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I have two 5 volt voltage regulators that I would like to connect to a 9 volt source in a parallel configuration. Is this advisable, problematic, or even commonly done?

I have tested this configuration and noticed that one of the regulators gets hotter than the other. Why is this?

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Production regulators have tolerances - in the order of a couple of percentage points (most are 3 or 5%).

So, a 5V regulator might be giving 5.15V (5+3%) and the other one 5.00V. If you parallel them, the one providing the load is the highest one. The lower still thinks that the voltage is too high and waits for it to drop.

Until the highest regulator gives up (its protection circuitry kicks in), it will provide all the current (and heat up). I used an extreme example of difference in voltage between two regulators. In practice they vary less. On the other hand, just a couple of mV (millivolts) is sufficient to unbalance the load carried by each

So, putting two regulators in parallel is not a good idea.

There are circuits on the 'net which connect an external transistor to boost the output of a single regulator. It's slightly tricky because, unless you calculate a couple of resistors really well, the protection circuit won't work well. And connecting an extra transistor usually means the voltage drop across the regulator has to be at least 0.7V more.

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IF you must connect the two regs in parallel then some form of current balancing is necessary to prevent one reg hogging the load Series resistors are easy but they will degrade your voltage regulation because they are on the 5volt regulated output You could start with 3r3 and decrease if your output volts isn't enough and increase if your temps are too uneven If this is not good enough then you could put in a bigger reg or place resistance in series with the input if your dropout volts are OK

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You can O-Ring two different potential voltage source by Diodes only, like this:

1) Anode of Diode 1 is connected to +9V
2) Anode of Diode 2 is connected to +5V
3) Cathode of Diode 1 & Diode 2 are shorted and connected to load.

Above solution will prevent circulating current and hence avoid heating.

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  • \$\begingroup\$ Why did this get down voted? \$\endgroup\$ – Hoytman Jun 18 '15 at 14:38

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