All FETS have 50% threshold tolerance on gate voltage and those std types with Vt= 2 to 4V need at least 250% times the actual threshold to make a good switch. Although BJT transistors have a vast range in hFE, however when used as a switch, it is the actual Vbe that controls the current. Normally they are typically rated as switches with base current = 10% of Ic unless they are super beta types hFE> 300. The critical power dissipation in switches is the ON resistance, such as Vce(sat)/Ic=Rce and RdsOn @ Vgs. But the secret in fast pinball bumpers is the low T=L/R. That means lower inductance and higher resistance but hotter or less efficient but faster. This is important for spring back solenoid bumpers and flippers. Dry contact switches produce very high back EMF so a clamp diode is needed but this slows the L/R=T time even with a stiff spring. So instead of a diode clamp by adding a Zener & diode in series makes it faster and limits the maximum voltage for the switch for safety. As far as substitutions, one needs to know the load, current, voltage and function. Otherwise, why would they stock seventy-five thousand different FETs. It's a matter of the heatsink method, size, cost, drive voltage, load current, availability, and compatibility. Copying newer designs can help.
Rule of thumb for replacing power transistors with MOSFets?
Started by ●March 15, 2023
Reply by ●March 16, 20232023-03-16
Reply by ●March 16, 20232023-03-16
On 15/03/2023 7:00 pm, John Robertson wrote:> I'm updating some game board designs and was curious if there is any > sort of printed guideline for replacing silicon driver transistors with > MOS? Both NPN and PNP. > > For example TIP122 Darlington transistors driven by another pre-driver, > that is the TTL interface. I've seen many a substitute, but would like > to have the theory explained so I can make better use of it and > understand exactly what I am doing. > > A good reference book you can recommend perhaps? Books and I usually get > along very well. > > Too many projects and not enough to hunt down all the solutions! > > Thanks, and sorry for the on-topic post... > > John :-#)# >Can you say what kind of loads they are switching and at what rates? piglet
Reply by ●March 17, 20232023-03-17
On a sunny day (Thu, 16 Mar 2023 09:13:58 -0700) it happened John Larkin <jlarkin@highlandSNIPMEtechnology.com> wrote in <5vf61i137r720gkme27pk26j93pp8so6cs@4ax.com>:>On Thu, 16 Mar 2023 07:15:30 GMT, Jan Panteltje <alien@comet.invalid> >wrote: > >>On a sunny day (Wed, 15 Mar 2023 12:00:02 -0700) it happened John Robertson >><jrr@flippers.com> wrote in <tut4jn$10ok6$1@dont-email.me>: >> >>>I'm updating some game board designs and was curious if there is any >>>sort of printed guideline for replacing silicon driver transistors with >>>MOS? Both NPN and PNP. >>> >>>For example TIP122 Darlington transistors driven by another pre-driver, >>>that is the TTL interface. I've seen many a substitute, but would like >>>to have the theory explained so I can make better use of it and >>>understand exactly what I am doing. >>> >>>A good reference book you can recommend perhaps? Books and I usually get >>>along very well. >>> >>>Too many projects and not enough to hunt down all the solutions! >>> >>>Thanks, and sorry for the on-topic post... >>> >>>John :-#)# >> >>This is how MOSFETs work: >>https://en.wikipedia.org/wiki/MOSFET >> >> >>If you want to use one: there are basically 2 types: some conduct with zero volts at the gate >>and some do not conduct with zero volts at the gate. >> >>To interface with 5V logic I usually use an IRFZ44A MOSFET (off when 0V at the gate). >>10 for $8.63 on ebay.. >> https://www.ebay.com/itm/324380901727 >>even cheaper at other places >>datasheet: >> https://datasheetspdf.com/pdf-file/799031/Samsung/IRFZ44A/1 >> >>look at page 3 figure 2 for at what voltage the thing starts conducting. >> >>At 5 V from TTL logic it can do several amps. > >Note that 74-series TTL does not pull up to 5 volts.Yes, but even at 4.5 V gate that IRFZ44A does 10 A
Reply by ●March 17, 20232023-03-17
On Fri, 17 Mar 2023 06:32:57 GMT, Jan Panteltje <alien@comet.invalid> wrote:>On a sunny day (Thu, 16 Mar 2023 09:13:58 -0700) it happened John Larkin ><jlarkin@highlandSNIPMEtechnology.com> wrote in ><5vf61i137r720gkme27pk26j93pp8so6cs@4ax.com>: > >>On Thu, 16 Mar 2023 07:15:30 GMT, Jan Panteltje <alien@comet.invalid> >>wrote: >> >>>On a sunny day (Wed, 15 Mar 2023 12:00:02 -0700) it happened John Robertson >>><jrr@flippers.com> wrote in <tut4jn$10ok6$1@dont-email.me>: >>> >>>>I'm updating some game board designs and was curious if there is any >>>>sort of printed guideline for replacing silicon driver transistors with >>>>MOS? Both NPN and PNP. >>>> >>>>For example TIP122 Darlington transistors driven by another pre-driver, >>>>that is the TTL interface. I've seen many a substitute, but would like >>>>to have the theory explained so I can make better use of it and >>>>understand exactly what I am doing. >>>> >>>>A good reference book you can recommend perhaps? Books and I usually get >>>>along very well. >>>> >>>>Too many projects and not enough to hunt down all the solutions! >>>> >>>>Thanks, and sorry for the on-topic post... >>>> >>>>John :-#)# >>> >>>This is how MOSFETs work: >>>https://en.wikipedia.org/wiki/MOSFET >>> >>> >>>If you want to use one: there are basically 2 types: some conduct with zero volts at the gate >>>and some do not conduct with zero volts at the gate. >>> >>>To interface with 5V logic I usually use an IRFZ44A MOSFET (off when 0V at the gate). >>>10 for $8.63 on ebay.. >>> https://www.ebay.com/itm/324380901727 >>>even cheaper at other places >>>datasheet: >>> https://datasheetspdf.com/pdf-file/799031/Samsung/IRFZ44A/1 >>> >>>look at page 3 figure 2 for at what voltage the thing starts conducting. >>> >>>At 5 V from TTL logic it can do several amps. >> >>Note that 74-series TTL does not pull up to 5 volts. > >Yes, but even at 4.5 V gate that IRFZ44A does 10 AAdd a pullup, or substitute an HCT part.
Reply by ●March 17, 20232023-03-17
On Thursday, March 16, 2023 at 12:16:14 PM UTC-4, John Larkin wrote:> On Thu, 16 Mar 2023 05:06:35 -0700 (PDT), Fred Bloggs > <bloggs.fred...@gmail.com> wrote: > >On Wednesday, March 15, 2023 at 3:00:19?PM UTC-4, John Robertson wrote: > >> I'm updating some game board designs and was curious if there is any > >> sort of printed guideline for replacing silicon driver transistors with > >> MOS? Both NPN and PNP. > >> > >> For example TIP122 Darlington transistors driven by another pre-driver, > >> that is the TTL interface. I've seen many a substitute, but would like > >> to have the theory explained so I can make better use of it and > >> understand exactly what I am doing. > >> > >> A good reference book you can recommend perhaps? Books and I usually get > >> along very well. > >> > >> Too many projects and not enough to hunt down all the solutions! > > > >Unless there's an availability problem with the bipolar, leave it. As far as any rules of thumb: don't touch a hot case with your bare thumb. > > > >> > >> Thanks, and sorry for the on-topic post... > > > >You're one of these whiners always complaining about the group content, while contributing little to nothing yourself. > > > Never miss the opportunity to redirect a technical thread to an insult > fest.Fet is not a word, it's an acronym, so it's not MOSFet, it's MOSFET. Post is off topic, because it doesn't have much to do with design, it belongs is sci basic basics.
Reply by ●March 17, 20232023-03-17
On Thursday, March 16, 2023 at 4:39:52 PM UTC-4, Anthony Stewart wrote:> All FETS have 50% threshold tolerance on gate voltage and those std types with Vt= 2 to 4V need at least 250% times the actual threshold to make a good switch. > > Although BJT transistors have a vast range in hFE, however when used as a switch, it is the actual Vbe that controls the current. Normally they are typically rated as switches with base current = 10% of Ic unless they are super beta types hFE> 300.The external circuit determines the current in a transistor switching application. The design merely ensures the base drive current is sufficient to support that current worst case, while keeping it to a minimum..> > The critical power dissipation in switches is the ON resistance, such as Vce(sat)/Ic=Rce and RdsOn @ Vgs. > > But the secret in fast pinball bumpers is the low T=L/R. > That means lower inductance and higher resistance but hotter or less efficient but faster. > This is important for spring back solenoid bumpers and flippers. > Dry contact switches produce very high back EMF so a clamp diode is needed but this slows the L/R=T time even with a stiff spring. So instead of a diode clamp by adding a Zener & diode in series makes it faster and limits the maximum voltage for the switch for safety. > > As far as substitutions, one needs to know the load, current, voltage and function. Otherwise, why would they stock seventy-five thousand different FETs. > > It's a matter of the heatsink method, size, cost, drive voltage, load current, availability, and compatibility. Copying newer designs can help.
Reply by ●March 17, 20232023-03-17
On Thursday, March 16, 2023 at 9:28:43 AM UTC-4, legg wrote:> On Thu, 16 Mar 2023 05:06:35 -0700 (PDT), Fred Bloggs > <bloggs.fred...@gmail.com> wrote: > >On Wednesday, March 15, 2023 at 3:00:19?PM UTC-4, John Robertson wrote: > >> I'm updating some game board designs and was curious if there is any > >> sort of printed guideline for replacing silicon driver transistors with > >> MOS? Both NPN and PNP. > >> > >> For example TIP122 Darlington transistors driven by another pre-driver, > >> that is the TTL interface. I've seen many a substitute, but would like > >> to have the theory explained so I can make better use of it and > >> understand exactly what I am doing. > >> > >> A good reference book you can recommend perhaps? Books and I usually get > >> along very well. > >> > >> Too many projects and not enough to hunt down all the solutions! > > > >Unless there's an availability problem with the bipolar, leave it. As far as any rules of thumb: don't touch a hot case with your bare thumb. > > > >> > >> Thanks, and sorry for the on-topic post... > > > >You're one of these whiners always complaining about the group content, while contributing little to nothing yourself. > > > >> > >> John :-#)# > Hey, your kettle's black. > > Just deleted at least ten of your most recent posts as being OT.Ask me if I give a damn.> > RL
Reply by ●March 17, 20232023-03-17
On Wed, 15 Mar 2023 12:00:02 -0700, John Robertson <jrr@flippers.com> wrote:>I'm updating some game board designs and was curious if there is any >sort of printed guideline for replacing silicon driver transistors with >MOS? Both NPN and PNP. > >For example TIP122 Darlington transistors driven by another pre-driver, >that is the TTL interface. I've seen many a substitute, but would like >to have the theory explained so I can make better use of it and >understand exactly what I am doing. > >A good reference book you can recommend perhaps? Books and I usually get >along very well. > >Too many projects and not enough to hunt down all the solutions! > >Thanks, and sorry for the on-topic post... > >John :-#)#Given sufficient voltage in the gate drive, just make sure RdsxIds is less than the old Vcesat that was present. I used to stick fets in older bipolar SMPS and could usually increase unit rating till something else in the power train started to sweat. Often a magnetic part. RL
Reply by ●March 17, 20232023-03-17
On Fri, 17 Mar 2023 04:59:22 -0700 (PDT), Fred Bloggs <bloggs.fredbloggs.fred@gmail.com> wrote:>On Thursday, March 16, 2023 at 12:16:14?PM UTC-4, John Larkin wrote: >> On Thu, 16 Mar 2023 05:06:35 -0700 (PDT), Fred Bloggs >> <bloggs.fred...@gmail.com> wrote: >> >On Wednesday, March 15, 2023 at 3:00:19?PM UTC-4, John Robertson wrote: >> >> I'm updating some game board designs and was curious if there is any >> >> sort of printed guideline for replacing silicon driver transistors with >> >> MOS? Both NPN and PNP. >> >> >> >> For example TIP122 Darlington transistors driven by another pre-driver, >> >> that is the TTL interface. I've seen many a substitute, but would like >> >> to have the theory explained so I can make better use of it and >> >> understand exactly what I am doing. >> >> >> >> A good reference book you can recommend perhaps? Books and I usually get >> >> along very well. >> >> >> >> Too many projects and not enough to hunt down all the solutions! >> > >> >Unless there's an availability problem with the bipolar, leave it. As far as any rules of thumb: don't touch a hot case with your bare thumb. >> > >> >> >> >> Thanks, and sorry for the on-topic post... >> > >> >You're one of these whiners always complaining about the group content, while contributing little to nothing yourself. >> > >> Never miss the opportunity to redirect a technical thread to an insult >> fest. > >Fet is not a word, it's an acronym, so it's not MOSFet, it's MOSFET. Post is off topic, because it doesn't have much to do with design, it belongs is sci basic basics.FET is a fetish to whine about grammarschoolmarm rules. A 2N7002 doesn't care what you call it. What's the last thing you designed with mosfets?
Reply by ●March 17, 20232023-03-17
On 2023/03/16 12:15 a.m., Jan Panteltje wrote:> On a sunny day (Wed, 15 Mar 2023 12:00:02 -0700) it happened John Robertson > <jrr@flippers.com> wrote in <tut4jn$10ok6$1@dont-email.me>: > >> I'm updating some game board designs and was curious if there is any >> sort of printed guideline for replacing silicon driver transistors with >> MOS? Both NPN and PNP. >> >> For example TIP122 Darlington transistors driven by another pre-driver, >> that is the TTL interface. I've seen many a substitute, but would like >> to have the theory explained so I can make better use of it and >> understand exactly what I am doing. >> >> A good reference book you can recommend perhaps? Books and I usually get >> along very well. >> >> Too many projects and not enough to hunt down all the solutions! >> >> Thanks, and sorry for the on-topic post... >> >> John :-#)# > > This is how MOSFETs work: > https://en.wikipedia.org/wiki/MOSFET > > > If you want to use one: there are basically 2 types: some conduct with zero volts at the gate > and some do not conduct with zero volts at the gate. > > To interface with 5V logic I usually use an IRFZ44A MOSFET (off when 0V at the gate). > 10 for $8.63 on ebay.. > https://www.ebay.com/itm/324380901727 > even cheaper at other places > datasheet: > https://datasheetspdf.com/pdf-file/799031/Samsung/IRFZ44A/1 > > look at page 3 figure 2 for at what voltage the thing starts conducting. > > At 5 V from TTL logic it can do several amps. > MOSFETS do not pose a current drain on the driver (very high impedance, these should work for a pinball machine I'd think). > Also these MOSFETS have a reverse diode that limits the peak voltages when switching an inductive load, > see figure 13 in that datasheet (may save you a protection diode, but of course you can leave it in too). > > So this is a "simple practicle man's few lines book" but you gotta start somewhere... > > More in my book 'The Fart Of Electronics' provided I ever can find a decent pen to write it. > > There also exists a book 'The art of electronics' I'v never read it but people get all exited about it it seems. > The author used to post here...I've been meaning to buy Winfield (& Horowitz)'s book for a while, and now you have tipped the balance. I'm sure that will be a useful addition to my library! Thanks! John :-#)# -- (Please post followups or tech inquiries to the USENET newsgroup) John's Jukes Ltd. #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3 (604)872-5757 (Pinballs, Jukes, Video Games) www.flippers.com "Old pinballers never die, they just flip out."
