Supply Current

Why is the current on a negative power supply rail always so small, and does this matter?

I am working on some simple DC experiments and just generally screwing around with electricity, and I was wondering why the amperage rating on negative power supply rails is always only a few hundred milliamperes. If I want to provide 24VDC for a circuit by using +12v and -12v (is this correct?) am I limited to the current of the -12v rail? How does this work? Thanks.

If you want 24 volts, you use a 24 volt supply, with (usually) – tied to ground and + tied to the load.

But you could bridge between –12 and +12 to get 24v, it depends on what the circuit is. Usually the first is the only solution, but sometimes the ±12 volt approach will work. If you do bridge between the ±12 volts supplies, you are limited to the current in the least capable supply, here the –12 volt one.

Usually the power demands for the –12 volt supply are light (for PCs) so that is why it is a low power supply.

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Why do bridge rectifiers take pulses of current from the supply?

Billy Butthead -
That's not what I was getting at, to be honest.
Without any capacitance, the current would just be a sinusoid, but increasing the capacitance in parallel with the load makes the pulses shorter. Why is that?

The capacitor will accept the charge from the rectifier and will then provide power to the load. Current will only flow if the voltage from the rectifier is greater than the voltage on the capacitor. When the capacitance is increased in relation to the load then the capacitor will not discharge as quickly.

So current will only flow as the charge on the capacitor is replenished. The longer the charge remains on the capactior the shorter the time needed to replenish the charge on the capacitor.

Just as your shorten the "pulse" if you increase the capacitance, you will lenghten the "pulse" should you increase the load