Circuit Breaker value for Presto 1D

[basplin]

I've read a few threads here about suggested fuse values for these heads, but I want to try and use a small circuit breaker with my Presto instead of a fuse. The part I'm questioning though has to do with these breakers not tripping once the level has been reached. Depending on the level they are subjected to, they could take up to an hour to break or just a few seconds. For example, with the .25A and .50A breakers I've found, they will each run forever at 100% of their rating, one hour at 175%, 4.5 - 28 seconds at 200%, and 6 seconds at 400%. I know fuses generally break with a peak over their ratings, but these don't do that obviously.

My question is how much can a Presto 1D (or 1C for that matter) take, and for how long? Which of the aforementioned breakers would provide adequate protection while allowing the strongest signal?

[piaptk]

I use 1amp fast blow breakers in all of mine and have never blown a head. I know some people use 2amp fast blow as well, but i like to play it safe.

[basplin]

Oh wow...and I here I was thinking that using a 0.5A fuse was "playing it safe", haha!

[EposLab]

500mA fast-blow fuse is a real safe value for the head and as long you cut with flat eq you will get loud cuts. But things change when you play with the eq and try to optimize the sound. When you boost the highs then the 500mA are not enough and you have to go up 1A but thats not good for the coils. The more the highs in the sound the more the heat on the coils. When Jesus ( Ilias @agnewanalog) rebuilt my 1-d head he made aluminium bobbins for the coils and use a specific resin while he was winding the coils. That resulted a "one-body" bobbin with fast heat absorption! Jesus suggested a 500mA as a real safe option. Also mentioned to keep the heat of the coils low (the cooler the better) . The copper wire he used can handle heat more than 70°C before it starts to melt the insolation , and since its impossible to measure the coil core temp , we agreed that better to never exceed the overall coil temp of 50°C !
While my lathe has 270LPI feedscrew i do not really make loud cuts. But since i try to optimize and boost the highs abit i use a 630mA fast blow fuse to keep my 1-d safe!

Bellow is a generic Fast Blow fuse Time-Current chart that might help you compare and choose the correct ressetable fuse for your application!

Fast-Acting-Fuse-Graph.jpg

*Sorry but i couldn't find a generic chart for the ressetable fuses as they differ by company!

[basplin]

This is fantastic, thanks for the info! I was also looking for a chart like this, very helpful...

[markrob]

Hi,

Actually, it is possible to measure the voice coil temperature using the temperature coefficient of the copper wire in the coil as a temperature probe. I've used this approach when working on my own DIY heads. See the attached paper for more info. I have made a simple circuit board to make this easier, but you can do on a breadboard on even on the bench.

Real Time Measurement of Voice Coil Temperature.pdf

Mark

[EposLab]

@basplin please note that this Time-Current chart is a generic for Fast Blow glass fuse and not for the ressetable one! You can just get and idea for the case you use a glass fuse!

@markrob thanks for uploading this document, It lokks very interesting!
As for my real time thermal measurement , i use my Flir thermal camera (the closest i can under the hood!)!

[Dogtemple]

Hi,

Actually, it is possible to measure the voice coil temperature using the temperature coefficient of the copper wire in the coil as a temperature probe. I've used this approach when working on my own DIY heads. See the attached paper for more info. I have made a simple circuit board to make this easier, but you can do on a breadboard on even on the bench.

Real Time Measurement of Voice Coil Temperature.pdf

Mark

Out of interest, what sort of temperatures were you getting or are to be expected from a voice coil in a cutting head?

[markrob]

Hi,

I've attached some testing I did some years ago on a Dayton DAEX13CT-4 exciter using the method I described in the earlier post. I ran the test under the same conditions with and without ferrofluid to see what the effect was on thermal efficiency. The files are PDF printouts of MathCAD calculation sheets. If you read through, you will see the final coil temperature and the calculated thermal resistance in deg C /Watt.

Hope you find that helpful.

Mark

DAEX13CT-4 With Ferrofluid.pdf

DAXE13CT-4 Without Ferrofluid.pdf

[Dogtemple]

Nice one thanks, I’ll check that out

[basplin]

so here's another question... rather than a fuse, if I wanted to put a milliamps meter to just monitor the current between the amp and the cutterhead, would I get a meter that measures AC or DC?

I know the amp outputs an AC signal (+ & -), but I'm wondering if DC would work if I just attached it on one lead, or AC if I attached across both leads? Or do I have this wrong?

[markrob]

Hi,

I would not lose the fuse. That is still your protection (as limited as it is). Adding current monitoring is a nice thing depending on the meter type and if you understand how to interpret the readings. If you decide to make a current reading you can do it by breaking one of the leads to the head and inserting the meter in between. The meter needs to read AC current. I would try to measure the average current over a long term like a 1-10 seconds to give you a better idea of the power the head is dissipating. Digital meters are not as easy to read in an application like this since the numbers can be changing rapidly. A bar graph LED meter or analog style is probably better. Inserting some analog meters in line like this can cause issues with the audio signal since they may not be a low linear impedance.

Mark

[basplin]

Hi,

I would not lose the fuse. That is still your protection (as limited as it is). Adding current monitoring is a nice thing depending on the meter type and if you understand how to interpret the readings. If you decide to make a current reading you can do it by breaking one of the leads to the head and inserting the meter in between. The meter needs to read AC current. I would try to measure the average current over a long term like a 1-10 seconds to give you a better idea of the power the head is dissipating. Digital meters are not as easy to read in an application like this since the numbers can be changing rapidly. A bar graph LED meter or analog style is probably better. Inserting some analog meters in line like this can cause issues with the audio signal since they may not be a low linear impedance.

Mark

Thanks markrob - this is exactly what I needed to know. Good to know on the fuse, I'll keep one inline. That's a good 'firewall' to have. I am hoping to use an analog meter, and it sounds like what you're saying is that one of these needs to have low impedance? What level impedance are we talking about here?

[markrob]

Hi,

Yes, you don't want to introduce very much extra resistance between the amp and the head. Its also possible that if the meter uses a rectifier to convert the AC to DC that it might cause some unwanted effects on the signal. It may be hard to find analog current meter that fits the bill.

Mark

[basplin]

Hi,

Yes, you don't want to introduce very much extra resistance between the amp and the head. Its also possible that if the meter uses a rectifier to convert the AC to DC that it might cause some unwanted effects on the signal. It may be hard to find analog current meter that fits the bill.

Mark

Thanks, markrob. So I'm reading that there seem to be two different kinds of A/C meters - one that uses a rectifier, another that uses an Iron Vane Movement. I'm assuming the Iron Vane Movement would be what I'd want for this application?

As for impedance, this is something I'm having trouble understanding, so apologies for the (potentially) stupid question. If something says impedance (@ 60HZ) is 0.065Ω, would this be considered high? My basic understanding has been the farther away from 0 the value is, the lower the impedance is, but maybe that's an over-simplification or I just don't understand.

[markrob]

Hi,

You want the current meter to have the least impact on the signal going to the head. Since its in series with the head, that means you want its impedance to be as low (close to zero) as possible. 0.065 ohms is very low and would not have much impact on your audio. You also have to select a meter that has the proper full scale range you need. I'm thinking a amp or maybe 2 would be in the right range.

Mark

[basplin]

2 amps is exactly what I was thinking as well. Thank you!