Strange values using a bridge rectifier in LTSpice. - Page 1
Forum section: electronics / general

author: jb82
author_rank: Contributor
timestamp: April 18, 2025, 09:06:39 am
content: I'm currently trying to simulate the following circuit:This circuit is part of an hangar for an autonomous drone and should power said drone once it has landed.On the left side, a load switch would be connected toSWwhich powers the drone, represented by a 1k resistor (ignore the MOSFET and the rectifier for now). This is the normal mode of operation and not the mode I'm interested in. I'm trying to simulate the following error condition:The docking procedure doesn't work as expected and the load switch gets accidentially connected to the charging contacts. This means, that, instead of the 1k load, there would be a 24V battery connected, resulting in the following circuit:simulating this, the rectifier and connected voltage divider give me a small voltage around 3V atP_ONthat can serve as an error indicator (load switch off + voltage present = battery is errorneously connected).However, the battery might also be connected in reverse resulting in the following:I would expect the MOSFETM1to block and the same ~3V atP_ON, but I get around -20V. Also, the voltage is ~0 before the 8.2k resistor and ~24 behind the 1k resistor, as if it got shifted down by 24 volts, effectively reversingI'm new to LTSpice, and I can't say if this is a LTSpice problem at all or simply a misunderstanding on my account. The most likely reason that comes to mind is a grounding issue (grounding is said to cause lots of confusion in LTSpice). Can anyboby explain what's happening here?This is the file I'm using:https://www.dropbox.com/scl/fi/q108ja8nodsphfxmf3d3o/Draft2.asc?rlkey=dxnjpxskor0a5x932aminb9xu&st=0o0l78qs&raw=1

author: Andy Watson
author_rank: Super Contributor
timestamp: April 18, 2025, 10:25:27 am
content: Quote from: jb82 on April 18, 2025, 09:06:39 amThere is no positive or negative indicated on your diagram, however, guessing by the orientation of the other components I would suggest that you have drain and source swapped on M1. Also, there is a "body diode" in M1, so it will conduct in one direction regardless of of the gate voltage.I'm not sure how you envisage PON working? What are you assuming as your reference/ground voltage?Also, just noticed the datasheet for the IRF7401, maximum drain-source voltage is given as 20V. You may want to re-think your choice of mosfet.

author: Kean
author_rank: Unknown
timestamp: April 21, 2025, 05:47:31 am
content: Presumably you want to measure the differential voltage across R3.In the first case with the battery polarity normal, the MOSFET will be turned on connecting battery negative to GND. Voltage at P_ON will be offset by the bridge rectifier diodes.In your second case with the battery reversed, the MOSFET will not be turned on nor will its body diode conduct. In this instance there is no properly connected ground reference from the battery to GND for P_ON to be measured against. The simulation is probably giving a reasonable value based on leakage current from battery positive to GND through the body diode.Use the voltage probe in LTspice to see the voltage at each net.