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question about a capacitor

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psecody

Member
Well I'm building a power supply that calls for a 1uf @ 40V and I don't have one an I kind of doubt radioshack will have one but I did find a 1uf @ 50V and I was wondering if these could be interchangable? Thanks I appreciate the help
 
Yes no problem what so ever.

The voltage rating of a capacitor relates to the voltage that a capacitor can withstand so there will be no problem using a 50V rated capacitor instead of a 40V one
 

john1

Active Member
Make sure its supposed to be AC wkg
or DC wkg.

Also if its DC wkg, it has to be the right way round.
If its AC working, it can be either way round,
but sometimes one side is preferred as neutral or ground.

Best of luck, John :)
 

psecody

Member
thanks alot guys I appreciate it, that what I thought that the voltage rating was just the max it could take but I wasn't sure. All the caps I'm using in this project are on the DC side of the circuit so I'm good to go in that area. Thanks guys.
 

Hero999

Banned
Note that you can use an AC capacitor in a DC circuit but not the other way round.
 

psecody

Member
ok I got another capacitor related question. I need a 2200µF 63V capacitor and I don't have one and I'm pretty sure that our radioshack doesn't have one and I'm trying to get away without having to order parts. So is there anyway I could combine capacitors to get that amount? The problem isn't really the capacity but the voltage, I don't have any rated over 50V so I didn't know if combining them would help this. I actually don't think that works but just incase I'm wrong I thought I'd ask. Thanks
 

psecody

Member
Ok sorry but I thought I'd post the link to the schematics I'm using **broken link removed**. Looking at the schematics the 2200µF cap is after the transformer so it looks like to me the highest voltage going to it would be 24V so could I get away using some 50V caps? Also the question from above about combining caps to get the 2200 value still remains. Thanks

edit: While I was looking at it, on the transformer why does it have to be a 24V 3A if the output of the power supply is 1.5A? Couldn't I just use a 24V 1.5A transformer? If not why? Sorry for all the stupid questions I appreciate ya'll taking the time to answer them I know ya'll probably have better things to do. Thanks
 
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Gayan Soyza

Active Member
If you don’t have 2200uF caps then you can combine two 1000 uF in parallel.
But the voltage for that 1000 uF must be 63V each.

In the circuit diagram you can use a 2200/ 50V caps. It’s not reaching to its peak value more than 50V in 24V AC transformer.
 

psecody

Member
Yeah thats what I figured after I started looking at the schematic. Thanks. But on the transformer does it have to be 3A? because I haven't found one of those in stores or online and I don't see why it needs to be 3A if the output is only going to be 1.5A. Thanks again
 

Gayan Soyza

Active Member
If you draw the same amount of current the transformer has, the power rating of the transformer is going to its maximum. So the transformer is getting heat too much.
So in good practice we take a transformer which exceeds the output require current.

Actually you don’t need a 3A transformer for that power supply. Because the LM 317T can produce only 1.5A maximum current. You can replace that with 1.5A one.

I think the designer had a 3A transformer at his home that’s why he has added that transformer.
 
In the circuit above the transformer secondary is 24V. This means it has a peak voltage of 36V (1½ x 24)* so a 40V capacitor will be OK.
It is possible to connect capacitors in series to raise their voltage rating but there are 2 disadvantages which makes it best to avoid series capacitors. :
1) the overall effective capacitance is halved
2) You need resistors across the capacitors to distribute the voltage.

Its much easier to use smaller value capacitors of the correct voltage rating connected in parallel.

A 2 Amp transformer would be adequate with a LM317 regulator. You could even use a transformer with a lower voltage though that would limit the PSU output voltage.


*The actual peak value is √2 (= 1.414) times the AC voltage. In practice 1½ times the AC voltage
 

stevez

Active Member
Gayan touched on an important point regarding the transformer capacity. Devices that handle current (transformers, resistors, transistors, etc) produce heat as a result of the flow of current. These devices may be expected to handle the rated current flow continuously or only for short periods of time. These devices can be built to handle the rated current for short periods - or continuously. It is likely that the device made to handle the rated current for short periods will be smaller and less costly. If you understand your needs you can better define the requirements of the devices. Quite often the needs aren't so clear - or the specifics about the devices to be used aren't available. One approach can be to oversize the device. Derating is another word that applies here - we might "derate" a 3 amp transformer made for intermittent service so that it can be used for continuous service.

Temperature is another significant factor. A transformer might carry a rating that is based on a 25 deg C ambient temperature. That's more or less room temperature. An equipment enclosure would likely be warmer. Good design practice would require one to de-rate the transformer based on the expected operating temperature.

As a hobbyist you might not get too wrapped up in all this but someone designing products or systems where reliability is an issue would need to understand and manage these factors. Many people just apply some factor of safety ( a 3 amp transformer for a 1.5 amp load is an example) or put the project together and look for problems.
 

psecody

Member
Thank I appreciate the answers. I can get the 2 Amp transformer no problem so I'll get to start on the power supply tonight. Thanks I appreciate it
 
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