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I'm building a digital power base for a Scalextric slot car track using the schema provided here (you might have to click the "Powerbase Circuit" tab).

Now, my question is, what type of capacitors should be used for C7 and C8 (the right-most ones next to U3 & U4). What is said about the caps in the text is only "Output protection is provided by suppression capacitors C7 & C8", but nothing about their type.

At my local electronics shop, the only bipolar capacitors they had was 1,5uF of the following type: "Visaton elko glatt 1,5uF 10%". It also reads "For high quality speaker networks". See this link for a pic.

Can I use these caps for this purpose or should I get another type with correct rating (of 1uF)? In that case, which type should I use?

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2 Answers

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They are suppression caps, not decoupling caps as they are used to provide protection from transients on the H-bridge drivers.
Most 1uF low ESR >16V rated caps should be okay for this. I would probably avoid electrolytic. A 1uF ceramic should be fine, but if you can't find any and it has to be through hole you could use a plastic film capacitor.

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Those look to be bypass caps. The value isn't that critical, but they do need to have low ESR and work to reasonable frequencies. They should be ceramic caps. 1uF ceramic caps are plentiful and cheap. Make sure the ones you get are rated for at least a bit more than the voltage they should be subjected to, which is too small to read on that schematic. Getting 1uF ceramic caps up to 20V will be no problem at all. Higher voltages are available.

Generally you don't get electronic parts at local electronic shops anymore. There are so many different ones that a physical store can't possible stock a reasonable variety. You get these kinds of things via mail order from electronics distributors like Mouser, Digikey, Newark/Farnell, and many others.

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You can click the schema and open it in pdf. It's 12V. Also note that the caps are not connected to the Track output, but between 12V and ground (at least that's how I interpret the schema). – Anttu Oct 19 '11 at 19:42
@Anttu: OK, then 1uF 20V or 16V caps will be fine. Those will be readily available in 0805 and some other packages. – Olin Lathrop Oct 19 '11 at 20:00
Ok, so if these caps are only for protecting noise from the 12V power supply to get through to the track, I assume I can live without them for now as I'm planning to use an computer PSU to drive the circuit? Btw, I'm building this on a stripboard, so standard hole mounted components suits my need better :) – Anttu Oct 20 '11 at 4:26
@Anttu: They are decoupling caps, not for power supply noise reduction. 1uF wouldn't do much for a good supply. Decoupling caps provide a small local high speed power reservoir which responds faster than the main power supply with its series leads can. You should not leave them off. Limiting yourself to archaic thru hole components will severly impact parts available to you. You can solder 0805 packages between adjacent holes with .1 inch pitch. – Olin Lathrop Oct 20 '11 at 12:16
Ok. It's just a little confusing with the terminology to me. As the schema text talks about suppression caps, but you are using the term decoupling caps. I assume the caps in this application fulfills both functions? Or is there any practical difference? However, found this cap at my local shop. Should be ok? – Anttu Oct 22 '11 at 9:03
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