not enough power?

[mischl]

hello all

i have a circuit with 230 VAC to 5 V DC regulation. see picture

now when i connect 7.5V DC from an external source at the input of the 7805, it works fine and need 90mA at the 5 V out.

but when i connect the transformator as drawed, the pic18f876a reset itself allways. the trafo output is 6V 1.0 VA.
the input ripple at the 7805 is a sine between 6.6V and 8.8V peak to peak which breaks down every 80 ms to about 5 V.
the outpt voltage at the 7805 is 5.0 V and breaks down every 80 ms to 3.6 V.

ok, the 80 ms result from power-up timer(72ms), start up environment and then ... ? probaply there is not enough power to generate the 5V 90mA with 6 V AC 1.0 VA or the bottom of the input ripple is with 6.6 V to less for the 7805...??
i've tried to turn off the mclr at the 18f876a, but can't found the fuse configuration...

thanks for any help

[Dave]

mischl, You will need at least 6.2 volts dc at the input of the 7805 for it to regulate at 5.0 volts on the output. A typical 7805 regulator has a 1.2 volt drop internal. Therefore if the input voltage drops below 6.2 volts the output will drop to the input voltage-1.2 volts.

Dave Purola,
N8NTA

[mister_e]

That's a perfect situation to do a search and find some LDO regulator.
LP2957, LP2954, LP38690ADJ,ADP3367,... ,...

OR a simple silicon diode in serie to drop 0.6-0.7 volts could work.. not a fan of that method but in theory it works

[mister_e]

the input ripple at the 7805 is a sine between 6.6V and 8.8V peak to peak which breaks down every 80 ms to about 5 V

??? about the capacitor before the regulator??? In theory 6VAC should gives you about 9 volts near stable depending of your capacitor size. Try increase capacitor value first. Forget the serie diode here.

Reliability and stability of a numeric design sit on the PSU first...

[mischl]

hello dave and steve

thanks for your replys. i need a bit of time to answer because i must test all seriously.

i'm no a step further. the problem seems not to be in PSU... i made a mistake in the software which have updated the outputs too fast and this problem sourced. normally the output at the 7805 is stable at 4.9 V

now a new thing is coming up. all is working but i'm not sure why this can be :
the circuit needs 16 mA @ 5 V. when i turn on a reed relais it needs 20 mA more, so 36 mA. the next relais add also 20 mA, all is fine.
now, when i turn on the third relais and the current increase to 76 mA, the 5 V looks at in the attachment...?

the relays are connected at portb, which have beside the relays only two led's (r=330ohm). the datatsheet says a maximum output current sunk by a pin is 25 mA, maximum current sunk of porta and b is 200 mA. but the whole circuit only needs about 90 mA when all is turning on.
the trafo output is 6V 1.0 VA.

as i say it. the circuit works, but i ask myself what is that?

thanks for any comments again

[keithdoxey]

i'm no a step further. the problem seems not to be in PSU... i made a mistake in the software which have updated the outputs too fast and this problem sourced. normally the output at the 7805 is stable at 4.9 V

the 5 V looks at in the attachment...?

thanks for any comments again

The problem is DEFINATELY in your PSU. The attachment shows the voltage dropping towards the end of the mains cycle due to Capacitor C5 in your PSU not being large enough to maintain the voltage above the 7805's minimum input requirement.

The fact that this only becomes apparent as you draw more current confirms this. More current discharges the reservoir more quickly.

Increase C5 to at least 1000uF, more if you have it available. I would probably go for 4700uF.

Regards

Keith

[mischl]

hi keith

you are right, a greater C5 helps. i have only a 1000 uF, and this smooth the ripple nearly

because the packaging is very great, are there other possibilities of the design held the size small? probaply a LDO or other trafo? or is there a basic failure on the schema ?

thanks a lot

[mister_e]

you have many choice here.

1. increase capacitor value (still good to place some in parrallel)
2. change your regulator to an LDO
3. change your transfo for an higher voltage model... but keep in mind that higher the voltage is... more heath the regulator will provide.

choice is yours.