Amplifier transformer input current on the primary (FBR and Caps)

[killingtime]

Hi,

I'm trying to design a soft-start switch for some of my amps. I've got two 500W class A\B amps using toroids powered from the mains, and the relay I've been using to switch them on is now toast.

I've been reading through the pages on this forum, and it would appear that toroid inrush current and perhaps the inductive kick at switch-off are to blame. The whole destroyed relay situation prompted me into thinking that I'd under specified the relay contacts, and so I've been thinking about what sort of current will be seen by anything connected to the primary side of the toroids.

The transformers feed a full wave bridge rectifier on the secondary side, and smoothing capacitors beyond that. The caps will therefore only be pulling current from the transformer at the peak of the voltage wave (ripple current). This means the larger the capacitor bank, the larger the current peak on the primary.

If the above is correct, does anyone have any idea what sort of peak current I could expect to see on the primary of a 500w amp at full load?

I was going to design something that would automatically limit the input current to say 13A using a shunt resistor on the mains side (nothing new), but if amp primary current peaks beyond this during normal operation at full load, then automatic resistor shunting will not work.

I don't have the kit to test. Just wanting ball park figures.


Thanks.

[killingtime]

OK,

I ran a simulation through LTSpice.



For a 504W RMS load, the primary input current peaks at 8.1A (240V AC Primary). By comparison, if you were to connect a 504W resistive load across the mains, then you 'd see a current peak of 2.97A.

I've used 20,000uF across the FBR, and chosen a secondary voltage of ~70V as this is what I believe my amps are using.

Blue trace is the primary voltage, green is the primary current, red is the load power developed in R2.

BTW, the transformer is based on an ideal, with 4 ohms of primary winding resistance. No secondary resistance, no leakage inductance or magnetising current. The inrush current peaks at 42A on the primary. This value was heavily dependent on the value of R1 (primary resistance).

EDIT:

Also listed some other smoothing cap values.

40,00uF : 9.5A Pk Pri Current.
60,00uF : 12.0A Pk Pri Current.
70,00uF : 13.4A Pk Pri Current.
80,00uF : 14.8A Pk Pri Current.

[AndrewT]

a 500VA transformer on the UK 240Vac system should run perfectly well with a 2A fuse fitted in line.
Since you have two 500W transformers fit a 5A fuse in the plug top and fit a T2A fuse in the feed to each primary inside the case.

Now design your soft start circuit to not blow any of these fuses at start up.

The fuse blowing problem should never be solved by fitting overly large fuses. Sort the inrush current instead.

A 500VA transformer can have a few ohms of resistance in the primary. The feed from the mains and back could add another ohm to that. Let's guess at a total primary circuit resistance from the distribution board and back of 5ohms (5r0).

The 240Vac mains has a peak voltage of ~340Vpk.
The peak current if one closed the ON switch at that moment of peak voltage would be ~ 340/5 ~ 70Apk.
That is the current that must be limited by the soft start circuit.

The charging of the capacitors comes after the transformer has started and should use a different slow charge circuit if you still have a problem with nuisance fuse blowing.