Hello! Have had a problem with my lm3886 chip amp since I built it. The problem is just that the fuse (2A) blows instantly when turning on the amp. But this just happens like every 15 times I use it. Thats why it is so weird. And the fuse for the amplifier is not the only one that blows. My house's fuse box also blows for the floor where the amp is placed. Adding a drawing of the the circuit before the transformer, and a picture of the amplifier for you to see.
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Hi!
Sometimes the inrush current of big transformers (especially toroidal) can cause the fuse to blow.
I cannot estimate it by picture, how is the wattage of your transformer?
Best regards
Sometimes the inrush current of big transformers (especially toroidal) can cause the fuse to blow.
I cannot estimate it by picture, how is the wattage of your transformer?
Best regards
Perhaps there are two different problems going on...
With a 2 Amp fuse, you shouldn't blow the house fuse, unless the circuit is so loaded to near the limit that the starting surge for the amp puts the house fuse over the limit. Of course, there might be something even worse going on, which is a little scary, so please be careful.
As to the 2 Amp-fuse itself blowing every 15 times...What timing is the fuse?
A fast blow fuse will typically have the problem you cite...the issue is charging the capacitors, and the phase of the AC line when you turn on the power.
Medium or Slow blow fuses are probably more appropriate for this fuse position.
Here's a place where I learned the hard way about this:
GT-101 Troubleshooting
Search the page for blown fuses 1...
With a 2 Amp fuse, you shouldn't blow the house fuse, unless the circuit is so loaded to near the limit that the starting surge for the amp puts the house fuse over the limit. Of course, there might be something even worse going on, which is a little scary, so please be careful.
As to the 2 Amp-fuse itself blowing every 15 times...What timing is the fuse?
A fast blow fuse will typically have the problem you cite...the issue is charging the capacitors, and the phase of the AC line when you turn on the power.
Medium or Slow blow fuses are probably more appropriate for this fuse position.
Here's a place where I learned the hard way about this:
GT-101 Troubleshooting
Search the page for blown fuses 1...
This is where I bought it, a datasheet is provided on the right side of the page, but I couldn't see anything about the wattage..
88016 - Toroidal power transformer 2 x 25V 10A 500VA - Europe Audio
Edit: The fuse I bought was supposed to be slow blow, but when I now look at the package it says: "type: FST" don't now if this means fast blow?
88016 - Toroidal power transformer 2 x 25V 10A 500VA - Europe Audio
Edit: The fuse I bought was supposed to be slow blow, but when I now look at the package it says: "type: FST" don't now if this means fast blow?
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Using a Mains Bulb Tester prevents the mains fuse blowing even when you have completely messed up the mains wiring.
A 500VA transformer fed from 220Vac needs a T6A fuse if you start it directly on line.
If you use a soft start you can use a T2A fuse. The soft start will need a resistance of aprox 50r and that will need to be bypassed after about 100ms to 300ms
A 500VA transformer fed from 220Vac needs a T6A fuse if you start it directly on line.
If you use a soft start you can use a T2A fuse. The soft start will need a resistance of aprox 50r and that will need to be bypassed after about 100ms to 300ms
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A 5-6 A slow-blow fuse may solve the problem with the fuse within the amp blowing, but it will do nothing to address the house fuse blowing.
It's rather surprising that the house fuse blows, actually. Usually they can withstand 100 % overload for over an hour. If the circuit is already heavily loaded, the inrush of the transformer may be enough to push it over the edge.
I suggest measuring the amount of current drawn by the transformer when the amp is running. When the amp is idle, there should hardly be any current draw. A few mA plus the magnetizing current of the transformer. If you see more than that, there's something wrong.
If nothing is wrong with the transformer or the rest of the circuit, a soft-start may be the way to go. Those are not trivial to design. The components need to be able to survive start-up, the soft-start needs to get out of the way once it's done its job, and it needs to re-engage if the power glitches.
I'd be surprised if you need a soft start circuit, though. I used to have two 500 VA transformers turn on in my power amp without a soft start. Even on 230 V, I never popped a fuse.
Tom
Thank you for many good suggestions, I will look into it and report back when/if I find the problem
Hi again, i luckily figured out the problem. After a simple check the amp fuse actually doesn't blow, I only thought it did. But the house fuse blows, and this is because it is only rated 10A. With some computers and other stuff connected it's not that strange it blows some times
Yeah, but it's a 10 A, extremely slow blow fuse. You're either running right at 10 A on that circuit and the amp happens to pop the fuse or something is wrong in the amp. Many European countries now use 16 A fuses. You may want to talk to an electrician to see if you can upgrade the circuit to 16 A.
Do you have the option for testing the amp on a dedicated circuit? Maybe unplug your fridge for a few minutes so you can use that circuit...
Another option would be to build a Light Bulb Tester. The light bulb will effectively work as an inrush current limiter. It should blip bright when you turn on the amp and extinguish after maybe a second. If it remains lit, you have a problem in the amp.
Another question for you: What's the capacitance of your supply caps?
Tom
Do you have the option for testing the amp on a dedicated circuit? Maybe unplug your fridge for a few minutes so you can use that circuit...
Another option would be to build a Light Bulb Tester. The light bulb will effectively work as an inrush current limiter. It should blip bright when you turn on the amp and extinguish after maybe a second. If it remains lit, you have a problem in the amp.
Another question for you: What's the capacitance of your supply caps?
Tom
You could add a CL-60 in series with the fuse and it will act as a soft start. Anything over a 400va x-former should include a soft start circuit for just this reason.
Do the CLs get hot enough during low-power operation to get out of the way once the soft start is no longer needed, though?
Another issue is that if the CL gets hot enough, it takes a while for it to cool off. Hence, when power-cycling the amp, you may still pop the fuse.
Tom
Another issue is that if the CL gets hot enough, it takes a while for it to cool off. Hence, when power-cycling the amp, you may still pop the fuse.
Tom
The Power Thermistor starts off cold.
It has a big resistance.
On start up the current is limited by the Thermistor resistance and it gets hot, internally. The resistance drops and the outside starts to warm up.The power dissipated reached a peak and dropped off a bit as the thermistor warmed up.
One the transformer has started, the start up current drops to near zero and now the smoothing capacitance charges up. The Thermistor warms up a bit more and the resistance drops a bit. As the caps reach full charge the current drops significantly and the warm thermistor starts to cool. The resistance goes back up to it's warm value.
The amp is now ready to run, with the warm thermistor and it's warm resistance in the feed to the primary.
The primary will see a variable voltage when ever the current draw on the smoothing caps changes.
The above describes a lot of variable thermistor resistance. It is not just a hot thermistor and a cold thermistor. The temperatures are all over the range from cold to very hot.
It has a big resistance.
On start up the current is limited by the Thermistor resistance and it gets hot, internally. The resistance drops and the outside starts to warm up.The power dissipated reached a peak and dropped off a bit as the thermistor warmed up.
One the transformer has started, the start up current drops to near zero and now the smoothing capacitance charges up. The Thermistor warms up a bit more and the resistance drops a bit. As the caps reach full charge the current drops significantly and the warm thermistor starts to cool. The resistance goes back up to it's warm value.
The amp is now ready to run, with the warm thermistor and it's warm resistance in the feed to the primary.
The primary will see a variable voltage when ever the current draw on the smoothing caps changes.
The above describes a lot of variable thermistor resistance. It is not just a hot thermistor and a cold thermistor. The temperatures are all over the range from cold to very hot.
A 500vA transformer needs a soft start circuit.
The inrush current is quite capable of tripping a type B MCB in a house distribution board.
Frank
The inrush current is quite capable of tripping a type B MCB in a house distribution board.
Frank
Thank you again for all replies.
A soft start would be good for this amp in this house, but I am soon moving to an apartment with hopefullly 16A fuse wich is normal. So changing the house fuse is not an option, that would be up to my dad in that case. The CL is interesting because of the limited space inside the amp and something I will consider in the future.
Sent fra min SM-G850F via Tapatalk
A soft start would be good for this amp in this house, but I am soon moving to an apartment with hopefullly 16A fuse wich is normal. So changing the house fuse is not an option, that would be up to my dad in that case. The CL is interesting because of the limited space inside the amp and something I will consider in the future.
Sent fra min SM-G850F via Tapatalk
Yes. That's how the thermistor-based inrush limiters work.
Um. No. Neglecting the magnetizing inductance, the main load during start-up is the "short circuit" imposed by the completely discharged reservoir caps. The inrush current will be limited by the DCR of the transformer windings and the ESR of the reservoir caps, along with any wiring resistance.
The magnetizing inductance will add a few mA to the inrush current. The reservoir caps may cause 10 A to flow in the transformer primary for a very short amount of time.
The thermistor may be warm from the inrush current, but it won't stay warm during idle or low-power operation. That's the problem. It'll increase the resistance of the primary circuit and increase the output impedance of the supply.
Even worse, the thermistor may end up lukewarm, in which case it will degrade the supply impedance and not provide any inrush limiting in the event of a short-term power outage or glitch.
The best solution is to short out the thermistor once the supply caps have charged.
Tom
Tomchr, I don't agree that the main current AFTER switching on is the charging of the capacitors.
When no limiter is present, I believe the biggest current is the transformer start up current.
Once the transformer has started then the transformer begins the charging of the capacitors.
The primary current during the charging phase is usually less than the transformer start up current.
It's the potentially enormous transformer start up current that really needs limiting.
The Power Thermistor is continually changing temperature and this resistance during the period after switch ON. We are agreed on this.
The added resistance of the Power Thermistor does impact on the source impedance of the PSU. We are agreed on this.
For best performance from the amplifier the PSU should see an near constant and low source impedance. It seems we agree on this as well.
In both of our different ways of assessing the Power Thermistor effects, we are agreed that it is best to bypass it soon after it has done it's start up current limiting job.
When no limiter is present, I believe the biggest current is the transformer start up current.
Once the transformer has started then the transformer begins the charging of the capacitors.
The primary current during the charging phase is usually less than the transformer start up current.
It's the potentially enormous transformer start up current that really needs limiting.
The Power Thermistor is continually changing temperature and this resistance during the period after switch ON. We are agreed on this.
The added resistance of the Power Thermistor does impact on the source impedance of the PSU. We are agreed on this.
For best performance from the amplifier the PSU should see an near constant and low source impedance. It seems we agree on this as well.
In both of our different ways of assessing the Power Thermistor effects, we are agreed that it is best to bypass it soon after it has done it's start up current limiting job.
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A single Power Thermistor of appropriate cold resistance is certainly the most compact system for limiting start up current.
Not the best, but good enough to allow a sensible size fuse to be used and not suffer nuisance blowing.
A transformer transforms impedance. Hence, a short circuit on the secondary will be transformed to a short circuit on the primary. When the power is turned on, there's no "oh, let me charge the transformer first, then see if there's any load on it" the transformer will impose a short circuit on the applied voltage and the fuse will blow. You can convince yourself of this pretty easily by modeling a power transformer in spice and running a simulation.
A discharged capacitor is pretty darn close to a short circuit... It's only a few 10s of mΩ at startup.
Tom
Yes, exactly that. More often with torroidal tx. It has all to do with momentary core saturation at switch on when there is residual magnetism in the core, so depends on the specific transformer. Unusual at only 500VA but perhaps the tx is conservatively rated.
I found this with two monobloc 750VA torroidal transformers in the same amp, would trip house mains breaker depending on the state of residual core magnetism and where luck would have the mains cycle. A soft start relay fixed it, placing a (correctly rated !) resistor in series with the primary until transformers had settled down.
I explored peak start solid state relays, but they were expensive - anyone know of an affordable part/source ?
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