What is the purpose of D8 in the spice schematic?

 

What is the purpose of D8 in the spice schematic?

sorry, i never updated the spice schematic, here it is - so the question is what is the impact on the power ratings of R1 and the current of the FET, U1. The diodes on this are going to a series of switches (replicated with 200k resistors here) on the chain shown on the wider diagram, I have estimated that the current into this chain is about 150mA tops, assuming the fuse is sufficently derated. I suspect that a few components will be overly stressed but just want some other opinion

 

I don't know what you expect to with that circuit but it draws between 2-4 millamps thru R1 from 24-40v.

 

I don't know what you expect to with that circuit but it draws between 2-4 millamps thru R1 from 24-40v.

hello, how have you got that value? I see you got that from using the 10k resistor - i was thinking the current through R1 maybe higher (base of Q1)

 

hello, how have you got that value? I see you got that from using the 10k resistor - i was thinking the current through R1 maybe higher (base of Q1)

Do you see any current path to ground that doesn't go thru at least a10k resistor?

 

Do you see any current path to ground that doesn't go thru at least a10k resistor?

ok, thanks - I realised I had 10k components instead of 100k, I can see now current with ltspice and about 3mA flow into R7 so that may explain why the PIC is not getting damaged from the high voltage (> 8V), due to the low current

This is the current flow through R1, similar to what you stated - how did it come to these values? trying to calculate these

I can see from spice the voltage drop on R1 going from 20 to 34mV which will determine the currents but still unsure where this is coming from

 

any pointers on this, i added up 8.2R + Rds on (5R) + 12.7K + 2.7k = 12713.2; so 24V /12713.2Ohm = 1.88mA, what am I missing?

 

When you make changes to your schematic, please repost it so were on the same "page". At the same time we can verify your statements.

Check your math...

eT

 

When you make changes to your schematic, please repost it so were on the same "page". At the same time we can verify your statements.

Check your math...

eT

So, I removed the 2k7 since its blocked off by C1.

8.2R + 5R + 10k = 10,013.2Ohm > 24/10,013.2 = 2.39mA and 40/10,013.2 = 3.99mA
This is closer to the spice result?

 

is my above answer closer to what is expected or am i missing something out.?

 

I've figured out the current into the pic from R7 (40-8.5V/10k), but still a bit confuse about the current in the 8.2 ohm.

 

I've figured out the current into the pic from R7 (40-8.5V/10k), but still a bit confuse about the current in the 8.2 ohm.

R7 (10k) current is drain current of U1,
R1 (8.2 Ohm) current is source current of U1 (the same current 3mA).

 

thanks guys.