Hi,
Firstly, before posting I have searched on Google and here on the subject of input capacitors and found most of what I need but have one or two remaining questions.
I am using an MCP1827 LDO regulator, the fixed 5V model with 5 pins, datasheet here:
http://www.google.co.uk/url?sa=t&rc...IiLgKgOCZFz-oqYj5odbQGg&bvm=bv.53899372,d.d2k
I put the circuit together, using the recommended resistor and capacitors, except that lacking ceramic capacitors of the right size (4.7uF and 1uF for input and output respectively) I substituted them for electrolytics. From a position of significant ignorance, I couldn't see how they could be reverse biased by the circuit and so assumed the fact they are polarised wouldn't matter, as long as they were connected the right way. I assumed, for both input and output, that they were just there to compensate for a drop in output from either the original voltage source or the regulator, should the load suddenly increase.
The power supply circuit worked for some initial testing, several times, with virtually no load, and then a couple of times running a Raspberry Pi Model A, so up to 500mA. I disconnected everything to add a switch then on reconnecting it failed. It turned out that the LDO was outputting around 1.2V, though within around 30 seconds this climbed to around 1.6V. At this point I tested the temperature of the LDO with my finger and it was extremely hot (I was using a DC power supply at 6v as input, the plan being to use batteries when the circuit is finished).
I did some further reading and now understand that the polarity does seem to matter.
My questions are these:
1. Much discussion of voltage regulators refers to regulators oscillating. However, it's not clear whether this means that the LDO has a tendency to output AC, which would explain why polarised capacitors would not work, or merely fluctuating DC, in which case they ought to be fine. Which is it?
2. The schematics in the datasheet for the LDO show non-polarised capacitors and refer specifically to a 1uF ceramic capacitor on the output. This is easy enough to do. However, the datasheet shows a non-polarised capacitor of 4.7uF on the input and I have struggled to find a ceramic capacitor of such a large value online. Rs components have them, but only as surface mounted and not through hole. So, does the input capacitor really need to be non-polarised, in which case what is the right sort of capacitor to use? The datasheet says it's needed if the circuit will be run from batteries or from a DC supply with several inches of wire, which is the case with my circuit.
Thank you,
Jim.
EDIT: The datasheet is really very helpful and includes significant discussion on the appropriate size and type of the output capacitor, i.e. ceramic and 1uF, and of the *size* of the input capacitor, i.e. 1uF - 10uF. The schematics clearly show a non-polarised capacitor on the input (as well as the output) but give no clue as to what sort of capacitor the input should be.
Firstly, before posting I have searched on Google and here on the subject of input capacitors and found most of what I need but have one or two remaining questions.
I am using an MCP1827 LDO regulator, the fixed 5V model with 5 pins, datasheet here:
http://www.google.co.uk/url?sa=t&rc...IiLgKgOCZFz-oqYj5odbQGg&bvm=bv.53899372,d.d2k
I put the circuit together, using the recommended resistor and capacitors, except that lacking ceramic capacitors of the right size (4.7uF and 1uF for input and output respectively) I substituted them for electrolytics. From a position of significant ignorance, I couldn't see how they could be reverse biased by the circuit and so assumed the fact they are polarised wouldn't matter, as long as they were connected the right way. I assumed, for both input and output, that they were just there to compensate for a drop in output from either the original voltage source or the regulator, should the load suddenly increase.
The power supply circuit worked for some initial testing, several times, with virtually no load, and then a couple of times running a Raspberry Pi Model A, so up to 500mA. I disconnected everything to add a switch then on reconnecting it failed. It turned out that the LDO was outputting around 1.2V, though within around 30 seconds this climbed to around 1.6V. At this point I tested the temperature of the LDO with my finger and it was extremely hot (I was using a DC power supply at 6v as input, the plan being to use batteries when the circuit is finished).
I did some further reading and now understand that the polarity does seem to matter.
My questions are these:
1. Much discussion of voltage regulators refers to regulators oscillating. However, it's not clear whether this means that the LDO has a tendency to output AC, which would explain why polarised capacitors would not work, or merely fluctuating DC, in which case they ought to be fine. Which is it?
2. The schematics in the datasheet for the LDO show non-polarised capacitors and refer specifically to a 1uF ceramic capacitor on the output. This is easy enough to do. However, the datasheet shows a non-polarised capacitor of 4.7uF on the input and I have struggled to find a ceramic capacitor of such a large value online. Rs components have them, but only as surface mounted and not through hole. So, does the input capacitor really need to be non-polarised, in which case what is the right sort of capacitor to use? The datasheet says it's needed if the circuit will be run from batteries or from a DC supply with several inches of wire, which is the case with my circuit.
Thank you,
Jim.
EDIT: The datasheet is really very helpful and includes significant discussion on the appropriate size and type of the output capacitor, i.e. ceramic and 1uF, and of the *size* of the input capacitor, i.e. 1uF - 10uF. The schematics clearly show a non-polarised capacitor on the input (as well as the output) but give no clue as to what sort of capacitor the input should be.
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