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June 30, 2009
AC / DC
 
 


It seems like every day I’m getting some fantastic feedback from readers of our Daily Blog...for me...the really great thing is that I’m learning about some new and really interesting applications...
 
I’m really excited that the Daily Blog has now turned into a dialogue rather than a monologue...keep the questions and ideas coming!!
 
Speaking of interesting topics (as much as fuses can be)...Steve Victor of Fedco Electronics, a value-added manufacturer and distributor of batteries in Wisconsin, recently wrote to me about my e-mail regarding the "not to exceed" voltage rating of fuses and circuit breakers..
 
Steve reminded me that there are really two different voltage types:  Alternating Current (AC) and Direct Current (DC).  A wall outlet has AC current while a battery (as well as solar panels and DC power supplies) produces DC current.
 
AC Voltage
 
Just like the name says, alternating current alternates up and down, rising up to about 170 volts and then going in the opposite direction to -170 volts.  Each time the voltage changes direction, it will pass through 0 volts on it’s way to the opposite peak.  This is important to note since at 0 volts...the "fuse spark" will extinguish itself and open the circuit.  In North America, this 0 volt situation happens 120 times per second!!
 
DC Voltage
 
Unlike AC Voltage, DC Voltage does not alternate.  This means the voltage never goes to 0 volts but rather stay constant.  Therefore when a fuse or circuit breaker opens up, there will always be a "fuse spark" present. 
 
For this reason, the DC voltage rating of a fuse or circuit breaker is generally considerably less than its AC rating (in most cases less than half).  Now some fuses and circuit breakers are designed specifically to open DC currents (such as automotive fuses and/or breakers) and are rated typically at 32VDC whereas electronic fuses such as the glass or ceramic tube fuses are typically rated at 250VAC. 
 
Does that mean you can’t use an AC in a DC application?  No...that just means that it hasn’t been tested any of the safety agencies (UL / CSA) at that voltage.
 
The calculations in converting the AC rating to the DC rating can be complicated at best and impossible at worse.  If you have an application that requires a DC rating, please contact OptiFuse so you can choose the right fuse for your application. 
 
Take-a-way points:
  • Voltage come in two flavors:  AC and DC
  • An AC circuit is easier for a fuse or circuit breaker to open due to the 0 volt crossing of AC voltage
  • Some fuses or circuit breakers are designed specially to interrupt DC voltage - such as automotive fuses

I realize that some of these topics are somewhat complex for non-technical people...but I appreciate you indulging us

Thank you so much for your continued support of OptiFuse and thanks for your participation to our learning experience...
 



Jim Kalb
President
OptiFuse
jimk@optifuse.com

 

 
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