H-Bridge vs. OPAmp for driving actuators? - RC Groups
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Feb 11, 2006, 05:40 PM
High-power Rocket Gliders
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H-Bridge vs. OPAmp for driving actuators?

What are the relative advantages/disadvantages of using an H-Bridge or an OPAmp to drive low-resistance (~50 Ohm) actuators? Koichi used both in his IR RXes, and I can't figure out when you'd want to use one or the other.

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Feb 11, 2006, 06:59 PM
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caseyjholmes's Avatar
By op amp do you mean FET transistor or similar?

ah, I See what you mean now. Inverters... basically a FET will work the same there wont it? Are you trying to drive the act from an MCU?
Last edited by caseyjholmes; Feb 11, 2006 at 07:12 PM.
Feb 11, 2006, 07:48 PM
Sticky Shepherd
Graham Stabler's Avatar
an op amp is an operational amplifier, a few of them have the rail to rail abilities to drive coils.

When I designed my FM RX (never commercially realized), I looked at the options of op-amp, h-bridge and also analogue switches (as the RFFS100 uses). The best in terms of availablity of components and pcb real estate turned out to be using pairs of dual fets as many others now do. If you use fets it is not necessarily the best idea to get the lowest on-resistance type due to the noise created by switching inductive loads.

There is at least one decent quad op-amp that produces a very neat drive circuit, I saw this first used by the late Eric Hook and you will now see them on some of Nick's larger receivers. Availiablity when I looked (about 2 years ago) was poor.

It is really just a matter of looking at what you can get your hands on and comparing weight, price, spec and pcb real-estate. The latter means weight in effect. Also layout can come into it if you intend to use a single sided PCB, some are a nightmare to route.

Feb 11, 2006, 07:49 PM
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caseyjholmes's Avatar
This is a good question.
I think there are a few difference I may be able to point out. Although, you could argue both sides of each difference. To me, it seems mostly personal preference and design requirements, as well as your skills in programming.

A single chip H-bridge will usually have several features as opposed to say a FET transistor. With an H-bridge, you get an enable line (most often. driven high or low to turn on/off motors/actuators), seperate power and ground inputs (not for directly switching power, but ment for amplifying it accross the switch like inverters do, and driving the motor high and low without disrupting or rippling direct power inputs to the entire circuit... aka noise reduction). Also, H-bridges usually have a set load limit, data sheets for examples on how to use it and why, specific frequencies and many other variables.

(Assuming you would use a FET instead of an OP AMP...)
A FET is simply that. It has a max voltage and max current rating, and will more than likely require external components other than just the FET and the MCU. The good thing with FETs is their tiny size, as opposed to an H-bridge IC like the sharp SI9988 at mouser electronics. (ssop8 as opposed to sot23 or smaller FETs). This accompanied by an MCU to make an esc is absolutely tiny, whereas an Hbridge is the same size as both the FET and the MCU together (not always the case).

Main differences I can point out clearly...
FETs are tiny, and are great for small size applications, yet they will likely require a more advanced program than an H-bridge. You'll have to define EVERYTHING with a fet, including PWM functions or simple high,delay,low,delay routines to get speed control. Then once you define the routine, you can adjust the speed in your program by calling the routine and differing the varible of delay. Same goes for actuators and proportion, accept for the fact that you need two FETs to drive an actuator reliably and smooth, in which case your program just doubled as you have to controll both fets at the same time, exactly opposite of eachother.

With an H-bridge...
(depending on the H-bridge you choose)
Enable the H-bridge enable pin through a port, then send the pwm pin of the h-bridge a single value. Done! Proportion, bi-directional ESC's and driving motors OR actuators down to pretty low levels (10 ohms or less, or more..depends on the H-bridge). The H-bridge you get may also have additional capabilities that more suit your purpose. You have an easier more precise grasp on the object you are trying to control, while worrying about the harder parts of your program instead of debugging speeds and delays. But, you have drawbacks like size, extra power consumption while sleeping, more pins to solder correctly and then as always the price will be anywhere from a buck more to $10 more than a FET or two.

For the purpose of driving actuators..
An H-bridge will drive a lower ohm actuator than a FET because of it's internal circuitry preventing shorts through the actuator to power and ground at the same time. Also, H-bridges ment for motors are able to drive coils as low as a couple ohms! You could almost throw any coil on an H-bridge and get good results. If you don't have a size requirement, you have extra money and patients to solder up a few larger chips than a FET.. Go with H-bridges.

If your project needs to be small, you can handle the extra programming and you only need basic operations (excluding enables and extra ganged inverters... ect), and your actuators will never be below a certain resistance (less flexibility), go with FETs.
just my 02.. anyone? Does that make any sense? =P
Feb 11, 2006, 08:08 PM
Sticky Shepherd
Graham Stabler's Avatar
Casey, your comments about h-bridge chips are relelvant to larger units that would never be used in a model. A h-bridge in its basic form consists of 4 transisters or fets. Forming two push-pull pairs. The good op-amps have push-pull outputs to they are just h-bridges by another name when you wire them right.

Fets do not need any more components than the micro controller, this is because the gate drive circuit for a fet is a push-pull pair and this is exactly the same as the output of a PIC. So just 2 dual fets and two MCU outputs per actuator. They do not need a more advance program, just two PWM outputs.


p.s. drive the bridge as per the schematic on the right for less chance of shorting while switching. Sketch supplied by JBerg.
Feb 11, 2006, 08:26 PM
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caseyjholmes's Avatar
I see what H-bridges you mean now.. old school 4 transitor H-bridges! How could I forget? (built many for beam robotics as motor signal amplifiers). Is This necissary? How much current does an actuator draw max?? (no amp meter over here). At 5v, you can run an act straight from the output right?

I was referring to H-bridges like on the back of the cygnal, atmel and MEGAbitty boards I have been toying with.. Pic below. It's only a few mm's in each direction, yet houses enables, pwm inputs and the like. Pretty small! You can almost fit one of these (vishay SI9988... not sharp) H-bridges in the same amount of pcb area as a set of FET's and corresponding caps. Downside is they are $4 each! I See many H-bridges such as this in the servos I rip up for gears and things. They make some tiny ones, But I was unsure what type of H-bridges Iter was speaking of The programming with two FETs won't be more advanced, just more room consuming and a bit more typing (the way I would do it, IF we were talkin the type of H-bridges I thought we were).

You guys'll have to forgive me here, as I was unaware you could pull proportion with an OP AMP on an actuator. (although now I can see all it takes is driving the pin) Also, some of the question wasn't quite clear to me concerning what types of devices were being duscussed. I tend to get a bit hyper about the fun discussions like these =)
Last edited by caseyjholmes; Feb 11, 2006 at 08:34 PM.
Feb 13, 2006, 02:32 AM
High-power Rocket Gliders
iter's Avatar
For the record, I was referring to Koichi's RX no.17 that uses the MAX4020 op amp and RX no.21 that uses h-bridges built out of NDC7001Cs. Koichi wasn't completely satisfied with the 4020 because it had high internal resistance (~50 Ohm). He mentioned AD8534 as a low-resistance replacement, but said they were difficult to obtain in small quantities. DigiKey seems to carry them now.

Long time ago, in the beginning of the DIY IR thread, epilot mentioned Zetex h-hridge chips. I assume he was talking about ZHB6718.

DigiKey doesn't seem to carry it, but IP1M10 seems to be another example of the kind of h-bridge ICs Casey is talking about, but perhaps more suited to our purpose - if I understand the datasheet correctly, a single SOIC-14 chip will drive two coils. It has DIR and PWM inputs and corresponding internal logic instead of raw transistors. SN754410 may be another interesting chip.

Graham, which hard-to-get op amp are you talking about that Eric Hook used? Also, what is an analog switch, and which one does RFFS use?

Pairs of dual FETs seem pretty straightforward, but I can't help thinking of the high part count and PCB complexity it creates. Assuming I want to drive 2 coils, all I (theoretically) need is 10 pins: 2 inputs and 2 outputs per coil, +5 and GND. With 2 FET pairs I end up with 24 pins!

Thanks for the schematic. I've never seen an h-bridge wired like this. I'm pretty new to DIY electronics, but for some reason I've always seen h-bridges similar to the one on the left. I think I've already burnt a couple of my 6718s, so I'll have to try your alternative layout!

Last edited by iter; Feb 13, 2006 at 02:38 AM.
Feb 13, 2006, 02:53 AM
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caseyjholmes's Avatar
The tiny H-bridge in my photo can handle a half amp output, and comes ina tiny smt package that should make placing two of them on a PCB nice and easy. I have several PCB designs using these h-bridges for examples too. I can send you some if they end up of any interest to you, as I purchased a bunch in bulk a while back so I'd have stock for the future. =) Each one will handle one actuator or motor in any speed and direction. Here is a data sheet http://www.vishay.com/docs/71326/71326.pdf
Feb 13, 2006, 03:56 AM
High-power Rocket Gliders
iter's Avatar
Thank you Casey! This is most generous of you. Email sent.

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