Rule of thumb for replacing power transistors with MOSFets?

TTL can drive low-threshold, aka logic level, fets. There might be a base voltage divider that you can booger to get maximum voltage swing.

I like FDV301 for low current, 0.2 amp loads. STD16NF06 for heftier stuff; it will easily sink 15 amps with +5 on the gate.

Just don't let that happen again.

Digikey, Mouser, and Newark are all months away from having stock on the STD16NF06...could use the STD25N10F7 I suppose - a bit of overkill.

Oh, well, I'll just look for TTL compatible I guess - I just thought there might be some theory behind it that I could brush up on. Not that I ever really understood transistors, too much magic in those parameters!

John :-#(#

There are lots of "logic level" low-thresold p and n channel fets around. I just named a couple that we use.

If you post a link to your schematic(s) we could check the circuits for drive compatibility.

On a sunny day (Wed, 15 Mar 2023 12:00:02 -0700) it happened John Robertson snipped-for-privacy@flippers.com wrote in <tut4jn$10ok6$ snipped-for-privacy@dont-email.me:

This is how MOSFETs work:

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.. cheaper at other places datasheet: 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...

Hey, your kettle's black.

Just deleted at least ten of your most recent posts as being OT.

RL

You have something personal against John?

Note that 74-series TTL does not pull up to 5 volts.

The foul morons have driven a lot of people away.

Never miss the opportunity to redirect a technical thread to an insult fest.

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.

Can you say what kind of loads they are switching and at what rates?

piglet

On a sunny day (Thu, 16 Mar 2023 09:13:58 -0700) it happened John Larkin snipped-for-privacy@highlandSNIPMEtechnology.com wrote in snipped-for-privacy@4ax.com:

Yes, but even at 4.5 V gate that IRFZ44A does 10 A

Add a pullup, or substitute an HCT part.

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

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?

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 :-#)#

Ordered the hard cover - 2015 3rd edition - (I prefer books) and also picked up the PDF from Academia.edu.

Wow! What can I say other than this is the book I've needed my whole career! To say it goes into detail but in plain enough language for a non formally (EET) student like me to understand.

Now I think I will have to get the X Chapters (2020) that Winfield spoke of here, after reading the 3rd edition first.

I'm sorry Mr. Hill no longer posts here, he was a great reference and I think he enjoyed interacting with the folks here that are actually interested in learning about electronics and sharing their knowledge.

John :-#)#