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H Bridge Driver


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Hope somebody could look at the attached schematic and let me know what we're doing wrong in the H bridge circuit. The two halves work independently, but once the two halves are joined together across the load they do not. We're getting + and - 10mA from the the two halves, like we should. However we get 0 when they are tied together. Thanks for any suggestions on what to change!

_1206110256_001.pdf

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I don't think you need the 1Kohm resistors on the top half of the circuit. I don't know what you mean about the 10mA. The circuit is designed to give you zero volts across the load. The 1Kohm resistors on the top half will drop the voltage across the phototransistor to zero volts. They should be biased at 50 volts like the bottom circuit. What you have is a lack of current to the top transistors which will make the voltage on each side of load lower.

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Tried removing the 1K ohm resistors like you suggested and current really dropped across the load.

What we're trying to do is drive a Bimor Pump model BPH-414D from MEDO so that once we trigger current to flow in one direction, the diaghram inside the pump will expand and once the current is reversed, the diaghram will contract.

This circuit works, but we would like more current across the load. So if you have anymore suggestions, please let us know.

We don't have much experience with h bridge drives and there is little on the internet about them. Any sites would be great that you know of.

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Since you have taken out the 1Kohm resistors on the top half of the circuit, you are ready to apply the highs and the lows. You will have 100 volts across the load when you are done. So watch out. The best way to do it is to use two inverters on one of the inputs. It looks like the top inverter isn't supposed to be an inverter. So use two here and one on the bottom part of the circuit. That is all. You will then get 100 volts with the current in one direction and when you change the logic of the input you will have 100 volts with the current in the other direction.

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Hi Qwert,
What is the frequency of your drive? Those piezo transducers in the pump are designed for only 50 or 60 Hz.
Why are you trying to drive the pump with a 195V square-wave when it is designed for a 120V sine-wave? Maybe the abrupt cycle edges and sustained over-voltage fractured the transducers.
Have you ever tried to drive the pump when it is dry without a fluid load? Maybe over-excursion damaged the transducers.

From what you have showed us, your circuit should work fine with the 1K resistors.
We need to know more about your circuit to see why it doesn't work:
1) Is the drive to each side complimentary (opposing)?
2) Which IC's are you using for the 5V inverter and non-inverter?

We need to know more about your measurements:
1) What resistor value did you use for RLoad to simulate a pump?
2) When measuring the RLoad current with the full H-bridge, did you set the ammeter to measure AC current?
3) Can you 'scope the H-bridge to tell us what its inputs and outputs voltages swing to?
4) Are the output voltage swings complimentary (opposing)?

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audioguru,

The reason for the H bridge design is so that we an control the pumping actioning, on and off when we want it.

Could we use the 120Vac out of the wall outlet into a switch or relay and still control the pump with a microcontroller? I think we may be making this more difficult than what's necessary. What would you do?

To answer your questions:

The frequency is 60Hz.
We have tried to drive the pump when it is dry without a fluid load and it pumps (you can hear the diaphragm working inside).

1) Is the drive to each side complimentary (opposing)? Yes.
2) Which IC's are you using for the 5V inverter and non-inverter? For the time being, we are just using a 5V supply from a 7805 chip and using that for testing (we will eventually be using the output from a microcontroller to give us +5V and 0V).

1) What resistor value did you use for RLoad to simulate a pump? 2- 2.2K ohm resistors in parallel
2) When measuring the RLoad current with the full H-bridge, did you set the ammeter to measure AC current? Our DMM's only measure uA, mA, or A with no reference to AC or DC current.
3) Can you 'scope the H-bridge to tell us what its inputs and outputs voltages swing to? The output voltage is 72Vdc with no input voltage and with the 5V input, the output dropped quite a bit to 6Vdc.
4) Are the output voltage swings complimentary (opposing)? Yes

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Hi Qwert,
I don't know how you tested the AC pump/h-bridge circuit with a DC 7805 regulator as an input and you don't know if your current measurements are AC or DC. I don't know how you measured 72VDC with no input (instead of a grounded input) when your DC supply voltage is only 50V.
So why don't you just power the pump from the mains like it is designed and drop the h-bridge.
Continue the discussion of your pump, a relay and its control on your other post.

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