Decoupling caps for battery powered PICAXE work...

The Zener is to clamp the Maximum voltage getting to the regulator to a safe, and constant, level - if regulator is rated at 35V max then a 30V Zener will enure that the regulator doesn't get fried.

I think your diagram was wrong - the Zener goes from positive to ground and the ordinary diode goes before it in the positive line - just to prevent reverse polarity connection blowing evrything up! Plus it was probably a 15V Zener (or higher) not 5V.

Ripple is from poorly regulated supplies (residual changes in voltage from the original AC) You want a voltage that is constant (like a calm mill pond) ripple is like waves when the voltage is disturbed -- mixing a few metaphors here!) Capacitors can smooth the voltage ripple down to an acceptable level.

"Ripple" is essentially the techy way of saying "Noise". Ripple on the supply line is electrical noise on the supply(hums, little repetitive spikes in the voltage etc). The capacitor on the input filters this out, meaning that the regulator input sees a more stable consistant voltage. It is quite normal to find caps in the area of 2200uF on the input to linear regulators such as 7805 and 7812, if they are being fed from a mains rectifier or the DC supply to the circuit is KNOW TO BE noisy.

As for the zener, I am a little at a loss for that idea.
Normally, a zener is reverse-bias connected across the supply with a load resistor. It is common to find them as cheap-but-effective over-voltage protection to the input of regulators etc, but normally they are connected across the input supply voltage NOT in series with the supply voltage, so not exactly sure what the idea is there - perhaps someone here can enlighten us both.

The "Normal" arrangement is a zener and a load resistor directly across the supply voltage. If the zener is rated for, say 15v, then if the supply voltage exceeds 15v, the zener will break-down, conduct, and essentially clip the input voltage to 15v, with the current drawn to keep the voltage to 15v being dissipated in the load resistor.

As I say - very cheap, but effective and a very common practice. I don't tend to use them like this that much though, preferring the idea of a pre-regulator or a PSU where you know the input voltage can never exceed the input maximum of the regulator.

JimPerry beat me to it!!!

With regard to the use of Zeners in series to limit the voltage input to the regulator, have a look at the schematic in post3 in this thread...
http://www.picaxeforum.co.uk/showthread.php?22014-Squeezing-functionality-out-of-an-OLED-display
It confused me at first, but the author explains his rationale.

Oh my God guys!!!

I have made this fix today. I have sent the machine back , express delivery to the guy. Are you all saying that my forum friend may have got the design (fix) wrong! I AM BUSTED!!!

I'm a nube to the electronics and all this s way over my head. Is there any chance some one could please draw up a replacement schematic as i dont get some of the phrasses used, so I can make a new pcb and send it to him ASAP . The last one lasted 6 hrs before it blew. If this one blows, I think I've lost a massive sale.

Thanks in advance.

JPU - I'm sure it will be OK. I had not heard of using zeners in series before, but I am reading about it now. I am sure it will be OK - your friend sounds like he knows a thing or six.

EDIT: On the zeners, now I can see how that works. Essentially, the regulator itself becomes the zener load, and when looked at like that, the circuit is the same as the more "Normal" zener arrangement I was referring to earlier. Rather then have the zener/resistor in parallel with the regulator input, it forms the zener/resistor concept with the zener and the regulator instead - very clever, actually, and I have never thought of a configuration like that. Used like this(zener in series), you MUST supply the circuit with at least the zener voltage, or nothing wil happen, but it is a great way to supply a regulator with a lower input voltage from a higher supply rail, as described in the last post of the link posted in post #44. I still prefer pre-regulators though, in my own humble opinion.

Grogster said:

JPU - I'm sure it will be OK. I had not heard of using zeners in series before, but I am reading about it now. I am sure it will be OK - your friend sounds like he knows a thing or six.

Click to expand...

Hi Grogster

It's 1:30 in the morning here and I think I ain't going to sleep for worry now!

The guy on the other forum actually introduced me to picaxe as he has helped me design the whole control unit for a battery operated motor based around a 08m2. He is very thorough but I did kind of rush him for a solution as I needed a fix on Sunday morning.

Thanks for your words of encouragement though.

JPU

Have a nice snooze.