try and use an IRFP450 (IRFP250 (0,075 ohms works even better if resists the voltages), if it however fit's in there, the IFRP450 has only 0.4 ohms ON-resistance, there better ones too, You may be able to replace the two IRF840 with one IRFP450, which will reduce input capacity quiet a bit, this might give you better pulses, anyway, you may check if your driver circuit is capable of driving the FET's fast enough into saturation.
Just some thoughts
Friedel
At 05:14 16/11/03 -0800, you wrote:
Naw, there are only two and I replaced em both. One of the 820s was shorted, but so was just about everything else in the tool (it is an oil well logging tool). They dump a high voltage pulse from an inductor into a ultrasonic magnorestrictive transducer at 20 cps. I don't seem to have enough trigger pulse to make em work. Would the 820 be more sensitive than the 840 with regard to the trigger pulse? Most of these type tools use an SCR; this is my first experience with a mosfet fired transmitter.
----- Original Message ----- To: "Multiple recipients of list CHIPDIR-L" <[EMAIL PROTECTED]> Sent: Sunday, November 16, 2003 6:04 AM
> On Sun, Nov 16, 2003 at 01:49:19AM -0800, Syd Levine (AnaLog) > enlightened us thusly > > I just replaced two IRF820 mosfets with IRF840 mosfets. The 820s were > > bad, but the device still won't work. Should the 840 work OK in place > > of the 820, or have I created a new problem with the substitution? > > > You're fine unless they are sharing current with other irf820s, in which > case they will hog it. They have similar voltage specs, but lower > resistance and hence higher current. > > > They need about 10V on the gate if you're switching. How were the 820s > bad? That will point you to your problem. > -- > > With best Regards, > > > Declan Moriarty. > -- > Author: Declan Moriarty > INET: [EMAIL PROTECTED] > > Fat City Network Services -- 858-538-5051 http://www.fatcity.com > San Diego, California -- Mailing list and web hosting services > --------------------------------------------------------------------- > To REMOVE yourself from this mailing list, send an E-Mail message > to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in > the message BODY, include a line containing: UNSUB CHIPDIR-L > (or the name of mailing list you want to be removed from). You may > also send the HELP command for other information (like subscribing). >
-- Author: Syd Levine \(AnaLog\) INET: [EMAIL PROTECTED]
Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
-- Author: Friedel Bruening INET: [EMAIL PROTECTED]
Fat City Network Services -- 858-538-5051 http://www.fatcity.com San Diego, California -- Mailing list and web hosting services --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
