Rectifier and 7805

Hi,

I have a AC transformer that deliver 20 VAC to the circuit, and it goes through a full wave bridge rectifier. The rectified output drives the DC motor and 7805 in parallel. I have a smoothing capacitor at 330 uf, I have a 330 uf that ties 7805 input to gnd, 100 uf that ties output to gnd, and a few ceramic cap. However, I am getting many “LOW” when I logic analyzes the 5V regulated output. In fact, the Arduino MCU and hall sensor are in the regulated output. The dips will cause falling interrupt to trigger a lot more than expected ( The rpm should be around 1800, and I get 50000 ish). I have tried changing capacitor and have not yet resolve the problem. When i turn off the motor, the hall sensors works as expected. Any ideas?

If your AC is 20 volts and the smoothing capacitor is rated at 25V then you have destroyed the capacitor. 20VAC is equivlent to around 35 volts once rectified (20x1.4)

I take it your circuit is wrong or did you really wire it like that?

You should be supplying +5V to the vcc of the hall effect sensor and the arduino 5V input -
without R2 - what's that for?

perhaps you could redraw it correctly , and then we could make some guesses

regards

Allan

And don't scale it so the text is illegible please...

Why not use a DC-DC converter - the 7805 has an absolute maximum input voltage
scarily close to your max input voltage(*), and will get very hot dropping 20+V.

BTW you don't have a free-wheel diode across the motor.

(*) Transformer secondary voltages are quoted for full load, you get 10 to 15% more voltage at low load,
and even more if the mains is unusually high voltage or a distorted waveform (harmonic content). Your
25V supply may vary between 27V and 35V in practice. You can also have mains transients from inductive
equipment float straight through the transformer (after all it merely scales the voltage, not limits it), so
the resistor/capacitor you have on the path to the regulator is partly helping to suppress those transients.

jackrae:
If your AC is 20 volts and the smoothing capacitor is rated at 25V then you have destroyed the capacitor. 20VAC is equivlent to around 35 volts once rectified (20x1.4)

While you're correct about the voltage being too high for the capacitors, your voltage calculation is a little off:- 20 x 1.4 = 28V
(And I thought I was the only one who made those mistakes. :smiley: )

allanhurst:
I take it your circuit is wrong or did you really wire it like that?

You should be supplying +5V to the vcc of the hall effect sensor and the arduino 5V input -
without R2 - what's that for?

perhaps you could redraw it correctly , and then we could make some guesses

regards
Allan

Unless I'm mistaken, R2 is a pullup resistor on the output of the Hall-effect sensor. And if I'm reading it right, the Hall-effect sensor and Arduino are supplied with 5V. I agree, though, that it is badly drawn and it's hard to tell what are junctions and what are crossovers. I like little humped 'jumpers' when wires cross over, and blobs on junctions. (Every bit helps when your eyesight's as bad as mine. :slight_smile: )

@sunnyksl, you should really have a diode from motor negative to motor positive, to protect the transistor.
What are the current ratings of the transformer and the motor? Do you have a link to the motor and/or it's specs?

P.S. Mark has just said some of the same while I was typing, but now that I've typed this.....

Crossing-over replies happen all the time - I think its a good think, it neatly shows a concensus
if there is one.

MarkT:
Crossing-over replies happen all the time - I think its a good think, it neatly shows a concensus
if there is one.

I agree. :slight_smile:

And while it might be a dead cap, (if not they won't last long if they're really rated at 25VDC), my guess is that the motor is loading the supply too heavily. That would explain why it works OK when the motor isn't running. If that is the case, it must be one hell of a mismatch to pull the voltage that low though.

OldSteve:
While you're correct about the voltage being too high for the capacitors, your voltage calculation is a little off:- 20 x 1.4 = 28V
(And I thought I was the only one who made those mistakes. :smiley: )

:blush:

Hi,
I have drawn your circuit to show all your components with some of the suggested changes.
Circuit1.jpg

Tom… :slight_smile:
Feel free to edit it as you go.

Motor connects to C1, not to C2.
Wise to have a diode in series with R1, so motor can't drain C2 (less ripple for the 7805).

The value of C1 sets motor voltage.
A higher value cap is less ripple, so a higher average motor voltage.
Leo..

Without a resistor before the first cap you get heavy current spikes into the capacitor - normally a
resistor is added to spread these spikes and reduce the ripple current to something that won't
bake the capacitor so much or waste so much heat in the transformer - dissipation in both goes
up as I-squared.