mwl168

Technical Assistance/Advice Thread

49 posts in this topic

Hi All:

Ignorant people like myself and newbies often need various technical advices and assistance when dabble in this wonderful hobby. Many of these are general topics that do not specifically apply to one specific device. These valuable advices and suggestions are scattered in various threads over time and difficult to find. I thought it may be a good idea to have a dedicated thread where these discussions and information reside. So I thought I'll start this thread and see if it gets traction. 

So here goes my first question - inrush current limiter and fuse. My thinking has always been that the main fuse need to meet  two main requirements; 1. provide protection to both the device and user when the device malfunctions and 2. able to survive the inrush current when the device is first powered on. To me, these two requirements are somewhat contradictory. A high current rating fuse that can survive the power-on inrush current, with the help of inrush current limiter(s), may not provide proper protection against device malfunction and vise versa.

For my PSU for Kevin's ES amps, is it better to use one single inrush current limiter and a higher current fuse or to use a lower current fuse and use two inrush current limiter in series? 

Also, do I need to worry about the voltage rating of the inrush current limiter (in my case a NTC thermistor)? What's the actual voltage drop on the inrush current limiter during power-on? 

I would really appreciate your experience, thoughts and advices.

Thanks!

Edited by mwl168

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I use the Firstwatt build guides as a reference, so with Kevin's stat amps using 300-400va transformers

(In my limited experience) Inrush is probably not needed but do it anyway, and use his .33nf X rated cap also.

with fuses I am comfortable with something in the 2-4 amp range, even though it will not protect the circuit boards

With the 1000va Uber amp It is Very much needed and I have many blown up parts to prove it.

17 minutes ago, mwl168 said:

Also, do I need to worry about the voltage rating of the inrush current limiter (in my case a NTC thermistor)? What's the actual voltage drop on the inrush current limiter during power-on? 

they go on line side right, or are you using them on secondary's?

voltage drop would depend on which NTC,  they come in all shapes and sizes, just as hard to pick as a transformer

check data sheet for your NTC

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1 hour ago, congo5 said:

they go on line side right, or are you using them on secondary's?

voltage drop would depend on which NTC,  they come in all shapes and sizes, just as hard to pick as a transformer

check data sheet for your NTC

Thanks for the replies.

My question is specifically targeted at when using NTC thermistors on secondaries, in my case, 330vac secondaries. The data sheet says 110vac/250vac but I am pretty sure Mouser listed them at 400vac when I ordered them (although I am unable to find that information now). I searched quite extensively and could not find any thermistors rated higher then 110/250vac. 

Since the thermistor is wired in series, would it really see the whole secondary voltage drop?

 

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30 minutes ago, mwl168 said:

would it really see the whole secondary voltage drop?

If you were using 400ma and had a 50R thermistor then the drop would be 20v, but is there more to it?  is it 10-50 ohms?

my general concerns are how to hold the part, (prevent shorts) and protect nearby parts from the heat, 200c

do you have part number?

when you use too small of one they explode nicely and light up with a nice big spark

43 minutes ago, Bespav said:

I have built this and it works, and is nice to bypass any series resistance, if it worries you.

Edited by congo5

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It's a 15 ohm 3 amps NTC thermistor, Mouser part number: 954-15D2-13LD. Right now I use one each on the two 325vac secondaries on my Antek AS-3T325 (rated at 300VA with 325v/450ma x2 and 6.3v/4a x2 secondaries). It energizes a GRHV PSU which I use to power my KGSSHV, HV Carbon and Blue Hawaii so I doubt the PSU ever draws more than 200mA on each rail. 

The thermistors are mounted on a PCB for exactly the reasons you mentioned - to give it proper cooling and prevent shorts and coming into contact with other parts and wires.

The soft start circuit looks interesting. I'll need to read the article carefully. My current PSU chassis may not have room for them though...

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I am by no means an expert on this, but I thought thermistors went in the primary wiring? Nelson Pass seems to use CL60s a lot, wired in the primary.

527-CL60

He may have changed, but this is what I remember. Hopefully some of the larger amp builders will chime in.

This thread has some discussion of this.

 

Edited by Pars
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30 minutes ago, mwl168 said:

My current PSU chassis may not have room

its tiny, like 1.5"SQ

 

32 minutes ago, mwl168 said:

It's a 15 ohm 3 amps NTC

that works, a bit large, 3A rating with lets say .5A steady state through it. so it will bleed off some voltage

it will not get hot enough to lower resistance to stated .48 ohms but doubt you will modulate voltage much with the draw from headphones

some have ratings for max surge / or capacitance values it can support so the inrush doesn't kill it.

18 minutes ago, Pars said:

Hopefully some of the larger amp builders will chime in.

Yes that would be Nice.

What I know is from messing with the Uber.

Ntc's on the line side help protect the house wiring and all the way through the primary winding and some past that.

On the Uber I have two, one 15A NTC per 500va on the primary.

But without any on secondary with 188,000uf I kept blowing bridges which when shorted  blew the NTC's ,fuses,and melted some wiring.

so added the project39 boards on secondary to handle cap inrush current

still thinking of adding a rail protection circuit to shut off power to amp boards if problem is detected, also then could add delay on start.

something like this:  http://wiringradar.blogspot.com/2013/03/short-circuit-protection-for-balanced.html

using mosfets for relays  http://www.geofex.com/article_folders/ampprot/dcprot.htm

 

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1 hour ago, Pars said:

I am by no means an expert on this, but I thought thermistors went in the primary wiring? Nelson Pass seems to use CL60s a lot, wired in the primary.

527-CL60

He may have changed, but this is what I remember. Hopefully some of the larger amp builders will chime in.

This thread has some discussion of this.

 

I do have either a CL60 or CL70 between the primary wiring and the IEC socket of the PSU. I added an switch on the HV secondaries and put two extra thermistors in series so together they function as  a manual HV delay switch when I use the PSU for the Blue Hawaii. 

And going back to my questions in my first post; 

Is it better to use one single inrush current limiter and a higher current fuse or to use a lower current fuse and use two inrush current limiter in series when the power-on inrush current is much higher than steady draw current of the amp? 

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The fuse should calculated as follows:

Total VA transformer rating divided by line voltage times 133%.

In rush configuration should be whatever is necessary to make this value work in practice, given the capacitance in the power supply. This might be a thermistor or may need to be a relay switching a resistor.

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Is it Better? I think it comes down to builders choice. what makes you comfortable.

a lower fuse is "safer" for the transformer but will a 2A save a meltdown vrs a 3 or 3.5?

if the transformer or our wiring shorts it will blow either size fuse, and neither will protect our amp boards.

I have yet to see a picture of Spritzer putting a thermistor in his amps, have you?

which two?  Two 10R thermistors in series are same as one 20R, Providing they have the same amp rating.   I have 5R up to 125R and 2A to 15A

nothing wrong with using two to get the resistance value you want or pick one larger one.   series though never in parallel

Rod Elliott has the best information that I have found on this.

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How about looking up Quicklag fuses....?

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The soft start circuit looks interesting. I'll need to read the article carefully.


The more powerful your trafo is - the more care about inrush you must take.
Simple NTC is not a good choice, why heat around components? You can better provide relay shunt with 2-3 sec lag.

Based on your home mains quality you must take care about DC component too. Powerful toroids can provide 50/60 Hz hum even with 0,2-0,3 Volts of DC in your mains.


If you aren't familiar with AC mains - be absolutely careful and better take good known PCB/Kit like this:
https://www.ebay.com/itm/132036677853

First turn-on must be made with 40-60 Wt incandescent lamp in series with your circuit. Continuous glow will tell you about errors.

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Following @luvdunhill'a formula (which he posted here before), I use ~3A fuses on all my stat amps (including a Carbon; NA voltage), never had any problems with inrush current. No lights dimming either — I have more trouble with commercial microwave ovens. I seem to recall Birgir confirming on the KGSSHV thread (or maybe the KGST one) that he does not use thermistors.

I'm sure it's a different story with something that draws way more currenrt, like a Circlotron.

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the rest of the power in the circlotron is pure AC for the filaments, so not much of a turn on whoosh either

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I was hoping to keep this discussion as a generic topic but it seems time to describe my specific case which triggered my question.

I built a dedicated PSU to be used with my KG ES amps (KGSSHV, HV Carbon, Blue Hawaii and in the future, the Grounded Grid and maybe CFA) as can be seen at the Carbon Build thread. The PSU contains a GRHV, GRLV and have two Antek transformers (300VA for GRHV and 50VA for GRLV).

 

I have a 3A slow blow fuse and a CL60 (IIRC) thermistor on the IEC. I also have one 15ohm/3A thermistor on each of the two 325v secondaries which are controlled via a second power switch.

When I use this PSU to power the HV Carbon, I keep the second power switch in the "On" position and only use the main power switch to turn on and off the amp and the 3A mains fuse has no problem handling the power-on inrush.

I tried using this PSU to power the Blue Hawaii this past weekend. I kept the 2nd switch in the "Off" position, turned on the main switch which energizes the GRLV and the filament supply to the 4 EL34's. After 30 seconds or so, I flipped on the second power switch which controls the 325vac secondaries to the GRHV. The 3A fuse blew within a second or so.

I replaced the 3A fuse, used a variac to power on the PSU and the amp worked fine.    

I was really puzzled - why did the 3A fuse blow when I flipped on the second power switch? Shouldn't the inrush current be lower in this case?

Anyone has a theory of what happened?    

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BH draws more power and you found the limit of a 3A fuse

what reads 120v on your meter from a Variac may be less VRMS than line

or    ?????????????????????????????????????????

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I never use thermistors and usually use 3.15AT fuses in all my amps.  2AT is fine for the KGSSHV at 230V. 

For a 300VA transformer you need at least 4A. 

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if the transformer or our wiring shorts it will blow either size fuse, and neither will protect our amp boards


I have had this fault actually happen and it was protected by a ground loop breaker using a thermistor and half of a bridge. See the arrangement in the F5 Turbo First Watt project.

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yea I always pull up his guides when wiring a chassis

but hadn't thought of that in reverse, what happens when the ground gets momentarily flooded with 120vac, waiting for the fuse to blow.

bad very bad, just another reason to follow his layout.

thanks

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5 hours ago, spritzer said:

I never use thermistors and usually use 3.15AT fuses in all my amps.  2AT is fine for the KGSSHV at 230V. 

For a 300VA transformer you need at least 4A. 

I'd agree with that rule of thumb. For 500VA I've generally needed 6.3AT.

The unfortunate thing is that this rating is way above what is needed once the transformer is energized. So in general I use a butch Omron relay with a time delay of a half second or so (which is overkill). The relay shorts out a power resistor in the range 4.7 ohm - 10 ohm in series with the transformer primary. The choice of resistor is quite important, since it has to deal with a high inrush current in the tens of amps range for some tens of milliseconds. So it has to be significantly overrated to be reliable in the long term. I tend to use an aluminium clad 25W resistor screwed to the chassis a-la-Krell.

Having said all that, I have never had any problems at all with the power-on of either the T2 clone or the BH original.

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Hey, man's, 200-300 VA trafos can be used without inrush limiters, especially with 20-30 mF caps after them.
Primary and secondary windings active resistance and inductance would effectively limit inrush currents.

Try to turn-on something bigger, say 1,3 kWt trafo with 200 mF and i have no doubt you would hear "click" from your 25 A protective breaker.



MWL168, let's speak about different question.
How much Joules of stored electrical energy we actually need per 1 Watt of output power?
2? 10? 50?

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Thanks all for chiming in. This is exactly what I was hoping for - to learn from your knowledge and empirical experience.

The issue Craig mentioned is the problem I was hoping to solve with the thermistors - the fuse rating needed to survive power-on inrush is way above what is needed once the amp is fully energized. Sounds like I have over simplified the issue. 

@Bespav: sorry I don't have the knowledge to answer your question.

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On 1/29/2017 at 11:05 AM, congo5 said:

I use the Firstwatt build guides as a reference, so with Kevin's stat amps using 300-400va transformers

(In my limited experience) Inrush is probably not needed but do it anyway, and use his .33nf X rated cap also.

 

I read the Firstwatt F5 and F5 Turbo article, the capacitor is listed on the schematic as .0033uf I think. So 3.3nf instead of .33nf?

Do you have a part number for the part you use?

Also, the schematic is different between the original F5 and the later F5 Turbo. I assume the F5 Turbo one is the one to follow?

Thanks! 

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