[SOLVED] Looking for a robust Voltage regulator

Hey all,

I've recently been using a L7805C voltage regulator to provide the logic voltage for my 3-axis stepper motor driver board (L297+L298). The L7805 is set up with a 0.33uF capacitor to gnd on the 12v side and a 0.1uF cap on the 5v output side. I should add that the voltage for the motors feeds off the same AC-DC 12 volt switched-mode regulator that I'm trying to step down to 5v with the regulator.

When I have all three axes' drivers powered, it runs fine. That is, until more than one motor is being driven at the same time. I figured it's maybe due to fluctuations in the supply from driving such large motors, but all I see when I scope the voltage rail is a little noise when the motors run. One thing I do know is that it works absolutely fine using a desktop power supply to provide the 5v.

I actually even tried having a separate voltage regulator for each driver, but that didn't really make a whole lot of difference.

Anyone have any advice on how I might go about fixing this? Power supply design isn't something I've ever dealt with past "throw on a voltage reg with a couple decoupling caps", so I'm not really sure what to do now. Is the L7805 too basic a voltage regulator? is there something I can add/change to make a more stable supply?

Not sure if it will help, but here's a picture of my stepper driver board alongside the FPGA that provides the driving signals. The 12v supply can be seen attached on the left hand side of the box. The L7805 is at the top right of the driver board next to the terminal block and the molex headers.


Edit for Extra info:
The transformer supplying the motors with current (and also the voltage I'm trying to regulate to 5v) is a 12v, 10A supply. The motors are ~2.5A each.

What is power of motors (current) ?

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I dont see capacitors before and after voltage regulator ?
Increase values to 470uF before and 100uF after, on end 100nF.

Heatsink for regulator ?

Transformer have enough power for all three motors ?

The motors are taking ~2.5A each at 12V from the power supply (same supply that I'm trying to regulate to 5V).
The caps are SMD, and underneath the Vreg.
I don't have a heatsink for the reg right now, it's yet to run hot enough to justify one.
The supply is rated at 10A so has more than enough current available.

I'll give it a go with those cap values. Why such large values though? Just to smooth transients caused by the motors drawing current?

Its not big caps. To smooth voltage 1000uF per 1A.

tpetar said:

Its not big caps. To smooth voltage 1000uF per 1A.

Click to expand...

I just meant big in comparison to what the datasheet recommends; they're 1000x larger. But I wasn't aware of that rule of thumb, so I'll give it a go. Thanks.

7805 have 1A max but if you have Vi-Vo in 3V range, and you have 7V difference (12V-5V) with 1A its 7W, put heatsink on it, and dont expect maximum from this part

There is 78S05 for up to 2A max, or you can use switcher voltage regulator as additional solutions.

:wink: