voltmeter read 7.5v WHY?

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MOSFET h-bridge[voltmeter read 7.5v , WHY?]

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Most of your gates are floating. Results are unpredictable.
 

Q1-Q2 are closed (off)
Q3-Q8 are opened (on, so motor- = 0v)
Q4-Q7 are closed (off)

Q5-Q6 are opened (on) but motor+ = 7.5v because some Volt opening voltage (here: 12-7.5=4.5v) is needed betwen the source and gate to open (switch on) a mosfet. This is the high-side-switch problem ... Use P-channel mosfets at high side or apply an additional higher opening voltage here.
 
Can you offer suggestions circuit?
 

http://www.bristolwatch.com/ele/h_bridge.htm

A popular and extreme powerful IC (TD340):

http://www.datasheetcatalog.org/datasheet/stmicroelectronics/7048.pdf

QUAD N-CHANNEL MOSFET DRIVE
INTEGRATED CHARGE PUMP FOR HIGH SIDE MOSFET DRIVING
VERY LOW GROUND EMI NOISE
MOTOR SPEED AND DIRECTION CONTROL (LOW SIDE PWM)
INTERNAL OR EXTERNAL PWM SOURCE
25kHz SWITCHING FREQUENCY ABILITY
SYNCHRONOUS HIGH SIDE RECTIFICATION
REVERSED BATTERY ACTIVE PROTECTION ABILITY
INTEGRATED 5V POWER SUPPLY FOR MICROCONTROLLER
INTEGRATED SECURITY CIRCUITS: UVLO, OVLO, WATCHDOG
60V MAX RATING
 
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If you want to use N-type MOSFET in the top of the H-bridge, you have to provide a higher voltage in the gates quite higher than in the sources (you want to set the MOSFET in the triode zone with a resistance as small as possible).

For that, you can build a charge pump or use an IC as suggested.

You can also use P-channel mosfet on the top row. This is OK for low to medium power. For hi power, N-channel MOSFET are preferred because they have slightly faster switching and less heat losses.
 
As already suggested above, you should drive the upper two MOSFETs in "high-side" configuration with suitable drivers. You can use drivers like IR2110, L6385E, etc.

I see you're using 2 IRF1405s in parallel for each leg of the bridge. So, I assume that you have a large current going through the MOSFET. For this, N-channel may be better than P-channel MOSFETs due to the lower RDS(on) and hence lower losses, plus higher current capacity of each individual MOSFET.

A driver employing IR2110 may be simple enough for you. You can also find high-low side drivers using only BJTs if you search online.

Hope this helps.
Tahmid.

---------- Post added at 12:19 ---------- Previous post was at 12:18 ----------

You can find a lot of information if you follow this thread: https://www.edaboard.com/threads/219997/
 

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