Help with buying capacitors - nf and VDC

liamon

Hi, 

I'm trying to buy some capacitors but not sure if I'm looking at the correct ones.

The schematic calls for:

- a 47n (non polarized)
- a 100n (non polarized)

I notice that 47n capacitors have VDC associated with them: 100, 400 etc. Can I just stick with getting any 47n or do I need to pay attention to the 100 and 400 as well? For example, a capacitor I got in a kit has 47nK100 written on it - can I get a 47n 400 VCD to replace it and will it do the same thing? 

Gagaryn

You can use any, voltage rating (the v) doesn't matter in a guitar. Higher v number general means the cap will be bigger so might have trouble fitting it in the cavity.

liamon

Thank you. Does the same go for an overdrive circuit?

Gagaryn

Yip!

liamon

Thank you!

timhulio

The 400v capacitors will be too big.

liamon

timhulio said:

The 400v capacitors will be too big.

Thanks - I have 2 of your pedals and they are freakin awesome. 

robinbowes

liamon said:

Thank you. Does the same go for an overdrive circuit?

Gagaryn said:

Yip!

Not strictly true. You must ensure that the voltage rating of the part is higher than the voltage used in the circuit.

Of course, this is rarely a problem in low voltage (9v) circuits as most cap are rated higher than 9v.

R.

Sporky

Caps should be rated for about double the highest voltage that'll be across them; get the lowest possible VDC that meets this. For ceramics and the like it'll usually be at least 100v or so, sometimes the lowest is nearer 1000v. Usually it doesn't matter, but some caps do behave differently (ie they don't act like their rated capacitance) at voltages significantly lower than their rating.

In a pedal it doesn't much matter as long as it's over the maximum applied voltage differential.

ICBM

robinbowes said:

Of course, this is rarely a problem in low voltage (9v) circuits as most cap are rated higher than 9v.

But you do have to be careful with electrolytics and especially Tantalums - 6.3V ratings aren't uncommon.

Danny1969

Yep low voltage Tantalum caps are very common now, especially in surface mount form as it's the smallest form factor for buck converter smoothing in laptops, tablet PC's etc ... and these to me seem about the most unreliable type of cap ever made, generally any dead short on a laptop board is a failed tantalum 

Here's a question, an electrolytic cap needs a DC polarising voltage to work but how low can that voltage be ? I mean as an example I've got a circuit where I've temporarily had to use the wrong opamp, so I have a DC offset on the output of around 200mV. As a stop gap I've capacitor coupled the next stage and as it's a very low impedance I've had to use an electrolytic . I was unsure if the 200mV would suffice but it seems to be fine and the offsets gone after the cap .... just made me wonder how low the polarising could be ?   

jpfamps

Danny1969 said:

Yep low voltage Tantalum caps are very common now, especially in surface mount form as it's the smallest form factor for buck converter smoothing in laptops, tablet PC's etc ... and these to me seem about the most unreliable type of cap ever made, generally any dead short on a laptop board is a failed tantalum 

Here's a question, an electrolytic cap needs a DC polarising voltage to work but how low can that voltage be ? I mean as an example I've got a circuit where I've temporarily had to use the wrong opamp, so I have a DC offset on the output of around 200mV. As a stop gap I've capacitor coupled the next stage and as it's a very low impedance I've had to use an electrolytic . I was unsure if the 200mV would suffice but it seems to be fine and the offsets gone after the cap .... just made me wonder how low the polarising could be ?   

The general rule of thumb with electrolytics used to be that they needed to see at least a 1/3 their rated voltage to perform correctly.

As a result of this there was an aversion to employing electrolytic in circuits with no polarising DC voltage; however this position seems to have been refuted and you regularly see electrolytics as DC blocking caps in op-amp based circuits. Other than at turn on where the +/- rails might not come up evenly, there should be no voltage across the cap.

I'd agree with tants being shite.

Adam_MD

Anyone know why tants get used in audio paths if they're shit?  Genuine question I'd like to know.  

Sporky

For a while the boutiques liked to suggest that they sounded better.

jpfamps

Danny1969 said:

Yep low voltage Tantalum caps are very common now, especially in surface mount form as it's the smallest form factor for buck converter smoothing in laptops, tablet PC's etc ... and these to me seem about the most unreliable type of cap ever made, generally any dead short on a laptop board is a failed tantalum 

Here's a question, an electrolytic cap needs a DC polarising voltage to work but how low can that voltage be ? I mean as an example I've got a circuit where I've temporarily had to use the wrong opamp, so I have a DC offset on the output of around 200mV. As a stop gap I've capacitor coupled the next stage and as it's a very low impedance I've had to use an electrolytic . I was unsure if the 200mV would suffice but it seems to be fine and the offsets gone after the cap .... just made me wonder how low the polarising could be ?   

The general rule of thumb with electrolytics used to be that they needed to see at least a 1/3 their rated voltage to perform correctly.

As a result of this there was an aversion to employing electrolytic in circuits with no polarising DC voltage; however this position seems to have been refuted and you regularly see electrolytics as DC blocking caps in op-amp based circuits. Other than at turn on where the +/- rails might not come up evenly, there should be no voltage across the cap.

I'd agree with tants being shite.

Danny1969

I think tants have a lower ESR than an Electrolytic so maybe for real low impedance stuff, like headphone coupling there's an advantage ? 

ICBM

Adam_MD said:

Anyone know why tants get used in audio paths if they're shit?  Genuine question I'd like to know.  

I think because electrolytics were thought to need polarising - however as jpfamps said, this appears to not be the case.

I did try tants recently when I was trying to trace a microscopic DC leak in a high-impedance circuit where there was a problem with charge buildup, but it made no difference. I wouldn't have bothered otherwise...

jpfamps

Tants are used because they have low ESR which can be VERY important in higher frequency applications (eg switching mode power supplies), but pointless at audio.

Modern electrolytics are far better now in this regard than they used to be, but tants are still smaller so useful when PCB space is at a premium.

Tants have virtually no tolerance of reverse polarity.

Adam_MD

Thanks for the education chaps.