The Cyber-Spy.Com Usenet Archive Feeds Directly
From The Open And Publicly Available Newsgroup
This Group And Thousands Of Others Are Available
On Most IS NNTP News Servers On Port 119.
Cyber-Spy.Com Is NOT Responsible For Any Topic,
Opinions Or Content Posted To This Or Any Other
Newsgroup. This Web Archive Of The Newsgroup And
Posts Are For Informational Purposes Only.
From: Crusty Curmudgeon
Subject: Re: Using the body diode of a MOSFET to protect against relay back EMF
X-Newsreader: Forte Agent 1.9/32.560
Date: Tue, 21 Jan 2003 09:41:07 -0600
NNTP-Posting-Date: Tue, 21 Jan 2003 09:39:44 CST
On Tue, 21 Jan 2003 14:59:03 +0000 (UTC), "burp" wrote:
>"Klaus Kragelund" wrote in message
>> According to the subject message - is this possible?
>> Specifically I'm thinking about a 2N7002 transistor which will be able
>> to carry the current my app. needs. 18Volts relay - 50mA relay
>> current. The transistor will function both as the switch but also the
>> catch diode.
>> When using the body diode the voltage at the drain terminal will
>> become perhaps 2volts negative - is this a problem? Furthermore the
>> ciculating current will no longer be limited to the standard
>> diode/relay loop - but will run in the entire power supply - but this
>> needn't pose a problem.
>Is wrong way around. Put on thinking cap.
>Mosfet on, positive current through relay.
>Mosfet off, still positive current through relay. Charg up stray capacitance
>in positive direction...... break down mosfet body-source diode and reset.
>Is mosfet rated for avalanche breakdown?
>Read data sheet. If so can mosfet survive event. Can mosfet survive multiple
>How much do mosfet cost versus transistor plus diode and two resistor?
>English too good for German.
>Perhaps bored but still have no sense of humor.
I'm not too wild about just a catch diode across the coil. The dv/dt on flyback
can still trash the gate. I like to add a series 100R and .001uF in shunt with
the coil to limit the dv/dt to something safe. It also keeps the voltage spike
induced in adjacent paths and wiring to a minimum.
"Make peace an attitude. Want it. Make peace a habit. Live it. Make peace a reality. Share it." Mattie Stepanek, age 11,
speaking from his wheelchair to school children in Independence MO. Mattie, a published poet and television celebrity,
spoke at the annual peace assembly at Community of Christ headquarters, May 2002.
Go Back To The Cyber-Spy.Com
Usenet Web Archive Index Of
The sci.electronics.design Newsgroup