Transformer fuse nuisance blowing

[halfgaar]

Hi,

I have an Amplimo 15-0-15 30VA transformer in use, with a recommended 0.16A slow blow fuse, which blows out occasionally upon switch on. It's hapenned twice now, once after about a week, and once after a few months.

How (un)safe is it to increase the fuse rating? Or, are there other solutions? I once did ask the shop about fuses with higher I²R ratings, but they only have one type.

[jarthel]

I've seen recommendations of 125 to 200% of actual current load.

you can connect a .1ohm 5W ceramic wirewound on the "hot" wire of the AC power cord (between the AC socket and the primary) and check the voltage across it to find actual current load. you can then adjust the fuse amperage.

[halfgaar]

Because the problem here is inrush current, measuring the normal current draw doesn't seem useful.

I need a fuse strong enough to survice the inrush current, but weak enough to let it blow on fault conditions. You're saying that sometimes, even 200% is used, meaning I have to use 0.32A? Seems kind of risky.

Would putting a low value power resistor in series with the primary help? I mean, It's only for a few opamps and a LED, it's not gonna draw serious power. I was thinking like 5 Ohms perhaps?

[mrshow4u]

Why not keep the fusing (0.16 A SB) and add a *Glo-Bar to the incoming AC feeding the power transformer.

*Glo-Bar is a NTC thermistor

[halfgaar]

Basicly, that would be the same as putting in a 5 Ohm resistor.

I have two of those 5 Ohm NTCs recommended for high VA Amplimo transformers. I used to have them in series with the primary of a 625 VA transformer for a 4 channel power amplifier. Their resistance doesn't drop at all, at least not from the idle current of the amplifier (idle load is 25W on the mains). In that particular amp, I replaced them with a decent softstart circuit.

[mrshow4u]

halfgaar said:

I used to have them in series with the primary of a 625 VA transformer for a 4 channel power amplifier. Their resistance doesn't drop at all, at least not from the idle current of the amplifier (idle load is 25W on the mains).

Hmmmmm, It sounds like they weren't getting hot enough. The "Dynamic Range" for thermistors to limit in-rush starts at maybe 100, 60, 40 ohms cold and gets down to an ohm or two Hot. The one's I've seen in power amps seem to get Hot right away and stay hot at idle. Maybe those thermistors just aren't the right ones? Maybe the R vs. T curve just isn't right. I love the Glo-Bar. You will too if you have torroidal transformers and power switches.

[halfgaar]

The ones I have are 5 Ohm cold, and stayed 5 Ohm... They have to be the right ones, as they are mentioned in the datasheet of the transformer.

I love the Glo-Bar. You will too if you have torroidal transformers and power switches.

The softstarter I use is a bit more robust. It starts the transformer with a ballast, and then bypasses that with a relay after a few AC cycles.

[quasi]

If we calculate 230v * 0.16 amps = 37va. This is not much more than the transformer rating of 30 va. So I would be quite comfortable increasing the fuse rating to 0.2 amps or even 0.25 amps slow blow.

My $0.02

Cheers
Quasi

[sawreyrw]

If the line voltage is 115, the current would be .26 A (min). I think you could use a larger fuse.

[halfgaar]

Line voltage is 230, as usual in Europe. 30 VA for 2x 15V would be 0.13, exactly half of what you'd have with 115. But, I would say it's 0.13 max, not min. 0.16A would seem to be the correct fuse.

I think I'll be more insistent about high I²R rated fuses, as the datasheet of the transformer specifies. I'm not too comfortable with increasing the fuse rating.

How much current is a standard 30 VA transformer likely to draw on the primary when it's shorted, BTW?