Hello!
I am adding a 'raised ground' to one of my power amps, a relatively common circuit that is simply a capacitor, 10R resistor and bridge rectifier in parallel. You can see it described on page 15 of this excerpt from The Valve Wizard's preamp book.
One curiosity for me is the 6A current rating that he suggests for the bridge rectifier. This seems like a helluva lot for something that is designed to blow a fuse under fault conditions! Most BR are rated for relatively high surge current, which seems like the more appropriate rating to looking at, rather than continuous forward current.
Should I just pay attention to the surge current rating when spec'ing a BR for this circuit in my own amp?
Thanks in advance for any help 🙂
I am adding a 'raised ground' to one of my power amps, a relatively common circuit that is simply a capacitor, 10R resistor and bridge rectifier in parallel. You can see it described on page 15 of this excerpt from The Valve Wizard's preamp book.
One curiosity for me is the 6A current rating that he suggests for the bridge rectifier. This seems like a helluva lot for something that is designed to blow a fuse under fault conditions! Most BR are rated for relatively high surge current, which seems like the more appropriate rating to looking at, rather than continuous forward current.
Should I just pay attention to the surge current rating when spec'ing a BR for this circuit in my own amp?
Thanks in advance for any help 🙂
I'm afraid I don't understand why. In order for 6A to be inadequate, there must be a DC signal consistently present on the bridge rectifier, between the audio ground and the chassis ground, at 1 to 1.1V (depending on the Vf of the part) and above 6A. The power consumption on the rest of the circuit would have to be pretty incredible at this stage, blowing the fuse, no?
The diodes have to conduct the worst case fault current until the RCD (aka GFI) trips.
Most 15 A mains trip circuits will take peak currents for 1-3 cycles of 10x the rated trip current.
You don’t want the ground lifter diode(s) to open circuit before the mains trips potentially leaving the chassis at hot (aka live) potential.
On safety issues it’s always important to make the following statement:-
The ground lifter does not break the safety ground connection the the amplifier chassis and associated metalwork. The incoming ground connection must bond securely to the chassis. The isolated secondary side electronics are what gets floated by the ground lifter.
Most 15 A mains trip circuits will take peak currents for 1-3 cycles of 10x the rated trip current.
You don’t want the ground lifter diode(s) to open circuit before the mains trips potentially leaving the chassis at hot (aka live) potential.
On safety issues it’s always important to make the following statement:-
The ground lifter does not break the safety ground connection the the amplifier chassis and associated metalwork. The incoming ground connection must bond securely to the chassis. The isolated secondary side electronics are what gets floated by the ground lifter.
Okay that's what was missing from my mental model. Thanks for clarifying 😀
On a similar but different topic, the 10R resistor being rated for 5W also seems overkill at first blush. For that kind of power, ~5.5V would develop across the resistor and the parallel bridge rectifier, by which stage of course the BR would be conducting in full and be drawing far more current than the resistor.
Am I equally wrong in that assessment 😛
Am I equally wrong in that assessment 😛
Mains shorts are very violent events (*), with very high current spikes. Magnetic forces alone could do damage to a small flimsy component carrying such a high current pulse. The rectifier internal bond-wires may well vaporize on sub-millisecond timescales... Basically you want all the protective earthed parts to be securely bonded together, normally < 0.1 ohms is allowed between any exposed metal parts and the earth pin of the mains cable (measured at high current, not with a multimeter!).
I witnessed one which took out 3 fuses including the main fuse for the section of the large building it happened in. It also destroyed a mains extension lead by coating its insides in a layer of copper. That gave me mild shell-shock for a few weeks...
I witnessed one which took out 3 fuses including the main fuse for the section of the large building it happened in. It also destroyed a mains extension lead by coating its insides in a layer of copper. That gave me mild shell-shock for a few weeks...
Perhaps I am miscommunicating: The only part of the circuit that will be 'raised' above earth is the audio signal ground. The chassis and the earth pin on the mains will remain at equal potential.
So in theory the bridge rectifier and the resistor may 'only' be subject to fault conditions like shorting a rail to audio ground. That is of course, in theory and this theoretical scenario will still present significant voltages/current. I guess it's probably a good to expect it to sustain at least the 3 and a bit Amps that a 2A slow blow mains fuse might decide to pump out before dying? Any AC component will be passed on it's merry way to the chassis both in normal operating conditions and under fault conditions through a 100nF NP0 ceramic cap.
It all depends where the fault current flows to - if a live wire comes adrift inside the case you want it to blow the breaker whatever it touches ideally - so bond the PSU ground to the case (in which case audio ground is also bonded so a lifter can't help). Another approach is double-insulation (and/or reinforced insulation), to prevent contact physically, but that affects which kind of transformer or SMPS you can use.
Fair enough - That makes sense to me. I may question how likely it is that this would be an issue to worry about in a real world situation - in this amp, the live wire is being kept short to transformer and if it did break off and short somewhere, the only metal within its reach would either be neutral or the chassis (bonded to the earth pin).
Internal transformer shorts can also happen unless its split-bobbin. Especially if the transformer overheats and insulation deteriorates.