Soft start is the term applied by most DIYers to the time/current required to get the toroid transformer working.Magura said:
Slow charging may be an appropriate term to apply to controlling the current through the rectifiers.
Magura,
you are confusing the issue (and probably Lordvader) by referring to your peak charging current of the smoothing capacitors, but calling it soft start.
Well the discussion has been very interesting thus far !
I'm definitely going to put some form of soft start. Still not 100% whether to use a thermistor per transformer, or a softstart per transformer.
I'm about to order 2 300VA transformers. However, these only have a single 240V primary. Where would the best place for the thermistor be in this scenario ? The only options would be to put the thermisor on the hot leg (in series with the fuse), or on the return leg.
I'm definitely going to put some form of soft start. Still not 100% whether to use a thermistor per transformer, or a softstart per transformer.
I'm about to order 2 300VA transformers. However, these only have a single 240V primary. Where would the best place for the thermistor be in this scenario ? The only options would be to put the thermisor on the hot leg (in series with the fuse), or on the return leg.
Hi,
A soft start can be either a Power Thermistor or a Power Resistor in the primary feed. The option is whether you decide to add a delayed bypass (500mS) to the Power Thermistor version. The Resistor version MUST have a delayed bypass (300mS).
I would recommend that the "resistance" be placed in the Live feed, to avoid the scenario of the soft start going open circuit and leaving the whole primary side live even though the amp appears dead.
If you have a large smoothing capacitance, you can also consider adding a slow charge circuit into the secondary before the capacitance. A Power Thermistor again with optional delayed bypass (3seconds) works well.
I recommend that each transformer has it's own T1.25A fuse and that the amplifier has an input fuse rated to match the downstream cables.
Do you use plug top fuses in Australia? This should be rated for the input flex current limit.
A soft start can be either a Power Thermistor or a Power Resistor in the primary feed. The option is whether you decide to add a delayed bypass (500mS) to the Power Thermistor version. The Resistor version MUST have a delayed bypass (300mS).
I would recommend that the "resistance" be placed in the Live feed, to avoid the scenario of the soft start going open circuit and leaving the whole primary side live even though the amp appears dead.
If you have a large smoothing capacitance, you can also consider adding a slow charge circuit into the secondary before the capacitance. A Power Thermistor again with optional delayed bypass (3seconds) works well.
I recommend that each transformer has it's own T1.25A fuse and that the amplifier has an input fuse rated to match the downstream cables.
Do you use plug top fuses in Australia? This should be rated for the input flex current limit.
AndrewT said:
Thanks for the reply, but that sentance kinda went over me ...
I do plan to give each transformer a 1.25A fuse, but what do you mean by input fuse to match downstream cable ? Aren't the 1.25A fuses the "input" fuses ?
And what are these plug top fuses ? Do they differ from the standard 3ag (i think that's right) type fuses ?
lordvader,
Just to straighten a couple of things out for you.
1. No we do not use plug top fuses, so don't worry about that.
2. There should be a fuse before the transformer and then each of the rails should be fused. The transformer fuse should be rated according to the safe operating limits of the transformer. The rail fuses should be appropriate to the amount of current draw you expect on each rail.
Just to straighten a couple of things out for you.
1. No we do not use plug top fuses, so don't worry about that.
2. There should be a fuse before the transformer and then each of the rails should be fused. The transformer fuse should be rated according to the safe operating limits of the transformer. The rail fuses should be appropriate to the amount of current draw you expect on each rail.
Hi,
In the UK, we are required to protect ourselves from our own folly.
Belt and braces, that are not universal around the world.
The general rule is that a fuse rating must be smaller than the smallest unfused cable that is suppled by the fuse.
At the distribution board there may be a 30A fuse or breaker. That demands that the down stream cable rating must be greater than 30A. In domestic, we use a twin and earth ring for each power circuit, each half of the ring is usually >=2.5sqmm with a current rating of 17A to 20A depending on cable environment. Two of these comply with the >30A breaker.
At the socket outlet in the wall, we fit plug top fuses between 1A and 13A. This allows the equipment cable to be smaller than the main distribution cables around the house. If we fit a 3A fuse in the plug top that allows an equipment cable of 1sqmm (~6A) to be used to feed the appliance.
Inside the appliance the manufacturer usually fits a further fuse (for the countries that don't have plug top fuses) This is usually fitted very soon after the input cable enters the appliance and can be incorporated in the IEC connector socket.
You have two transformers. If you fuse each @ T1.25A then these could draw around 2.5A from the mains and not rupture the input fuse in normal operation.
The input fuse (IEC?) would have to be >=3A to allow operation for all normal circumstances.
A 300VA will probably need about 50r to 70r in the primary feed to avoid nuisance trpping on start up. You may have to experiment with the resistor values. I use a bank of 10r 5W radial resistors for this and add as many as I need to give reliable start up.
Does that make sense?
Following up on rail fuse rating.
I find that a rail fuse of about half the peak current into your minimum resistive loading allows reliable operation and even sustained full power testing. i.e. F3A allows 6Apk to a resistive load. That is equivalent to 48Vpk into 8r0 = 144W into 8r0.
In the UK, we are required to protect ourselves from our own folly.
Belt and braces, that are not universal around the world.
The general rule is that a fuse rating must be smaller than the smallest unfused cable that is suppled by the fuse.
At the distribution board there may be a 30A fuse or breaker. That demands that the down stream cable rating must be greater than 30A. In domestic, we use a twin and earth ring for each power circuit, each half of the ring is usually >=2.5sqmm with a current rating of 17A to 20A depending on cable environment. Two of these comply with the >30A breaker.
At the socket outlet in the wall, we fit plug top fuses between 1A and 13A. This allows the equipment cable to be smaller than the main distribution cables around the house. If we fit a 3A fuse in the plug top that allows an equipment cable of 1sqmm (~6A) to be used to feed the appliance.
Inside the appliance the manufacturer usually fits a further fuse (for the countries that don't have plug top fuses) This is usually fitted very soon after the input cable enters the appliance and can be incorporated in the IEC connector socket.
You have two transformers. If you fuse each @ T1.25A then these could draw around 2.5A from the mains and not rupture the input fuse in normal operation.
The input fuse (IEC?) would have to be >=3A to allow operation for all normal circumstances.
A 300VA will probably need about 50r to 70r in the primary feed to avoid nuisance trpping on start up. You may have to experiment with the resistor values. I use a bank of 10r 5W radial resistors for this and add as many as I need to give reliable start up.
Does that make sense?
Following up on rail fuse rating.
I find that a rail fuse of about half the peak current into your minimum resistive loading allows reliable operation and even sustained full power testing. i.e. F3A allows 6Apk to a resistive load. That is equivalent to 48Vpk into 8r0 = 144W into 8r0.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.