One H-Bridge for several motors? - Page 1
Forum section: electronics / beginners

author: T_guttata
author_rank: Regular Contributor
timestamp: December 15, 2021, 08:21:17 pm
content: HiYou could use an H-bridge for several motors, if you put the motors in parallel with a switch in between (see picture). If the switch is mechanical, you could choose between one or several motors. It should also work, if you take a relay.However, it will not work when using a MOSFET as a switch, because the MOSFET will either be High Side or Low Side.Is there another solution for this problem beside using one H-bridge for every motor?Thanks for your help.

author: Kim Christensen
author_rank: Super Contributor
timestamp: December 15, 2021, 08:53:17 pm
content: Well, you could leave the righthand half bridge as is, as depicted by Q2 & Q4, and drive each motor's "minus" terminal with it's own half bridge. (8 transistors total)These solutions are not as flexible as a dedicated H-bridge for each motor so it would depend on the application.

author: T_guttata
author_rank: Regular Contributor
timestamp: December 15, 2021, 09:29:10 pm
content: Dou you mean like this?:Mosfet count: 2n+2 ?

author: Zero999
author_rank: Super Contributor
timestamp: December 15, 2021, 09:49:32 pm
content: Yes, relays, or using half bridges for the other side will work and there's no reason why MOSFETs can't be used. The circuit attached to the original post needs to have protection diodes.

author: Kim Christensen
author_rank: Super Contributor
timestamp: December 15, 2021, 10:03:10 pm
content: QuoteYes, that's what I was thinking.

author: T_guttata
author_rank: Regular Contributor
timestamp: December 15, 2021, 10:04:45 pm
content: Yes, the protection diodes are not shown.MOSFETs can not be used as indicated with SW1, 2, 3 in the first diagram. Why?- A MOSFET will only block current in one direction- There is no voltage referenceWhat's the disadvantage using one half bridge for all + terminals and half bridges for each - terminal of every motor?

author: Kim Christensen
author_rank: Super Contributor
timestamp: December 15, 2021, 11:23:27 pm
content: QuoteIf you want to run more than one motor at the same time, then those must all run in the same direction at that time.When you toggle the common bridge, you'd also need to toggle all the bridges of the idle motors to keep them in the same state.The common bridge would need beefier transistors if you want to run multiple motors at the same time.

author: thm_w
author_rank: Super Contributor
timestamp: December 15, 2021, 11:47:40 pm
content: Quote from: T_guttata on December 15, 2021, 10:04:45 pmYou can use back to back FETs as a switch.But most likely the best solution here is to have 6 half bridges, as they are a commonly available and fairly cheap part (can be cheaper than 3 relays depending on the current).Can you go into specific details about why you want to do this? What are the currents of the motors?

author: mc172
author_rank: Frequent Contributor
timestamp: December 15, 2021, 11:54:36 pm
content: If your only goal is to reduce MOSFET count, you could use a +V rail and a -V rail and drive each motor with a half bridge connected across each rail. You then get direction control of each motor independently for free.

author: T_guttata
author_rank: Regular Contributor
timestamp: December 16, 2021, 06:37:02 am
content: Quote from: mc172 on December 15, 2021, 11:54:36 pmDon't understand that. To reverse the motor you need to control 4 switches?The main idea is to reduce MOSFET count. Let's assume you want to control 20 devices. I was hoping you could do that with 4 MOSFETs for the main H-bridge + 1 MOSFET for each device --> 24 MOSFETs. With the method indicated in the second diagram, there would be 42 MOSFETs needed (+ 21 transistors to switch the P-channel MOSFETs with a logic level signal).Quote from: thm_w on December 15, 2021, 11:47:40 pmWith back to back FETs, there is still the voltage reference problem? Depending on the state of the main H-bridge, these FETs will be either on the lower side of the load or on the higher side of the load. What do you choose then?

author: Psi
author_rank: Super Contributor
timestamp: December 16, 2021, 08:00:35 am
content: If you use relays to switch the motors in/out of an H-bridge there is one important thing you can do in softwareThat is, only energize the relay's with no current flowing (H Bridge off).This means you don't need to spec a super crazy big relay able to handle motor-start inrush current/arcing.You can use a much more normal/cheaper relay.Also, keep in mind if you have a shared half-bridge that is common for multiple motors, then you have limitations for what you can do if you want to run more than 1 motor at the same time.You can wire each motor up in whatever polarity you want, but if the common half-bridge is currently 'high' running motor 1, then all you can do is set the other half-bridges to 'low' to turn on any other motors, in whatever rotation direction that happens to be based on how you wired them up.

author: Zero999
author_rank: Super Contributor
timestamp: December 16, 2021, 08:49:02 am
content: Quote from: T_guttata on December 15, 2021, 10:04:45 pmWhat's the supply voltage? BJTs are typically only rated to 6V maximum, in the reverse direction, when the hFEis very low, so it will require a large base current to turn on. Another problem is the BJT will act as an emitter follower, for one polarity, which will have a high voltage drop.QuoteWhat do you mean by that?QuoteYes, that will work, but the direction of the motors can't be independently controlled, only their speed and if they're on, or off.

author: T_guttata
author_rank: Regular Contributor
timestamp: December 16, 2021, 11:25:51 am
content: Regarding the "voltage reference problem" I have attached a schematic. I roughly indicated the voltage levels for the mosfets, assuming V_GS = 10V to switch a MOSFET.If Q10 (Q7) is a relay, it should work. If Q10 (Q7) is a MOSFET, it will not work.I hope I could clearly state what I mean.

author: Zero999
author_rank: Super Contributor
timestamp: December 16, 2021, 01:18:26 pm
content: Quote from: T_guttata on December 16, 2021, 11:25:51 amAnd how is the situation any different for BJTs? Worse still, when the voltage is reversed, the BJT will go into zener breakdown and be damaged.Yes, A relay will work and diodes to the supply rails are a good idea, to avoid arcing.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 16, 2021, 01:26:20 pm
content: This makes no sense.The switches are already present in the H bridge. Just use them. I.e., build three H bridges. Job done.Any savings in number of power transistors is wasted by added complexity if you build a single H bridge with separate semiconductor switches.Relays + their driver transistors are also more expensive.Copy-pasting is always easy, on the other hand.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 16, 2021, 01:51:49 pm
content: Yes, have seen exactly that done: I've worked on a product that used an array of precision (stepper based) pumps, driven by one or two inverters, and a massive plate of relays to select among them.Wasn't exactly an elegant solution, but neither was the pump array. Honestly not sure what a better solution would be, mind. Maybe mechanical mux, but I wonder if they had to eschew that solution for other reasons (backlash, reliability, positioning flexibility).I got involved as it was decided to move to a fully electronic method, so I designed a board with a bunch of inverters on it, and they use a stack of those boards to run everything, no relays. Which means they can run them all at the same time if they want, though there might be other limitations there (like total power consumption).On the upside, it's not much more cost than switching the outputs -- you need the full switching capacity regardless, it's just the extra control circuitry (which often comes for free e.g. integrated stepper controller + driver chips) and supply bypassing. Well, and anything else per-inverter, like EMI filtering.Anyway, for solid state switching, to be clear, you can use extra inverter legs like above, or anti-series MOSFETs as switches in the more direct way -- typically wired source to source, so that only one isolated gate driver is needed to switch them. That's really the reason you don't see this option used all that much -- it takes more drive circuitry, it's not just a logic level input or something. The most common place you do see it, is with AC/DC SSRs: these use an LED into a photovoltaic pile, which generates a feeble current (some ~uA), enough to turn on the transistors in some milliseconds. They don't scale up well (huge gate charge --> precariously slow switching time), and do have substantial capacitance, so are best for DC and mains-frequency application, not so much high frequency switching (notice the inverter delivers fast rise/fall times!).Tim

author: T_guttata
author_rank: Regular Contributor
timestamp: December 16, 2021, 06:09:49 pm
content: Off course I would use 3 dedicated H-bridges, if there are only 3 motors. But what if there are more, 10, 20 or even more?I was hoping, there is a more efficient way. High current mosfets are easily $1 each.

author: james_s
author_rank: Super Contributor
timestamp: December 16, 2021, 06:23:35 pm
content: How much current do you need? There are integrated H-bridge ICs. No matter what you do there are going to be parts needed that will cost money.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 16, 2021, 06:29:44 pm
content: Quote from: T_guttata on December 16, 2021, 06:09:49 pmStill, back-to-back dual MOSFETs with optoisolated (or otherwise level-shifted) gate drive is expensive. Mechanical relays are expensive as well.Four MOSFETs is likely the cheapest solution even if you need 100 motors. Note that MOSFET cost scales down as you buy more.If you run large enough power so that the power stage MOSFETs cost you $1 ea even in large quantities, that means roughly kW level of motor power, how much do the motors cost? Is the driver power stage of any significance here? For 100 motors, it would sound like a $10k mechanical system, with NRE cost of at least $100k; is $400 for driver board MOSFETs an issue? The bare boards themselves would cost more than that, given large size.Is there a mass market product to be cost-optimized, driving tens of motors? If no, it's clearly a special purpose device where 99% of the cost is not electronic component BOM, but NRE costs. Do what is simplest to design and verify. This is using the least number of different types of circuits in one design and using the main switching transistors to do all the switching work.

author: Kim Christensen
author_rank: Super Contributor
timestamp: December 16, 2021, 06:36:00 pm
content: QuoteWell you could drive 25 motors with a 5x5 matrix for a total of 20 mosfets. But individual motor control would be limited.QuoteSure, but what about motor costs? H-bridge costs and MOSFET costs will be swamped by motor costs. Also, when building things like this you might want to think about future flexibility. You might regret going the maxtrix route when you later decide you want to control motor X & Y independently at the same time but cannot because of the hardware design which now needs to be changed as well as the software.

author: T_guttata
author_rank: Regular Contributor
timestamp: December 16, 2021, 06:58:28 pm
content: Ok. I got it.Quote from: Kim Christensen on December 16, 2021, 06:36:00 pmJust for curiosity, do you have a diagram showing the principle?

author: Kim Christensen
author_rank: Super Contributor
timestamp: December 16, 2021, 07:18:08 pm
content: QuoteFor example, you'd turn on one PNP transistor in the row and one NPN in the column to power one specific motor (out of 25) in one direction:Scratch that idea.. I see where I went wrong with that now. Thanks for pointing that out James.

author: T_guttata
author_rank: Regular Contributor
timestamp: December 16, 2021, 08:05:27 pm
content: Thanks! This actually looks very interesting. To drive 25 devices with only 20 MOSFETs sounds great. That's only 1/5 you would need with regular H-Bridges.

author: james_s
author_rank: Super Contributor
timestamp: December 17, 2021, 03:38:58 am
content: I'm not sure that will work. A multiplexed matrix similar to that is commonly used in pinball machines to drive lamps and solenoids but with a single PNP for each row and NPN for each column. Incandescent lamps require a diode in series with each one, otherwise you end up with current taking multiple paths through some of the other loads and it turns into a nightmarish resistor puzzle.

author: Kim Christensen
author_rank: Super Contributor
timestamp: December 17, 2021, 04:49:52 am
content: QuoteOoops! I do believe you are correct!

author: Ian.M
author_rank: Super Contributor
timestamp: December 17, 2021, 05:19:44 am
content: Yep. You'd need a pair of steering diodes for each motor, and although one axis of the matrix could use half H-bridge drivers, the other would need twice as many lines so the high side and low side drivers to the steering diodes could be kept separate.It *might* be worth doing for single coil latching relays or low power motors if you needed to minimize the number of I/O pins used, but isn't viable for high power motors as a dual power diode + heatsinking will almost invariably cost more than a MOSFET half H-bridge + heatsinking due to the much higher dissipation in the diodes.

author: mc172
author_rank: Frequent Contributor
timestamp: December 18, 2021, 12:50:32 am
content: Quote from: T_guttata on December 16, 2021, 06:37:02 amPicture attached.

author: T_guttata
author_rank: Regular Contributor
timestamp: December 18, 2021, 01:25:16 pm
content: Ok, I understand. If I need 30V across the motor:V1-=V2+ --> 0VV1+ = 30VV2- =-30VOne terminal of the motor is always @ 0V?Are there common power supply architectures which produce + and - voltage from a battery?

author: Psi
author_rank: Super Contributor
timestamp: December 18, 2021, 10:40:38 pm
content: Either you have- two batteries and 2 fets- one battery and 4 fets.- one battery and a virtual ground system to turn that into a +/- supply, and 2 fets (very very uncommon for motor control)Usually a 2nd battery costs more than 2 extra fets, so the 4 fet approach is more common.That said, if you already had a +/- supply for other reasons then the 2 fet approach is cheaper.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 19, 2021, 08:25:59 am
content: Two batteries in series won't work because it's practically impossible to balance the loads well enough to prevent battery unbalancing. Having to add a complicated battery management system capable of dealing with large unbalances all the time shoots the costs through the roof.With primary cells (non-rechargeable), this approach is sometimes used, though. Even then you'd like to have decent balance in order to prevent some cells going too low / negative in voltage, causing electrolyte leakage.A mains device which uses isolating transformer anyway, a second negative rail comes for "almost free".I built a motor driver once with such bipolar supply. But this was a special case, a 2-phase motor ("stepper") which normally requires 8 MOSFETS (vs. 6 for a 3-phase motor). Also this was a part of a sensitive analog imaging system so I wanted to reduce EMI no matter what, so made a linear class B motor driver, driven by DACs with arbitrary (sine-like) waveforms. This was significantly easier with 4 transistors and dual supply.

author: wizard69
author_rank: Super Contributor
timestamp: December 19, 2021, 09:08:05 pm
content: Quote from: T_guttata on December 16, 2021, 06:37:02 amThe first thing that comes to mind is why? We need more context here, but this strikes me as one of those things and engineer comes up with that while technically possible in practice is a bad idea.QuoteThen buy 20 motor controllers, or suitable MOSFETS, or halfbridge MOSFET modules, or single chip motor controllers or single chip H-bridges. There are lot of options depending upon the motor sizes and voltages involved. I just don't see the point in a kludgy design that becomes a long term nightmare.I much rather see a one motor one driver approach.Quote