| Electronics > Repair |
| Transistor Jfet HP equivalent |
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| bfsy:
Can this be replaced by 2N4392 ??? |
| Ground_Loop:
--- Quote from: dacman on January 29, 2017, 03:35:54 am ---If you have some that are working, you could measure their conductance and cutoff voltage and then look for a cross. About all JFETs are D-MODE (depletion mode). --- End quote --- I did exactly this trying to match some JFETs in my HP 3456. Had to pull a good one and run it on my curve tracer. |
| Kleinstein:
--- Quote from: edavid on April 22, 2022, 11:31:22 pm --- --- Quote from: HUGOMAN on January 29, 2017, 11:18:57 pm ---i don't have yet Dca pro, just only i have chiness product it displays for the 1855-0743 N-Jfet I= 2,0 mA vgs= 1,4v. --- End quote --- That sounds close to a J113. They are not too hard to find. --- End quote --- The combination of Idss of 2 mA and 1.4 V threshold would be more a smaller JFET like J201/J202. The 2N4393 / J113 are larger JFETs with a higher Idss ( ~ 12 mA) when the threshold is at 1.4 V. In the 3458 ohms circuit it looks like the JFETs don't need to be very low resistance and there also seems to be plenty of gate votlage to turn them off reliably. So a somewhat larger resistance (and thus ideally lower leakage) jFET would be more reasonable, possibly even the rather high resistance 2N4118. |
| Wallace Gasiewicz:
As I remember the jFETS in an HP 3456 are used for the most part to turn things on and off. I think the same goes for the 3458. The important thing is to have the "on" value the same. To me this means the "on" resistance of the FETs should be low and the same on all the FETs. As I recall the resistance from source to drain was about 1.2 ohms, tell me if I am wrong (please do tell me so we do not confuse the issue for the OP). You can get a bunch of cheap 310 jFETS and sort through them. (not from China) If the FETs are not matched you will get unusual behavior including unstable readings in the least significant digits. They are switched "off" hard by large voltages, so I do not think small differences in gate response is very important. |
| Kleinstein:
1.2 ohms on resistance would be unusual low. The usual swtiching types are 2N4393 and similar (PN4393,J113, BSR56,...) with an on resistance of some 30-50 ohms. The higher threchold types like J112 offer slightly lower resistance, but have less headroom to turn off. Lower resistance types are used only when really low resistance is needed - even the J108 is at some 8 ohms, so 1.2 ohms would be an exceptional large one. Lower resistance also comes with more leakage. The J310 type would not work the specs are 25 V max., so too low in voltage. Matching of the on resistance is usually not that critcal, a few 100 ohms more or less would hardly matter in most uses. The R_on of the 2N4117 in the 5 K range can be a problem is some cases. Today the problem is the rather limited choice of JFETs in a TO92 case. To18 case often works, but often has the case tied to gate and they are a bit more expensive. |
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