I am trying to use an H-Bridge circuit as a fwd/rev relay to drive a low (1 Ohm) resistive load. The set-up I am using is the revised bridge from Eugene Blanchard's website. Anyway, I am activating the bridge using a function generator set on a square wave. I am separating the square wave into negative and positive components using diodes, and then using these pulses to activate the logical inputs of the circuit. However, the negative component of the f.g. output will not activate one half of the bridge. Does anyone know how to invert this signal, or perhaps modify the H-bridge to accept negative activation pulses? The goal output here is a high current square wave across the resistive load. Thanks in advance!
James, I am assuming that you want to re-create the bi-polar input signal across your resistive load, so that when the input signal is above ground, one side of the H bridge pulls high and when the input signal is at ground or below ground, the other side of the H bridge pulls high.
From looking at Blanchard's H Bridge, jumper the low side inputs so that they are driven off of the other leg of the bridge as Blanchard has shown. Now you only have two high side control inputs to worry about. One of the high side control inputs should be tied directly to the input signal. The other leg of the H bridge gets it's control input inverted with this circuit. If the Ascii art gets killed, it is just an NPN transistor with it's emitter grounded and a 10K resistor from the collector to +12V. A 4.7K resistor is used to tie the signal generator output to the base of the transistor. The output signal is at the junction of the 10K resistor and the collector of the transistor. The resistor values I supplied may need some fooling with. No additional diode stuff should be needed for separating the parts of the square wave.
+12V ----- | | \ / 10K or so \ / |------------------- To other leg high side | input / C |/ 4.7K | sig in --/\/\-----| NPN B | |\ \/ E \ | | GND
From looking at the drive circuits on that bridge, I don't think that bridge will work very well, especially not at high frequencies. I would expect problems to start showing up at frequencies above a few KHz.
Thanks for all the replies! For Dan, the schematic of the bridge can be found here.
I made the circuit using a printed circuit board and some standard parts, the MOSFETS I am using are IRF4905, and IRF3205. For Bob, I'm going to try the circuit you have suggested this afternoon, and hopefully it works. I've been trying to replace the PNP transistor on the high side, which didn't work. As far as problems when the frequencies are < 1KHz, I dont expect to be using this circuit any higher than probably 100 Hz. I think also I'll get rid of the diodes I have on there.
The inverter circuit you suggested worked like a charm. Thanks for the idea! Anyway, am I correct in assuming that this circuit setup would not need any negative component in the f.g. output to the bridge?
Hi James, The NPN is an OK choice for that location, it will switch the FET on as quickly as possible. The switch off is likely to be slow and dirty without changing over to a "totem pole" driver configuration (NPN and a PNP). It does make dead time (the time between turn off of the high side and turn on of the low side FETs or vice versa) harder to control externally.
Because the high side and low side FETS are controlled separately, I would suggest doing your debugging with a current limited power supply. If the current limiting is set to to a value that the FETS can safely carry and there is not a lot of capacitance on the DC bus, errors and problems will result in the supply limiting rather than the FETs catching fire.
Once the circuit is working the way you want it, adding capacitance on the DC bus is a good thing.
I'm glad it worked for you. Yes, you are correct that it does not need the function generator to go negative, only near to ground. An LM555 timer would drive this just fine instead of a function generator.
Motor controller design is more difficult than most other electronic design. There are many problems involved in dealing with large inductive loads, the more so the more amps are being switched. Most digital design is a breeze in comparison. I've designed a number of motor controllers, and am not really happy with any of them, for various reasons. I've literally blown the ends off of large 50W extruded load-resistors. They fire like a bullet :).
For a non-electrical engineer, I would suggest buying a commercial controller.
You might take a look at the Open Source Motor Control project. It is now a Yahoo group. The original project was the power section for a robot drive, as I remember it was in the 50+ amp range and I believe that they got a good working board. These guys sell bare boards from the project and have some of their own stuff as well:
The project has wandered off into a brushless multiphase project that isn't relevant to what you want but the original is probably a decent starting point.
If you want to learn it and blow up FET's on your own, International Rectifier has a couple of good app notes and sells driver chips and FET's. I took this path: it was interesting. It was an easy way to burn a year of spare time and a pot full of FET's and I am an EE.
I did find that this circuit attenuates an 8V at a 2 Ohm load cycling at 100Hz after a couple of minutes. The real disadvantage that I am having with this thing is that since the voltage to the motor activates the MOSFETs as well, when you lower the motor voltage less than the gate voltage (~4V), the mosfets don't fully open, so they heat up and there is serious motor voltage attenuation. Hopefully I won't have to drive it at a lower voltage than 5V. There isn't an appreciable amount of inductance in the load that I have this thing hooked up to, and braking isn't really necessary for the operation. What really would be nice is some sort of IC that would send voltage to the 4 MOSFETs on a trigger I guess. I'm going to have to use the existing circuit until I can find a way to do this. Somebody on another forum suggested I use a PWM IC. The reason I don't use one is that there is an internal oscillator in the chip, where the frequency is regulated by a RC setup. Is there some sort of IC that can use an external trigger to pulse voltage to the MOSFETs? Or even A and B?
I don't have a solution to your problem, but there are a couple of things you should be aware of.
First off, 4v is near the threshold level of any MOSFET, and you need to have at least 5v gate-source voltage to turn them on adequately, so you'll have problems unless you use a special charge-pump circuit to generate the gate voltages, along with a "separate" supply to drive the pump ckt. Logic-level MOSFETS will have a lower Rdson at Vgs = 5v than regular MOSFETs.
Secondly, you need to check the minimum Vdd for any off-the-shelf h-bridge chip. Not many will work at Vdd as low as 5v. Many require at least 10v minimum, I assume many due to the Vgs levels required.