Hi All,

i have tried to simulate the below circuit in spice to check the voltage at C1 which is going to a PIC, I am getting around 700mV on the drain of NDS0605 and < 200mV on the C1 node, seems a bit low? The drain in the schematic is connected to a voltage divider seen on the spice schematic.

any thoughts?

thanks

 

In the LTspice schematic, the junction of R2 and R3 should be connected to the collector of Q1.

 

I am getting around 700mV on the drain of NDS0605 and < 200mV on the C1 node, seems a bit low?

What do you expect?
It's the forward drop of the load diodes D1-D5 in parallel and 700mV is typical for a silicon junction diode.
(The diode model is undefined so I assume it's the generic LTspice silicon diode).

Note that using suffixes for the component values makes the schematic a lot more readable, such as 100k for 100000 and 10n for 0.00000001

 

What do you expect?
It's the forward drop of the load diodes D1-D5 in parallel and 700mV is typical for a silicon junction diode.
(The diode model is undefined so I assume it's the generic LTspice silicon diode).

Note that using suffixes for the component values makes the schematic a lot more readable, such as 100k for 100000 and 10n for 0.00000001

 

What do you expect?
It's the forward drop of the load diodes D1-D5 in parallel and 700mV is typical for a silicon junction diode.
(The diode model is undefined so I assume it's the generic LTspice silicon diode).

Note that using suffixes for the component values makes the schematic a lot more readable, such as 100k for 100000 and 10n for 0.00000001

I am still getting to grips with Spice, when I put in component values it brings them onto the screen like this..? The diodes actually are defined (the .include is not shown, the forward drop is looks to be .775V from the spice model below, I removed the diode loads and the voltage at the divider went up to about 7V, I was wondering what impact the diodes would have but look to have the answer, for series diodes the total drop is the same as for a single diode and for multiple diodes in series you add up the total drops.?

*SRC=S1J;DI_S1J;Diodes;Si; 600V 1.00A 3.00us Diodes Inc. Rectifier
.MODEL DI_S1J D ( IS=7.31e-018 RS=42.0m BV=600 IBV=5.00u
+ CJO=42.4p M=0.333 N=0.775 TT=4.32u )

 

In the LTspice schematic, the junction of R2 and R3 should be connected to the collector of Q1.

I have added that, but it looks like the answer is the diode connected to the drain.

 

for series diodes the total drop is the same as for a single diode and for multiple diodes in series you add up the total drops.?

I think you meant for parallel diodes it's equal to a single diode (at the reduced current for each diode).
For series diodes you are correct.

 

I think you meant for parallel diodes it's equal to a single diode (at the reduced current for each diode).
For series diodes you are correct.

yes, it was a typo - thanks

 

I have added that, but it looks like the answer is the diode connected to the drain.

The current through the diodes will now be much greater - about 0.8A.

 

actually, looking at this again I thought the voltage drop would be on the cathode end? like below @ D7 - not sure why 700mV is showing up on the drain of U1 below, even with the 4 diodes removed and using 1 i still get 700 ish mV out.

 

You do still have D8 from the drain to 0V.

 

You do still have D8 from the drain to 0V.

The drain is connected to the anode of D8 and then onto R7? or is U1 not getting switched on correctly...

 

The drain is connected to the anode of D8 and then onto R7? or is U1 not getting switched on correctly...

Because D8 is connected between the drain and 0V you will get only 0.7V.

 

of course, understood, there is actually a series of switches connected to the diode also so i really should have included them as a form of resistance.

 

I am getting around 700mV on the drain of NDS0605 and < 200mV on the C1 node, seems a bit low?

This was the original question. It has now been established that the voltage is 700mV because of the diode(s).
Has the question been answered, or do you have another?

 

well yes, i understand the voltage and diode situation, putting some load onto the diodes I have about 8.4V at R7 on the spice schematic above, which is too high since its going into a PIC GPIO input, but thats what I am figuring out, but the current maybe low so the PIC might not necassarily get damaged?


Looking at R1 in the spice schamatic, its rated for 250mW - this particular block is part of a chain shown below and the fuse at the source is rated for 500mA, so assuming this fuse is sufficently derated we could assume that about 150mA is the highest current expected to be distributed into the chain below, that would give a max power in the resistor R1 of about 180mW? (at 24V) - if the power is bumped up to 40V and R1 stays the same I am trying to establish the current increase (and if components are sufficently derated) Using 40/8.2 would result in almost 5A which would trip the fuse.

Assuming the current increase is linear then @ 40V there would be ca 250mA flowing into R1, giving power dissipation of 500mW (exceeding its power rating) and likewise the drain current of the FET can only handle 180mA so that too would be stretched.

Looking at the datasheet for NDS0605 (U1), the VGS maximum is +/- 20V, at 24V the Gate is 12V moving to about 21V @ 40V so VGS looks to be ok but the drain current i think is too much?

 

Hi

So what part of the circuit in post #16 are you attempting to simulate and what are you trying to determine?

eT

 

trying to establish the current impact on the resistors around the FET (U1 in Spice schematic), I am trying to figure out the impact of 40V rather than 24V, since the current increase will be expected to be linear we could assume worst case for it to be about 250mA (unless I am wrong) - this would allow us to establish roughly the power dissipation of R1 in the spice schematic as well as whether the FET U1 is going to be able to handle the extra current from 40V, it is currently rated for 180mA max at the drain.

The 500mA fuse is to handle an entire chain of switches and controls in this design as below, and the fuse should have been derated so i am assuming around 150mA is the baseline current flow.

 

Ok...well...for starters(no pun intended)...you've shown diodes for what appears to be LEDs in the design schematic. The current current draw and voltage drop will be different for an LED than a standard diode.

eT

 

Ok...well...for starters(no pun intended)...you've shown diodes for what appears to be LEDs in the design schematic. The current current draw and voltage drop will be different for an LED than a standard diode.
eT

for the moment ignore them diodes, they are on a seperate chain (i only included that so you can see the distribution of current flow, the total of which should be much less than 500mA), so going back to my original post what would the impact on current flow into R1 on the spice schematic be if the voltage was 40V, likewise the drain of the FET U1. I worked out about about 250mA so would like to see what others would propose.