Newbie questions - Please help guys

[Bassybert]

Hi guys, this forums great, and I can't wait to get cracking making my own pedals. i've got a few questions to ask beforehand though, and i'm sorry if these sound primitive but:

what is the difference between using a 1/4 watt resistor and a 1/2 watt of the same value i.e. 47k ?

what is the difference of using different voltage caps, Is it because of the voltage running through them? I'm only using 9v powered effects so is it ok to use as low a voltage as possible that is just above 9v?

i've orderd some TL072CDR opamps to use with the tonepad Pulsar tremolo and an 8 way dil socket which fits the pdf layout perfectly. However the TL072CDR is too small for the socket. I was wondering whether it is possible to get different size opamps and i've got the wrong one by mistake.

Thanks in advance

Jim

[toneless]

1/4 watt resistors are ok and they are prefered for the smaller size than 1/2...
Capacitors depends from the effect but for most stompboxes 16v are ok.

TL072 has 8 pins and and your socket 8 pins,too.
Where is the problem?Ask the person in the shop that you are buying things...

HEY!!!You didn't bought the small ICs like the one that are used in your PC or something...

Nick

[smoguzbenjamin]

Welcome to the forum!

A 1/4W resistor will allow less wattage through than a 1/2W. Watts or power = Voltage x current. In a formula: P=V.I It basically says at what ratio of voltage and current it will fail. 1/4W resistors are usually smaller but I use 1/2W resitors 'cos I get those cheaper :mrgreen:

A cap's voltage rating states the maximum voltage you can apply across it. A 500V cap will only charge up to 9v if you apply that across it. It can't go past that limit. Boss uses 10V caps but I'd recommend 16V because you'll need to make stuff fool-proof, it saves you replacing caps and allows you to run the circuit at a slightly higher voltage should you want to.

Did you get an opamp with long legs? Maybe you got a surface mount chip.  

Hope this helps!

[Mark Hammer]

Speakers are resistors in a way.  They are rated at different wattages to indicate how much power you can expect these things to handle without burning the voice coil up.  Bear in mind the voice coil is just fine wire and you can only expect it to handle so much heat (especially with all that rubbing around in the magnet gap) before it burns right up like a fuse.

Resistors AND speakers are like fuses in that respect too.  Try to pass too much current through them and they burn up, leaving an open circuit.

The wattage ratings specify how much "work" you can expect a resistor to do before overcome by heat and fusing out.  IN most instances, pedal circuits really only need 1/8W resistors for what it is they do, although there are plenty of instances where you can legitimately expect greater demands on the resistor's heat-dissipating capacity based on how much current you expect to pass through it.  For instance, using a 1/2W unit for those small-value resistors (22 to 100R) you see in series between a battery and pedal is probably a smart idea since almost ALL the current the pedal uses passes through that single resistor.  Similarly, it is common to see what is called a "Zobel network" on the output of solid-state power amps, consisting of a series resistor and cap.  Given how much current may come from the amp's output, even if it is a modest headphone amp, it is a smart idea NOT to use a 1/8W unit and a better idea to use a 1/2W.  More if the amp is a higher powered unit.

Why pay attention to capacitor voltage ratings?  Have you ever watched a spark jump a gap?  Hold a live arc-welding rod close to a grounded metal surface and you see the relationship between voltage/current/distance and the ability of current to jump a gap.  The more electrical "stuff" there is to jump that gap, the bigger a gap it can jump.

Ideally, you want DC to be impaired at jumping the gap that exists within any capacitor, and want the cap to store charge rather than let that charge march through unimpeded.  When  the types of voltages you can expect that cap to see are near or higher than the voltage rating of the cap, the cap can not be expected to live up to its stated capacitance because the cap is in jeopardy of simply letting DC march through.

Bench wisdom is to use caps rated for double the voltage you expect to find anywhere in the circuit for a guarantee that caps will remain on-spec.  So, if you are running a pedal off 9v, the next highest voltage ratings are likely to be 16v and 25v.  Sixteen volts is probably fine, unless you plan to feed your pedal/s with a regulated 15v external supply, but 25V is probably safer.

Do you need to observe such caution for ALL parts of a circuit?  Not necessarily.  Caps in the signal path are unlikely to even need to be as high as 6.3v rated much of the time (when was the last time you saw a 6v p-to-p audio signal in a 9v battery powered pedal?), and caps that bleed off bandwidth to ground in filters also don't need to be rated all that high.  On the other hand, you don't take chances with anything that is expected to filter a supply voltage.

The Pulsar doesn't really need anything fancy, so if you accidentally ordered a TL072 in a non-DIP format you can safely use any other dual op-amp with a similar pinout that you have handy.  Hell, a cheap (?) 1458 from Radio Shack will do just fine.

[Bassybert]

Thanks guys, that's really cleared things up. The reason the TL072CDR's don't fit my socket is because they are surface mount, but this wasn't explained to me when I bought them. I've got some new ones ordered so I should be able to start building in a couple of days.

Mark, i've had problems finding decent caps here in the UK, as most companies only deal with large bulk orders and we don't have the demand for specialist hobbyist shops like the US does so i'm either stuck with certain parts, or have to spend £100 or more for it to be beneficial.  I've found some decent caps for my needs but they are rated at 100v although small enough to fit into a packed board so i'll use them.

And thanks for giving such thorough and simple explanations to my questions. It's reassuring to know that people are so willing to help others who are just starting out. Thanks

Jim    

[Maneco]

Welcome to the forum,
regarding parts,you should look www.smallbearelec.com ,he's dedicated to stompbox electronics,and a cool guy to buy from,he ships worldwide ( i live in Uruguay)

Maneco

[Paul Perry (Frostwave)]

EPE Magazine and Elektor both carry ads for UK parts suppliers. But, small bear are probably the closest "full service" mail order suppliers, even though they are across the pond.

[gez]

www.esr.co.uk

http://www.rapidelectronics.co.uk

[Mark Hammer]

Incidentally, as Pete Snowberg has very expertly shown here, there can be a big difference between conditions that result in instant failure of a device, and conditions that increase the risk of failure or being off-spec down the line.  

We know that caps can change over time.  Personally, I know diddley squat about what the difference between a cap's spec'd operating voltage and the voltages it normally encounters means for the long-term ability of that cap to be within the tolerances printed on its casing.