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(Topic ID: 108771)

Two Opposite MOSFETs


By UvulaBob

6 years ago



Topic Stats

  • 14 posts
  • 4 Pinsiders participating
  • Latest reply 6 years ago by G-P-E
  • Topic is favorited by 2 Pinsiders

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  • Taxi Williams, 1988

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#1 6 years ago

I ordered some IRF9Z34N MOSFETs and put them into my Taxi. I turned the machine on, with only a few lamps connected to the board. I heard a pop, and immedately turned them off. I found that the two MOSFETs connected to lamps had blown, as well as their pre-driver transistors. So, I ordered some replacement ones.

Those replacements arrived, and before putting them in, I compared them to the non-fried ones I had in there from my first order. What I found was weird.

IMG_1936.JPG

With the red lead on the middle pin, and the black lead on the right pin on the top ones, I get .55 volts. I get nothing when the same is done on the bottom ones. When I reverse the leads, though, I get .55 volts. It seems to me that if these are both IRFZ34N's, then one pair of them is acting funny. Is it?

#2 6 years ago

Wait. I figured it out. One pair of these is IRFZ34N, and the other is IRF9Z34N, which appears to move current in a different direction.

http://pdf.datasheetcatalog.com/datasheet/irf/irf9z34n.pdf
http://pdf.datasheetcatalog.com/datasheet/irf/irfz34n.pdf

That could explain why the two that were connected to lamps proceeded to explode their pre-driver transistors.

I'll have to take it up with the people I ordered these parts from and be a bit more aware of the parts I get in the future.

#3 6 years ago

The IRFZ34 is a N-channel and the IRF9Z34 is an P-channel -- quite different little creatures.

Yeah, the "anonymous source" knows that a tube of IRFZ34's somehow ended up in the IRF9Z34 bin...
"He" thought he had accounted for all of them but was apparently wrong.

Shoot that "anonymous" source an email.. I... um... I mean 'they' will take care of this.

#4 6 years ago

Would anyone happen to know if a 74F08 would be a good replacement for the 7408 ICs that sit in between the PIA and the pre-driver transistors? All signs point to yes, but I'd like to get someone else's confirmation.

#5 6 years ago

Maybe, maybe not. 74F08 (F stands for "fast") have much faster edges and consume much more current than the 7408. The current probably won't be a big deal, but the fast edges can sometimes cause problems or even EMI (electro-magnetic interference).

That said, if it was me, and I needed a 7408 and only had a 74F08, I'd give it a try. I can't see any harm in it (unless you damage the PCB traces wile desoldering/soldering - be careful).

Good luck!

#6 6 years ago

I've seen the 74LS08 recommended as well. Would that be a better choice?

Here are the three datasheets for these ICs. Can anyone help me understand how I can determine this stuff for myself? What should I be looking for beyond simple input and output voltages and currents?

7408: http://pdf.datasheetcatalog.com/datasheets/166/500074_DS.pdf
74F08: http://pdf.datasheetcatalog.com/datasheet/fairchild/74F08.pdf
74LS08: http://pdf.datasheetcatalog.com/datasheets/70/375337_DS.pdf

#7 6 years ago

LS would probably be better.

Here's a web page with a cool chart to show if a logic family will work with a different logic family:
http://www.piclist.com/techref/logic/family.htm (See table 9.2)

#8 6 years ago

Man. That is one hard-to-read page. I feel like it's a page taken out of some kind of CS class curriculum.

#9 6 years ago

Bottom line: F or LS will work. HCT OR ACT would be good too. It's hard to go wrong...

#10 6 years ago

You must use 74F in the solenoid driver side of the board. Elsewhere it doesnt seem to matter on the driver board.

#11 6 years ago

Plain 7400 series parts are getting harder and harder to stock and cost has gone wayyy up.

74F or 74S for high current loads.
74S has been phased out and replaced by 74F.... but are both identical (as far as we're concerned).

74LS or 74HCT for low current loads.
74LS is slowly being phased out in favor of 74HCT. Seems each week I get another DMSMS report listing more ICs hitting the obsolete status.

Solenoid drivers have a 560 ohm pullups on outputs. That amounts to about an 8mA load which maxes out the 74LS parts and exceeds the limits of 74HCT parts. Need to use high current 74F or 74S here.

Switch drivers have 4.7K ohm pullups on outputs. That amounts to about a 1mA load -- any of the above parts would work. I would advise the 74LS08 as he has a little bit more 'umph' than the 74HCT08.

#12 6 years ago
Quoted from G-P-E:

Plain 7400 series parts are getting harder and harder to stock and cost has gone wayyy up.
74F or 74S for high current loads.
74S has been phased out and replaced by 74F.... but are both identical (as far as we're concerned).
74LS or 74HCT for low current loads.
74LS is slowly being phased out in favor of 74HCT. Seems each week I get another DMSMS report listing more ICs hitting the obsolete status.
Solenoid drivers have a 560 ohm pullups on outputs. That amounts to about an 8mA load which maxes out the 74LS parts and exceeds the limits of 74HCT parts. Need to use high current 74F or 74S here.
Switch drivers have 4.7K ohm pullups on outputs. That amounts to about a 1mA load -- any of the above parts would work. I would advise the 74LS08 as he has a little bit more 'umph' than the 74HCT08.

Ahh...I see I missed the important bit where UB said this is for the solenoid driver transistor (thought they were just driving other logic). G-P-E's right - the main spec that matters for that is that the minimum sink current be high.

#13 6 years ago

These aren't for solenoid drivers, though. These MOSFETs are to replace the lamp matrix transistors on the lower-right side of the board. Q52, 54, 56, 58, 60, 62 and 64, as per this guide:

https://pinside.com/pinball/forum/topic/williams-system-11-bullet-proofing-thread

So I'm trying to figure out if the existing 7408's were damaged when I powered the machine on. I don't have anything in the now-empty spots where the TIP42/MOSFET replacements go. I thought I could put the system in lamp test mode, put my logic probe against the different outputs of the 7408's and see the steady on/off of the lamp test, but no dice. I see a steady high on the pin that takes in the blanking signal, a pulse on the pin that goes to the PIA, and a low on the output. So either I'm not performing my test correctly (since I don't have actual transistors or lamps ultimately hooked up to the output pins of these 7408s) or they're both smoked.

I won't have access to another System 11 until Monday at the earliest, but since GPE has them for 35 cents a pop, I may as well get them and some sockets to go with them.

#14 6 years ago

"I thought I could put the system in lamp test mode, put my logic probe against the different outputs of the 7408's and see the steady on/off of the lamp test, but no dice. I see a steady high on the pin that takes in the blanking signal, a pulse on the pin that goes to the PIA, and a low on the output."

You're testing it correctly.
The high from the blanking (normal) anded with a pulse from the PIA (normal) and the 7408 having a steady state -- not normal. I'd replace the 7408.
The good thing is that you have pulsing from the PIA - he's not dead.

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