Why is the current on a negative power supply rail always so small, and does this matter?
Sunday, October 19th, 2008 at
12:10 am
Ashley K asked:
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.
Tagged with: Current • Electricity • Screwing
Filed under: Engineering
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sussex electric
the current is dissipated by the load. it doesnt come from the negative but from the positive .. well sort of lol .. it will become clearer later on.. one of the things you have to leave to trust
brighton electrician
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.
.
sussex electrician
The amperage rating on the negative power supply is usually low because is only used to feed a few IC´s, e.g. operational amplifiers and low power signal devices, and not heavy loads, also the number of devices which need this voltage is usually very small in a given equipment.
You are right, if you want to provide 24 VDC using the -12 VDC rail, you are limited by the current rating of that rail. Think of that as if you are connecting two bateries in serie to feed a load, thus, both batteries have to be able to feed the load current in order that it works.