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Ok, counter question: what´s the point of a question that is that incomplete and thus can´t be answered?
For me it´s similar to: What´s the result of: A x 3 = B
Only if you know the value of A you can get the result of B.
The same is here.
Only if you know the conditions like V_DS and current you can get the value of R_DS_ON.
What answer do you accept as a good answer?
2 Ohms is correct, 14.3 Ohms is correct, 233 Ohms is correct....all values are correct. Each value for a given condition.
But as long as you don´t give a condition .. don´t blame it on the datasheet, don´t blame it on me, don´t blame it on those who didn´t answer at all.
Remark: This is my reply to the original post #3 of neazoi - before editing..
Is it the RdsON I should look?
Also is 200mA the max switching current at 12v?
R = resistance
ON = ON
And I assume you mean the resistance between Drain and Source (ds)
If so then yes, look for RdsON.
But you can´t have both 200mA and 12V at the same time. (only in a special case).
If you have 200mA through drain and source ... and have 12V across DS then according Ohm´s law R = V / I = 12V / 0.2A = 60 Ohms.
Are you sure you have 200mA and 12V at the same time?
I think Klaus was being fair/honest, not rude. MOSFET datasheets are slippery characters, I find them pretty hard to understand. MOSFETs Vds falls when they are turned on, like BJT Vce (sat). Off, Vds = 12V; ON, Vds = Iout x Rds(on). The 12V is only present at start of turn on: Vds falls as Ids rises - these are part of the switching losses people try to avoid with zero voltage switching or zero current switching, etc. More able members can offer a more exact and accurate definition than me.
For 12V, 200mA question, you might be better served working out gate drive needs for e.g. 200ns or 2us or whatever rise time and fall time, then calculating PD for e.g. 12V x 200mA and checking max. PD at top of datasheet, if you don't want to go through rigmarole of calculating junction temperature rise, then scrolling down to SOA chart and checking 2N7000 can handle x volts and x amps for x pulsed time or for DC.
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Must add, BJT and MOSFET datasheets create need to extrapolate from graphs provided as they rarely match one's own expected operating conditions, and take with large pinch of salt and read small print about test conditions at bottom of each page/section. ...few people have 1% duty cycles...
Rds(on) should have three or four confusing and seemingly contradictory graphs... One approach (maybe it's sloppy, or just plain wrong) is select highest Rds(on) at highest temperature with operating conditions closest to what you expect in circuit, i.e. worst case values, from graphs and calculate Ids x Ids x Rds(on) = Vds(on). Good luck.
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Not ideal as manufacturer to manufacturer there are small differences... Vishay datasheet comes closest to your requirements of what is Rds(on) at Vgs 5V, Ids 200mA = ~3.5 Ohms at 25°C. Not clear what at 125°C as neither example in graph is like requirements. 0.2A x 3R5 = 0.7Vds, ...and PD of 140mW so within limits of 160mW at 100°C and 400mW at 25°C. No SOA in Vishay but ST yes, OnSemi no. One says derate 2.8mW/°C rise. Good to compare.