h-bridge output too high?

[qweets]

.
hello : ) hope your having a relaxing Christmas,

I recently bought a H bridge to power a small 10v dc 800ma motor.
L6202 Datasheet pdf - DMOS FULL BRIDGE DRIVER - SGS Thomson Microelectronics

i have my h -bridge working it takes its own (IC) power from the powersupply VS pin 9 which also feeds the motor.

the voltage range for the VS input pin 9 is :
12v min
36 typical
48 max

so I have to feed the h-bridge a minimum of 12v. I am using a 18v 1.1 amp Power supply which seems to be fine. but the output for the motor from the h-bridge is aprox 17.6v ( with variable polarity) depending on the logic (DIR) inputs.

The frequency of the PWM i am using is aprox 31kh.

Am I wondering how may I reduce the h-bridges 17.6v output down to feed my 10v 800ma motor and keep the high frequency PWM intact?

Would i need a special resistor that can handle the high frequency and amps?


thanks for any help on this.
peace
: )

 

[LunchMeat]

Hi, Your motor is rated for 10V operation and 800 mA. These are likely continuous ratings values. This means that the maximum power input is 8 watts (continuous). If you adjust your PWM waveform such that the maximum duty cycle causes less than (8 watts/17.6V) of current or ~450 mA (RMS) than you should not exceed the maximum continuous rating for your motor. You will have to do this under load for your motor setup to ensure you set the PWM duty cycle correctly. You will have to monitor the motor current when you first adjust the maximum duty cycle. You could monitor the voltage across a small resistor ( .1 ohm ? ) in series with the motor to check the current(remove the resistor after you have set the max PWM duty cycle). Use two channels of an oscilloscope and find the difference across the resistor and calculate the rms current. I would not trust a DMM for this measurement since most won't accurately display RMS current (or voltage) much past a few kHz, some out to 20 kHz, but not 30 kHz.
If you can't limit the PWM duty cycle, another approach would be to use back to back zener diodes in series with the motor to limit the voltage across the motor to ~10 volts. However, this would dissipate 7.6v*.8A watts ( 6 watts !) in the diodes. Pretty hefty diodes and heatsinking would be needed. And since the voltage drop is fixed, even at 100 mA the power dissipation is still 0.76 watts.
And of course, you can use a resistor to limit the motor voltage at max current to 10 volts. This would be approximately 10 ohms. But like the PWM approach it doesn't limit the voltage across the motor at the low current levels and like the zener approach it 'wastes' a lot of power through heating the resistor. But it doesn't waste as much as the zener approach ( 100 mA and 10 ohms yields 0.1 watts of waste power )
Good luck.

 

[FvM]

If you have a 18V supply anyway, the choice of L6202 may be reasonable, otherwise, you selected the wrong H-bridge device.

A suitable way to reduce the output voltage is to reduce the PWM duty cycle to about 56% maximum. Using a resistor to reduce the voltage sounds absurd, you could use a linear motor driver instead of PWM from the start.

 

[qweets]

oh darn it. : /

i can reduce the duty cycle, but i think i would feel more comfortable with an appropriate h -bridge , it needs to be able to handle approx 31kh PWM.

i tried a SN754410 that was easy to solder up, i liked it allot, however it did not like the high frequency pwm only lower around 700hz.

so i wonder what to try now? any ideas guys?

thanks for your help.

---------- Post added at 20:47 ---------- Previous post was at 20:31 ----------

hi,

i think i may have found a replacement a L298HN Dual Full Bridge Driver, in a "MULTIWATT H" package, hopfeully that will fit my breadboard.

L298HN Datasheet pdf - DUAL FULL-BRIDGE DRIVER - SGS Thomson Microelectronics

it seems to be able to handle up to 40khz ( hopefully that is regards to the PWM in)
a Vs range from 2.5 > 46 volts ( hopefully that is for both the motor and h-bridge, then i can use a 10 volt regulator)
with a logic in of 5v.

hope i haven't missed anything , these data sheets blur in front of my eyes.

3rd time lucky i hope!! lol : P

 

[qweets]

hello.

can anyone help with deciphering this picture below? it is taken from the schematic of the h bridge A3953SB-T i am to try out today. the pixelation is very bad and i cannot see if there is a capacitor and resistor between 3 and 5 (earth)? and also between 11 and 13 and 16 & 12? the pdf is here below but its not very clear either. thanks for any help on this
**broken link removed**