's spec, and indeed whether that's actually right. Generally speaking, if the primary is fused correctly then overload to the secondary ought to blow the fuse. If it's a very low power unit though, it's sometimes difficult to find a fuse weak enough to give protection but strong enough to carry the running load ... preferred values, and all that.

* Edit - that said, I also recall some witticism along the lines of "a transistor protected by a fast-acting fuse will blow first to protect the fuse"

 

[deveng]

This is not possible, IMO. A shorted section of the primary will prevent the ratio from increasing and in fact the secondary voltage will drop.

What probably happened here is a standard scenario where the load on the regulator has increased excessively (shorted capacitor or some active element), leading to regulator failure, secondary short circuit, primary overheating, loss of insulation of the primary copper windings and ultimately blowing the fuse on the primary side.

This would be my guess too, or the regulator simply failed short irregardless of the capacitor condition.
The fuse is labled as TF141C (Microtemp?). Transformer will not be fixed so this is just post mordem review. Sure seems like terrible protection design.

TF141C
15A/125V
141degress C