Author Topic: Full bridge showing half the voltage  (Read 1914 times)

Offline plasma

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Full bridge showing half the voltage
« on: November 30, 2019, 06:42:20 AM »
Hi I built this circuit, the diagram is just one leg, a inverteted signal is sent to the other leg. The Pwm and bridge share the same ground and 12V supply.
When measuring the output, I only get 6V, the problem has stumped me so I though to ask here.

Offline plasma

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Re: Full bridge showing half the voltage
« Reply #1 on: December 03, 2019, 07:29:48 AM »
Sorry the picture didn't come through.
The resistor at the top, pull up is 212ohms. The N MOSFET that triggers the others is switched with a hex Schmitt trigger, that produce one signal for one direction, and a invertered one for the other. The RC inverter brunches to one inverter, the other two, 58% duty cycle. Measuring on AC, the two outputs read 12V.

The problem is when I measure the voltage of the bridge with a DC blocking cap, to a bridge re iffer with a output cap it read 6V.

Offline klugesmith

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Re: Full bridge showing half the voltage
« Reply #2 on: December 03, 2019, 07:41:00 PM »
The problem is when I measure the voltage of the bridge with a DC blocking cap, to a bridge re iffer with a output cap it read 6V.
We still need to guess what you are thinking. The quoted sentence, and paragraph before it, mention many components and connections not seen in your schematic. 

Things which seem obvious and skippable to you are, apparently, not so for many forum readers.  Slow down and draw.  The meter whose reading puzzles you is part of the circuit, so please show it.
« Last Edit: December 03, 2019, 07:42:53 PM by klugesmith »

Offline plasma

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Re: Full bridge showing half the voltage
« Reply #3 on: December 04, 2019, 12:11:32 AM »
This is the whole setup, thanks

Offline davekni

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Re: Full bridge showing half the voltage
« Reply #4 on: December 04, 2019, 04:26:38 AM »
If your new schematic is accurate, the bridge rectifier you are using to measure voltage is connected to the gate-drive on the right side rather than to the right-side H-Bridge output.
David Knierim

Offline klugesmith

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Re: Full bridge showing half the voltage
« Reply #5 on: December 04, 2019, 07:41:21 AM »
Good catch, Dave.

Also: the bridge rectifier and capacitor circuit between DMM and probes looks like it should fairly replicate the DMM on AC-volts mode.
Without worries about the frequency response of DMM's true-RMS circuits fed with 50 kHz square waves.
But subject to: 
1. bridge rect diodes need to be fast enough
2. The DMM's DC-volts reading is 2 diode-drops smaller than voltage between the probes.

When those have been taken care of, what's the indicated AC voltage betwen the H-bridge outputs?
How about between one H-bridge output and ground?  Between the other H-bridge output and ground?  Between power and ground?

This seems to be an exercise in measuring mixed voltages (50 kHz AC on top of significant DC).
When you are satisfied with your AC-coupled rectfier, to look at the AC component of mixed voltages,
how about making a low-pass filter (R and C) for looking at the DC component of mixed voltages?
Like between the H-bridge outputs and each other, ground, and power?

Of course the DMM's internal AC and DC volts modes should take care of all that stuff, but apparently that's in question.

Last time I made a front-end circuit for DMM, was to measure variations in output current of an embedded buck converter running faster than 100 kHz.  12V in, 1.2 V out at around 15 amps.  Often the power distribution path in PC board can serve as a current sense resistor, but in this case it was a square shaped plane with 16 loads, fed in the center.  Wanted to monitor the voltage drop between 2 points having a single effective sense resistance, no matter how the current was distributed among the loads.  Had to use the buck converter's main inductor as the sense resistor.  Its average voltage drop, many tens of mV, is proportional to its DC current.  Successfully measured with DMM on mV scale, after a passive filter attenuated the high frequency 12 volt square wave which also appears between the probe points.
« Last Edit: December 04, 2019, 08:10:51 AM by klugesmith »

Offline plasma

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Re: Full bridge showing half the voltage
« Reply #6 on: December 04, 2019, 11:20:25 AM »
Made a low pass filter from 0 and 1 ohm and 2uF ,its only what I had on hand. The voltage through the recover dropped from 5.4V to 4V. Does that mean there is a lot or most is DC.
Running it through a transformer 1:2  works OK were each one out of four make 10V give or take?
Measuring with out the recifer AC is about 100mV and DC about the same.
Measuring from source to ground is around 11.5V

Offline shrad

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Re: Full bridge showing half the voltage
« Reply #7 on: December 04, 2019, 02:57:02 PM »
also, the rectifier bridge shows three diodes in a row, and one in reverse... if this is a typo don't mind my comment, but if it reflects real life circuit there will be an issue there also

Offline plasma

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Re: Full bridge showing half the voltage
« Reply #8 on: December 05, 2019, 11:38:59 PM »
Made a mistake, open circuit leads on the bridge, reading DC value from positive rail to ground, shows 3V.
From bridge leads to ground and positive rail, show around 1.4V AC setting and 1V DC.
Its a 2.5A supply and 212ohms for the gate rail , Must be a short! Tested with 11.6 ohm resistor just for the supply shows 11.5V.

Offline plasma

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Re: Full bridge showing half the voltage
« Reply #9 on: December 06, 2019, 05:19:26 AM »
Thanks for the help. I measure from drain to source on AC setting, all except the top right were 6-8V ,it was 0.1V replaced it and now get 15V from the recover part.

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Re: Full bridge showing half the voltage
« Reply #9 on: December 06, 2019, 05:19:26 AM »

 


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