I have an old vintage tape recorder that uses N-channel FET switches (2SK362) and P-channel FET switches (2SJ104). These are now obsolete, and I was also wanting an FET that had lower on-resistance----currently the N-channel devices are 80Ω and the P-channels are 40Ω. Can anyone suggest alternative replacements? These are both in a TO-92 package.
Nothing like answering your own question! The best FET switches I found were:
N-channel-----BS170, 1.2Ω on, ~33¢ each
P-channel-----TP0606N3-G, 3.5Ω on, ~91¢ each
N-channel-----BS170, 1.2Ω on, ~33¢ each
P-channel-----TP0606N3-G, 3.5Ω on, ~91¢ each
BS170 is an enhancement MOSFET, while 2SK362 is a (depletion-mode) JFET. Does that matter?
Yes. An N-channel JFET or depletion-mode N-channel MOSFET is on at VGS = 0, while an enhancement N-channel MOSFET needs a substantial positive gate-source voltage to turn on. Something similar applies in the P-channel case, but with the sign of the gate-source voltage reversed.
Well, I found an N-channel JFET (Vgs=0v=ON) ----J109D26Z----that met all the other requirements (dissipation, max Vds, max Vgs, and max Id) and had an on-resistance of 12Ω---definitely an improvement!! But I'll be damned if I can find a p-channel JFET that can better the SJ104's 40Ω on-resistance. Any suggestions???
Very low resistance P channel jfets have never been made - device physics.
I used to buy a selection of the J109 die with 5 Ohm rds on, those were the days
I used to buy a selection of the J109 die with 5 Ohm rds on, those were the days
dotneck335 needs JFETs or depletion-mode MOSFETs, BS170, BS250 and BS107 are all enhancement MOSFETs as far as I know.
If the control voltages are changed it will work (depending on the circuit). I am afraid there is not much choice left in through hole discrete parts. As dotneck335 proposed in another thread the new ADG6412 maybe can be used as an excellent analog switch. Four of them actually. It would help to know the exact function of the old JFET switches. I mean, I use BS170 to drive LEDs and micro relays but that can also be done with bipolar transistors.
Just trying to think in solutions. Maybe ADG1519 can be used on an adapter PCB. All in all it will cost time and money to keep old stuff running with todays parts.
Just trying to think in solutions. Maybe ADG1519 can be used on an adapter PCB. All in all it will cost time and money to keep old stuff running with todays parts.
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Of the P channel JFETs you can not find 🙂
Seriously, even many SMD JFETs have meanwhile disappeared. Let alone that they had equal or better specifications compared to 2SJ104.
Seriously, even many SMD JFETs have meanwhile disappeared. Let alone that they had equal or better specifications compared to 2SJ104.
interfet.com is one of the older/steady sources of Jfets. I once gave them a spec of an obsolete toshiba Jfet to be used as current limiter/ fuse and they had a hard time matching the spec. as they also supply a lot of military parts, they will probably stay in business. The big silicon vendors are all pulling out.
From what I see I deduct that IC based stuff is the future and todays popular and successful ICs better are bought in time so now in a small amount for repairs. It makes sense to pick out the sturdy ones and not the ones that are known to have higher than usual failure rates. Some chipamp ICs are already hard to find.
Finding popular spare parts long after they have become discontinued is difficult, expensive and time consuming. I regularly see repairs of old devices where the parts alone are more expensive than a new device will cost.
Finding popular spare parts long after they have become discontinued is difficult, expensive and time consuming. I regularly see repairs of old devices where the parts alone are more expensive than a new device will cost.
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I also thought of the ADG6412 as an alternative, but it's just not practical for retrofit into an existing PCB (at least on the decks I have). I also considered modifying the control voltage, but, again, it just is not practical in this case. The parallel idea might be worth a shot, as I think one could possibly fit 2 FETS together in the space for one.
But is the gate control voltage of your tape deck enough to completely shut off this much higher Vgs (OFF) low RDS (ON) replacement? Maybe diode(s) in series with the gate will prove helpful if such will prove to be the case.
The N-channel gate control is -15 volts for off, so this should be sufficient; it also DOES have a series gate diode---but I do NOT see how this help turn it off? Seems like it would just LOWER the negative voltage at the gate----???