Hi everybody, if I wanted to turn on/off a transistor with a 555 timer to control 12dc @20amps at high speed which transistor do I need to look for? I can't seem to decide.
I've already pulled out all my hair
and chewed off all my fingernails, said my prayers, kicked the dog, drank a beer and beat
the wife. Any suggestions?

 

what does "high speed" exactly mean?

 

Apologize to the dog and the wife (not necessarily in that order) and use a MOSFET.

Is the load grounded or can it float?

As kubeek asked, what is "high speed"?

 

FROM EARLIER POST REGARDING WHICH TRANSISTOR SHOULD I USE:
OK, I BANDAGED UP THE WIFE, GAVE THE DOG SOME WATER AND
DRANK ANOTHER BEER. TO ANSWER YOUR QUESTIONS, I THINK SPEED UP TO 10 CYCLES P SECOND AND YES THE LOAD WOULD BE GROUNDED . YOU SUGGESTED A MOSFET, COULD YOU SUGGEST THE
ONE YOU WOULD CHOOSE?

tHANKS

 

Just post in your previous thread. Don't start a new one. I'll answer you there.

 

From your other thread, for a grounded load you could use a P-MOSFET rated at about 30-40A and 20V or more. Where are you located so I can suggest an appropriate device and vendor, if you don't know of one.

 

Hello,

I have merged the threads.
You can reply to your own thread using the post reply button

Bertus

 

From your other thread, for a grounded load you could use a P-MOSFET rated at about 30-40A and 20V or more. Where are you located so I can suggest an appropriate device and vendor, if you don't know of one.

we live in south central Idaho. I want to thank you for your input..

 

SUP65 P-Channel, 15mΩ@30A@-10Vgs, Logic Level

There's not much for choices for P-Channel in Through Hole TO-220 mounting (rather than surface mount), that one is about $3.

ETA: Will switch to 1kHz with -10V Vgs, could do 10kHz with -12V Vgs.

 

SUP65 P-Channel, 15mΩ@30A@-10Vgs, Logic Level

There's not much for choices for P-Channel in Through Hole TO-220 mounting (rather than surface mount), that one is about $3.

Maybe he can use an n-channel MOSFET. We need to know what the load is, and if high side switching is required, or if low side switching will work.

ETA: Will switch to 1kHz with -10V Vgs, could do 10kHz with -12V Vgs.

I'm curious why you said this. The on resistance between -10V and -12V Vgs is negligible.

 

The grounded load was post #14, The switching speed, I followed the wrong line in SOA, used the top line, rather than the dotted/angled line downward. Would need 20V Vgs for 10kHz.

 

Could you please explain a bit more in detail how you get the Vgs from the SOA graph?

 

The grounded load was post #14, The switching speed, I followed the wrong line in SOA, used the top line, rather than the dotted/angled line downward. Would need 20V Vgs for 10kHz.

That was post #4, and I missed the comment about the grounded load.
The SOA graph says nothing about Vgs, other than

VGS > minimum VGS at which RDS(on) is specified

 

Yeah, I messed that one up. It's for dissipation of single pulse.

Unsure what was going in my head on that one.

 

Yeah, I messed that one up. It's for dissipation of single pulse.

Unsure what was going in my head on that one.

Commonly called a brain fart. I have them frequently.

 

This one should be fine.
Wire a gate resistor to 12V rail, and short it with a BJT.

 

This one should be fine.
Wire a gate resistor to 12V rail, and short it with a BJT.

You should mention that the transistor will dissipate 36 watts when it's on.
If you are going to make recommendations, you need to think them out first.

 

You should mention that the transistor will dissipate 36 watts when it's on.
If you are going to make recommendations, you need to think them out first.

I am not sure what you are talking about.

The transistor will not dissipate anything for itself.

The package is well suited for quite more than 36 Watts.

What is your problem?

I use these transistors actually.

 

The OP is only looking for 10Hz so switching losses are pretty much zero. The one suggested by thatoneguy is a good choice, requiring a fairly small heatsink for the 6W worst case if it was permanently on. 36W for the Fairchild worst case would need a significantly bigger heatsink, maybe also a fan.

 

I don't really follow how you derive these 6W or 36W.