[tomb]

Looking at Tangent's STEPS article, he lists slo-blo BUSS fuses in everything from 0.1A to 0.5A for the line side of the transformer. However, he doesn't discuss how these ratings are selected, and they don't have a one-to-one correspondence with the various STEPS VA rating selections.

That's question enough with BUSS fuses. However, Polyfuses are even more confusing. They have distinct advantages for constructing DIY walwarts (SOHA, TREAD-in-a-walwart, etc.): small size and no need for replacement. Unfortunately, the specs are confusing. There is a "Hold Current" and a "Trip Current," typically sized at 0.2 & 0.4, 0.4 & 0.8, 0.6 & 1.2, and so on. The "Trip Current" is always twice the "Hold Current."

If you read the literature, "Hold Current" is defined as that current required to maintain a closed circuit. Does that really mean a current less than that will drop out the transformer? In any event, with Trip Current twice as much, I am confused on how to size one.

In our headphone amps for instance, if a Polyfuse is sized for 0.1A (equates to about 500ma @ 24V on the line voltage side of 110V), does that protect the transformer if it doesn't trip until 0.2A? (about 1A @ 24V) If the headphone amp doesn't draw a full 500ma, does that mean the transformer will drop out from too little current on the Polyfuse?

So, how do I size a Polyfuse to "HOLD" the necessary current, but at the same time, "TRIP" before damage takes place, depending on transformer rating?

 

[amb]

Quote:

Originally Posted by tomb If you read the literature, "Hold Current" is defined as that current required to maintain a closed circuit. Does that really mean a current less than that will drop out the transformer? In any event, with Trip Current twice as much, I am confused on how to size one.

I think the hold current is just a maximum rating that is guaranteed not to cause the polyfuse to trip. You can run at a lower current and the fuse will maintain continuity.

Quote:

In our headphone amps for instance, if a Polyfuse is sized for 0.1A (equates to about 500ma @ 24V on the line voltage side of 110V), does that protect the transformer if it doesn't trip until 0.2A? (about 1A @ 24V) If the headphone amp doesn't draw a full 500ma, does that mean the transformer will drop out from too little current on the Polyfuse?

The transformer will not drop out from too little current. And yes a 0.1A polyfuse would certainly offer plenty of protection for a 1A transformer. Note that the polyfuse is even slower than the usual "slo-blo" buss fuses.

 

[tomb]

Thanks, Amb - I needed a sanity check. This stuff is not documented on Raychem or Littlefuse, etc.

 

[Tomo]

Hey, Tomb.

My Magnequest Transformer came with a sheet telling me the max current rating and VAC. Didn't your tranny come with something like it?

This tranny happened to be for 1.5A 15V Linear Regulator. I bought for some Class-A amp. Anyways, the sheet from Magnequest told me to use a fuse with rating around 200mA.

I tried using 125mA fuse for this tranny and it immediately blew. So I had to go back to Radio Shack for 250mA fuse. I think the tranny draw more current when turning on and off.

You are right, this fuse business is quite complicated. Anyone has a definitive documentations? (That is not EE Textbook.)

T

 

[amb]

Quote:

Originally Posted by Tomo I tried using 125mA fuse for this tranny and it immediately blew. So I had to go back to Radio Shack for 250mA fuse. I think the tranny draw more current when turning on and off.

This is why slo-blo fuses are usually spec'ed for this application - to allow turn on surges (yes, the transformer itself causes a surge as well as charging the capacitors).

 

[Tomo]

Cool,

I thought we use slow-blow for some other reasons. (125mA fuse was a slow-blow ... It blew anyway.)

But I am more interested in knowing which fuse go for which VAC tranny. People at manufacturer's outlet stores can only give me vague answers.

Thanks, Amb. (in advance

)

Tomo

 

[amb]

Yeah, your 125mA fuse was probably just too small for the application. The transformer is only part of the total equation, you'd have to figure out how much current draw (steady state as well as dynamic) of the entire circuit in order to come up with a suitable fuse rating. You can calculate that numerically as a starting point, but you then need to refine it with actual testing and then give it some headroom, so you don't blow fuses needlessly and still have enough protection against malfunctions.

 

[tomb]

Yep - ditto what Tomo said. There is little, if any, documentation on sizing fuses for xfmr's. The Polyfuses seem most convenient, but that "hold current" was really throwing me. I think if you sized the Polyfuse for the transformer's max ampacity, it would be safe. After all, we're not looking for loading our transformers/walwarts much more than 50%. So combined with the already slow reactive nature of the polyfuse, that should be enough for inrush, too. (I hope.)

It's a little worse for a polyfuse if that guess is not correct, though. It's not like unscrewing the fuse holder and popping in another fuse: the polyfuse has to be soldered in place.

 

[Tomo]

Hehe,

I am sure you can fit a poly-fuse into place no one would dare to think!

T

 

[luvdunhill]

yea, I'm still confused on this one as well. The dual primary/secondary toroid I'm considering recommends 100mA SLO-BLO for 230V operation and says to double the fuse current for 120V operation. Then it gives max secondary range numbers, and then has a recommendation for 125mA SLO-BLO and recommends doubling for 120V... I'm assuming since I will be putting the fuse in the IEC inlet, then I need a .2A fuse, not a .25A fuse which I'd use if the fuse was after the toroid...right?!

 

[tomb]

The primaries are at 120V, while the secondaries are at 24V (or whatever). Put another way, the VA rating of the transformer is Volts X Amps. Therefore, the primaries (where you want the fuse) will have five times less amperage than the lower-voltage secondaries.

So, if your transformer is rated for 1A at 24V, you'd want a 1/5 = 0.2A fuse on the line side (primaries). (24VA/120V = 0.2A)

I think they meant 0.2 on the primaries. They just interchanged 0.2 with 0.25 - the exact number probably doesn't fall right on top of fuse increment. It's also not that critical for a transformer.

 

[luvdunhill]

Quote:

Originally Posted by tomb The primaries are at 120V, while the secondaries are at 24V (or whatever). Put another way, the VA rating of the transformer is Volts X Amps. Therefore, the primaries (where you want the fuse) will have five times less amperage than the lower-voltage secondaries. So, if your transformer is rated for 1A at 24V, you'd want a 1/5 = 0.2A fuse on the line side (primaries). (24VA/120V = 0.2A) I think they meant 0.2 on the primaries. They just interchanged 0.2 with 0.25 - the exact number probably doesn't fall right on top of fuse increment. It's also not that critical for a transformer.

okay, this helps. So the transformer is 25.0 VAC, so current is 25.0VA/120V = 0.2A, so this is where you got the 0.2A fuse rating. cool thanks!!! So, I'll be feeding 22V to each TREAD and a maximum of 0.341A with the Amveco 62055 toroid.

Just to make sure I understand, if I take the Avel Y236006 transformer (which I cannot find where to buy, but the specs are on partsexpress.com), it is 15VA, so current is 0.125A and I would feed 25V to each TREAD and a maximum of 0.3A and use a slo-blo fuse of 125mA...

 

[tomb]

Quote:

Originally Posted by luvdunhill okay, this helps. So the transformer is 25.0 VAC, so current is 25.0VA/120V = 0.2A, so this is where you got the 0.2A fuse rating. cool thanks!!! So, I'll be feeding 22V to each TREAD and a maximum of 0.341A with the Amveco 62055 toroid. Just to make sure I understand, if I take the Avel Y236006 transformer (which I cannot find where to buy, but the specs are on partsexpress.com), it is 15VA, so current is 0.125A and I would feed 25V to each TREAD and a maximum of 0.3A and use a slo-blo fuse of 125mA...

YES.

 

[Tomo]

Hey,

Thanks guys. I don't need to make a fool of myself (even though I am) every time I go shopping.

(Hey, I deserve to "look" intelligent sometimes!)

T