Voltage regulator questions - capacitor on the input, difference between different regulators

Hi Tom,

The spec does specify that you may need a 100nF cap on the input (see page 20), you can additionally add the 10uF if you find you need it (it depends on what the impedance of your source is; there is no harm in adding a small amount of capacitance such as 10uF and the 100nF. To know why you're best picking up a book, because there is some theory including Ohm's Law and reactance that would need to be covered to do it justice - see here  I Want to Do Electronics!  Where do I start?

But in summary you can use 10uF on the input (and 100nF) and 10uF on the output with that particular device because that is what page 20 text and the diagram on page 21 show. If they start mentioning ESR and capacitor type then that is important too.

Regarding your second question, the 3A part is a similar part in the same family, rated for higher current. It may cost slightly more.

As for if it will get hot - this is all down to the Power equation and Ohm's Law. You'll find that the amount of power dissipated as heat depends on your source voltage (e.g. 9V), and the output voltage (e.g. 5V) and the current through the voltage regulator and current through your load. You can calculate the power being dissipated as heat using Ohm's Law and the Power Equation, i.e. V=IR and P=IV.

If your circuit is consuming 1A then the power dissipated by the voltage regulator as heat is approximately (first level approximation):

V=9-5 = 4

P=IV = 1*4 = 4W

So your regulator would dissipate 4W and would be very hot and would need a heat sink. That answer is the same for both of the regulators.

I strongly recommend you checking out that link, because it will help with these types of questions.

Thanks! That's very helpful. Yeah I guess I'll have to pick up some book...

Until then, just to make sure - if I need a 100nF capacitor on the input, and you're saying I can add 10uF as well, I should be able to just drop the smaller one and just go with 10uF only? That should be ok, right?

And in terms of heat - yeah, I'm planning to add the heat sink and some holes in the case for my thing to allow air circulation and stuff. Hopefully it won't melt

Hi Tom,

The 100nF and 10uF are used together, the reason is that they perform differently due to the material used in their construction (ceramic vs electrolytic). Both are advised. Figure 1 in this PDF highlights the difference http://rohmfs.rohm.com/en/products/databook/applinote/ic/power/switching_regulator/cera_cap_appli-e.pdf

Hi Tomasz,

If you can get by with one Amp you may want to check out these Recom Convetors. Since they are switching regulators they do not waste as much energy in IR loss. Further there is only a 1.5 volt drop out and probably no need for heat sinking. With efficiency up to 94% more of your battery power will go to your application.

http://www.newark.com/recom-power/r-785-0-1-0/dc-dc-converter-5w-5v-1a-sip/dp/62X9542

Incidently if you need more than one amp these devices can be paralleled.

John

Here is a link to Recom's DC to DC Book of Knowledge just in case you want more information on using switchers for converting voltages and some other really neat things.

http://www.recom-power.com/de/americas/downloads/book-of-knowledge.html

John

Thanks! That's really helpful, I'm learning a lot. I wasn't aware of the fact that the linear regulators have such low efficiency. It would drain my batteries in no time. I really need to buy some books and learn about this stuff...

Anyway, I guess I'll go with the switching regulator you've suggested. I'm pretty sure that I'm not going to drain more than 1A - I have to power up a small LCD screen that's supposed to draw 500mA, and a Raspberry Pi with no peripherals (I think it's supposed to draw around 300mA).

Worst case I'll connect them in parallel (one for the screen, one for the raspberry). But I think just one should be fine. I'll measure that later.

However, there still is a question of what capacitors to use... neither the spec nor the "book of knowledge" really specify that (or I can't find it again). Maybe it's some common knowledge that I'm lacking. Can you by any chance give me some advice on that?

Also in the spec it mentions to add a diode to prevent current going back through the regulator, but since I'm only powering a small LCD and a Pi I probably don't need that, right?

Hi Tom,

Now that there is a bit more information!! beware that you can't assume 1A is sufficient if you're using the Pi 3.

According to this link the current consumption (I expect with nothing attached) during a reboot reaches 720mA.

If you make the assumption 1A is enough for the Pi 3 I think this is just waiting to fail once your parts arrive.

If your LCD screen really does need 500mA as you say, then you definitely need a higher power DC-DC converter

Anyway it is unclear what Pi you're using.

Also, there are LiPo USB power bricks which would power the Pi and display (save on needing 6xAA batteries).

I'm using Rasperry Pi Zero - from what I've read, it's got a really low current draw, ~200mA. I've read on some blog how someone was testing it's draw and it peaked at around 450mA under really heavy load. So that's why I hope to fit in the 1A with both (worst case I can dim the backlight on the LCD screen by 50%, that's supposed to drop it's draw to around 370mA).

Hi Tomasz,

The Data sheets are not real clear on this I agree. The problem seems to be that the need is very application dependent. I would recommend a 4.7 uF to a 10 uF on the input of the Converter. While they say it is optional I would also put a 47 uF on the output as well as a 0.1 uF ceramic. This is in line with the recommendations from shabaz. The diode is also optional but if you have one why not afford protection if it is available.

John

Hi Tomasz,

For the fun of it I put 8 used AA batteries in a pack and powered a 5 V Recom Converter. Here is a picture of the set up. The little voltmeters each draw 20 mA.

I did not put any capacitors on the input or output in this case.

John