Picaxe voltage regulator.

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pandaweaver

New Member
Hi,
I've had experience of Picaxe chips before, but not using voltages above 6v dc.
So i was wondering, what regulator, and how would i set it up on a breadboard, that would be able to put voltages of 24v to 12v down to 6v for the chip. This is not to power the chip, this is to be used on the inputs.

Also, if a voltage pulse is sent through, ie it is at 6v then goes to 12v then back to 6v, a pulse, would this come through on the regulator, albeit 3v then 6v then 3v?

Just a little confused about using higher dc voltages with it, I don't want to fry it!

Thanks

Alex
 
T

Tom2000

Senior Member
Alex,

Many 5 volt regulators are spec'd to accept fairly high input voltages. So, theoretically, you might find a catalog regulator that's happy with 24 volts (or more) on its input. But you must be careful of the regulator's power dissipation when you're feeding one of them with higher input voltages.

As an example, let's say you're driving a 5 volt regulator from a 24 volt source. And further, your circuit is drawing 50 milliamps from that 5 volt supply.

If you're not using a switching regulator, all that extra voltage will contribute to the regulator chip's heat dissipation.

In the example above, the regulator will dissipate (24 - 5) volts times 50 milliamps, or 0.95 watt. In a small package without a heat sink, that's enough heat to blister a fingertip.

A possible strategy in a situation like that might be to place two regulators in series. One husky regulator might reduce the input voltage to, say, 12 volts. Then a second regulator would reduce the 12 volts to 5 volts.

Another solution would be a single husky regulator and a massive heat sink.

To answer your second question, regulator ICs are very good at reducing power line glitches, as long as you pay attention to input and output capacitance as specified on the data sheet. You should see nice, clean 5 volt power on the regulator's output, for just about any input.

For harsh environments, you might need further input protection from glitches that exceed your regulator's maximum input voltage specs. This is particularly true for automotive applications, as many have pointed out in these forums.

(I'm sure the 'noisy environment' folks will chime in here with some practical advice, and I surely hope they do, because I don't have much experience with real-world harsh environment applications to give you.)

HTH,

Tom
 
D

Dippy

Moderator
This crops up a lot.

Can someone do a summary of the popular (linear) regulators including dinosaurs and post it please?
Also, as Tom has mentioned, include some basic info on calculating power/heating losses.
And a little lesson on dropouts too. (Electrical not college).

Just a little 'chime' in addition to Tom's notes. Voltage regulators will, on the whole, smooth out transients well. However, they do have response times such that high frequency spikes can get through. These can be common in older car electrics or if you have a sinle power supply shared with something switching big or inductive loads.
It just depends on the size/frequency of the pulse.
This one of the reasons that the idea of using Switched Mode followed by a linear is usually pointless unless great care and skill is used.

If you get yourself a Data Sheet on a decent (non-Ebay) regulator you will be able to see transient responses shown graphically.

Protection in automotive applications is an longer subject which will require surge/transient protection potentially over a wide range. This would take a few pages to describe properly and I think you'd better take all this a stage at a time or else you'll overload.

Please don't say that you intend to use this in a car....

Above all: when you have an idea for the complete spec of the regulator you want then READ the DATA SHEET!!!!!!!!!!!!!!!!!!
They usually give application notes and suggested component values which are very helpful.
 
B

BeanieBots

Moderator
I think there is some serious confusion going on here!

Pandaweaver has stated that he is NOT intending to POWER his PICAXE from these voltages but wishes to monitor high voltages on its inputs.
If this is the case, then a voltage regulator is NOT the way to go. I would suggest a modest R/R potential divider for such cases. Maybe with a clamp diode to the positive power rail.

The example given of a pulse going from 6v to 12v and back to 6v, would give:-
5v to 5v to 5v if a 'standard' REGULATOR was used.
If a potential divider was used, it would give the same ratio, namely 3v to 6v to 3v for a divide by two version.
The PICAXE would like to see 0v to Vcc (normally 5v) to 0v.
To do that would require an offset voltage. IMHO, the simplest solution for THAT example would be to fit a zener diode in series with the source (say a 6v2) with a light load of about 10k to 0v and a current limit resistor into the PICAXE input of about 4k7.
 
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