How to Read a Capacitor
Know the units of measurement., Read the capacitance value., Look for a tolerance value., Check the voltage rating., Look for a + or - sign.
Step-by-Step Guide
-
Step 1: Know the units of measurement.
The base unit of capacitance is the farad (F).
This value is much too large for ordinary circuits, so household capacitors are labeled with one of the following units:1 µF, uF, or mF = 1 microfarad = 10-6 farads. (Careful — in other contexts, mF is the official abbreviation for millifarads, or 10-3 farads.) 1 nF = 1 nanofarad = 10-9 farads. 1 pF, mmF, or uuF = 1 picofarad = 1 micromicrofarad = 10-12 farads. -
Step 2: Read the capacitance value.
Most large capacitors have a capacitance value written on the side.
Slight variations are common, so look for the value that most closely matches the units above.
You may need to adjust for the following:
Ignore capital letters in the units.
For example, "MF" is just a variation on "mf." (It is definitely not a megafarad, even though this is the official SI abbreviation.) Don't get thrown by "fd." This is just another abbreviation for farad.
For example, "mmfd" is the same as "mmf." Beware single-letter markings such as "475m," usually found on smaller capacitors.See below for instructions. , Some capacitors list a tolerance, or the maximum expected range in capacitance compared to its listed value.
This isn't important in all circuits, but you may need to pay attention to this if you require a precise capacitor value.
For example, a capacitor labeled "6000uF +50%/-70%" could actually have a capacitance as high as 6000uF + (6000 *
0.5) = 9000uF, or as low as 6000 uF
- (6000uF *
0.7) = 1800uF.
If there is no percentage listed, look for a single letter after the capacitance value or on its own line.
This may be code for a tolerance value, described below. , If there is room on the body of the capacitor, the manufacturer usually lists voltage as a number followed by a V, VDC, VDCW, or WV (for "Working Voltage").This is the maximum voltage the capacitor is designed to handle. 1 kV = 1,000 volts.
See below if you suspect your capacitor uses a code for voltage (a single letter or one digit and one letter).
If there is no symbol at all, reserve the cap for low-voltage circuits only.
If you are building an AC circuit, look for a capacitor rated specifically for VAC.
Do not use a DC capacitor unless you have an in-depth knowledge of how to convert the voltage rating, and how to use that type of capacitor safely in AC applications., If you see one of these next to a terminal, the capacitor is polarized.
Make sure to connect the capacitor's + end to the positive side of the circuit, or the capacitor could eventually cause a short or even explode.If there is no + or
-, you can orient the capacitor either way.
Some capacitors use a colored bar or a ring-shaped depression to show polarity.
Traditionally, this mark designates the
- end on an aluminum electrolytic capacitor (which are usually shaped like tin cans).
On tantalum electrolytic capacitors (which are very small), this mark designates the + end.(Disregard the bar if it contradicts a + or
- sign, or if it is on a non-electrolytic capacitor.) -
Step 3: Look for a tolerance value.
-
Step 4: Check the voltage rating.
-
Step 5: Look for a + or - sign.
Detailed Guide
The base unit of capacitance is the farad (F).
This value is much too large for ordinary circuits, so household capacitors are labeled with one of the following units:1 µF, uF, or mF = 1 microfarad = 10-6 farads. (Careful — in other contexts, mF is the official abbreviation for millifarads, or 10-3 farads.) 1 nF = 1 nanofarad = 10-9 farads. 1 pF, mmF, or uuF = 1 picofarad = 1 micromicrofarad = 10-12 farads.
Most large capacitors have a capacitance value written on the side.
Slight variations are common, so look for the value that most closely matches the units above.
You may need to adjust for the following:
Ignore capital letters in the units.
For example, "MF" is just a variation on "mf." (It is definitely not a megafarad, even though this is the official SI abbreviation.) Don't get thrown by "fd." This is just another abbreviation for farad.
For example, "mmfd" is the same as "mmf." Beware single-letter markings such as "475m," usually found on smaller capacitors.See below for instructions. , Some capacitors list a tolerance, or the maximum expected range in capacitance compared to its listed value.
This isn't important in all circuits, but you may need to pay attention to this if you require a precise capacitor value.
For example, a capacitor labeled "6000uF +50%/-70%" could actually have a capacitance as high as 6000uF + (6000 *
0.5) = 9000uF, or as low as 6000 uF
- (6000uF *
0.7) = 1800uF.
If there is no percentage listed, look for a single letter after the capacitance value or on its own line.
This may be code for a tolerance value, described below. , If there is room on the body of the capacitor, the manufacturer usually lists voltage as a number followed by a V, VDC, VDCW, or WV (for "Working Voltage").This is the maximum voltage the capacitor is designed to handle. 1 kV = 1,000 volts.
See below if you suspect your capacitor uses a code for voltage (a single letter or one digit and one letter).
If there is no symbol at all, reserve the cap for low-voltage circuits only.
If you are building an AC circuit, look for a capacitor rated specifically for VAC.
Do not use a DC capacitor unless you have an in-depth knowledge of how to convert the voltage rating, and how to use that type of capacitor safely in AC applications., If you see one of these next to a terminal, the capacitor is polarized.
Make sure to connect the capacitor's + end to the positive side of the circuit, or the capacitor could eventually cause a short or even explode.If there is no + or
-, you can orient the capacitor either way.
Some capacitors use a colored bar or a ring-shaped depression to show polarity.
Traditionally, this mark designates the
- end on an aluminum electrolytic capacitor (which are usually shaped like tin cans).
On tantalum electrolytic capacitors (which are very small), this mark designates the + end.(Disregard the bar if it contradicts a + or
- sign, or if it is on a non-electrolytic capacitor.)
About the Author
Jennifer Burns
Professional writer focused on creating easy-to-follow pet care tutorials.
Rate This Guide
How helpful was this guide? Click to rate: