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I have a custom electromagnet that we are using in a system under development. It is powered with 2.25 VDC and draws about 1 A at that voltage level. We need to periodically reverse the current direction on this electromagnet and are planning to use a Toshiba TC 78H653FTG dual h-bridge driver to allow us to use the microcontroller to reverse the direction.

The main system power is from a 12 VDC battery. Ideally, I'd implement a 2.2, 2.3 or 2.5 V linear regulator but none are available that provide the required current.

I'm considering using an LM7805 5 V regulator with a passive voltage divider but hate the idea of the power being wasted using this method. What other options do I have?

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    \$\begingroup\$ Well, the obvious alternative to a linear regulator is a switching one... \$\endgroup\$
    – Eugene Sh.
    Commented Jul 7, 2022 at 19:31
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    \$\begingroup\$ Can you expand on why switching regulators (or off-the-shelf modules) do not fit your application? \$\endgroup\$
    – nanofarad
    Commented Jul 7, 2022 at 19:31
  • \$\begingroup\$ @nanofarad - A switching regulator would probably work. The ones I looked at were either out of stock or required a lot of external passives to setup. I'm also getting back into design after decades away and, well, things have certainly changed in the switching area so I'll look at this more closely now. Thanks for the idea! \$\endgroup\$
    – WalterCEden
    Commented Jul 7, 2022 at 20:03
  • \$\begingroup\$ many modest smps controllers, ex LM2596, show the same external component count in the datasheet's examples as a well-implemented lm317 does in it's datasheet. \$\endgroup\$
    – dandavis
    Commented Jul 7, 2022 at 20:22
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    \$\begingroup\$ There exist complete ICs called reversible DC motor drive with speed control, it incorporates bridge and PWM, i used such one successfully. However in term of supply problems, may be it is easier to use MCU to generate PWM with appropriate duty cycle and supply it to general purpose H-bridge. \$\endgroup\$
    – Vladimir
    Commented Jul 7, 2022 at 20:25

2 Answers 2

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Instead of regulating the power before the H bridge driver, use the h bridge driver to regulate the power to the electromagnet. This will require some careful code to avoid asserting a voltage in excess of the electromagnets rating, and some setup of the H bridge driver to limit the allowable current, but it is by far the most efficient and cheapest solution. Consider the block diagram for the intended toshiba H bridge, it already has all the necessary compenents to build an SMPS, with exception of the inductor: toshiba h bridge datasheet

Now of course it isn't this easy. the Toshiba H bridge isn't rated to your input voltage (and if your input voltage is actually a lead acid automotive battery on a vehicle, it gets a lot higher than 12V too!), you'll want an "automotive" h bridge chip that's probably rated to 20~28V to survive this application. Secondly you'll need a smaller regulator to power your 5V microcontroller.

datasheet snip from: https://toshiba.semicon-storage.com/us/semiconductor/product/motor-driver-ics/brushed-dc-motor-driver-ics/detail.TC78H653FTG.html

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    \$\begingroup\$ "with exception of the inductor". Some solenoids will make a suitable inductor. It really depends on how much eddy current losses they experience. Unless the solenoid is specifically designed for use in a circuit with a switching supply, though, you'll have to try it out and see if it works -- or just use a separate inductor. \$\endgroup\$
    – TimWescott
    Commented Jul 7, 2022 at 19:56
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    \$\begingroup\$ If the microcontroller has a built-in PWM controller, then you could probably do OK by grounding one side and running the other bridge at a duty cycle of \$\frac{2.25 \mathrm V}{12 \mathrm V}\$. You need some basic understanding of switching amplifier design, though. \$\endgroup\$
    – TimWescott
    Commented Jul 7, 2022 at 19:58
  • \$\begingroup\$ @Bryan - can you share a little more of what you are describing. I want to be sure that I understand. My first thought is that you're suggesting using the driver to create a square wave output that would drive the electromagnet. Help me understand a bit more please. And, we aren't using an automotive battery but you're comment is still valid. It isn't exactly 12VDC but not more than 14VDC. I think I already said this in another comment but we use the LM7805 to supply 5VDC to the main board, MCU and it's available to this daughter board. \$\endgroup\$
    – WalterCEden
    Commented Jul 7, 2022 at 20:15
  • \$\begingroup\$ @WalterCEden Essentially the H bridge contains all the switching components a synchronous buck converter (voltage-reducing switch mode power supply) would have. You would supply a control law (fixed PWM duty would do as per timwescott's insightful comments) and the AVERAGE voltage across your coil would be 2.25V. Again Tim comes with the insight as your electromagnet probably won't be happy with PWM (eddy currents) so you should add some external inductance. Have a look at microchip's page on sync bucks (microchipdeveloper.com/…). \$\endgroup\$
    – Bryan
    Commented Jul 7, 2022 at 20:22
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    \$\begingroup\$ @WalterCEden ugh that link is terrible and assumes familiarity with buck converters in general, chase down wikipedia as well: en.wikipedia.org/wiki/Buck_converter . Essentially yes though, you're correct that I suggest applying square waves to the magnet, though we should probably use an inductor and capacitor to limit eddy currents in the magnet. The square wave would be rather high in frequency, preferably above audible. \$\endgroup\$
    – Bryan
    Commented Jul 7, 2022 at 20:29
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There are many linear regulators that would "work" for you (LD1086D or LM317T for example), but it would be a huge pain to use them because of the massive amounts of heat generated. 12V in and 2.25V out at 1A means that you would have to get rid of almost 10W of heat, probably requiring fans and rigorous thermal design.

A much simpler and better approach would be to use a switching regulator. You could make one yourself using the many options available on digikey, or you could buy one of the many available online, like this.

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  • \$\begingroup\$ Thanks fro the suggestions. yes, it's a lot of energy being wasted. I'll look at your suggestions. We use the 12VDC because the overall system has several 12VDC motors, solenoids and pumps involved. We already use the LM7805 to get the 5VDC to power the BeagleBone. It does require a good heat sink. We get sufficient battery life even with this waste. It's an early proof of concept so the ultimate production units will be better. \$\endgroup\$
    – WalterCEden
    Commented Jul 7, 2022 at 20:07
  • \$\begingroup\$ heat kills reliability over time, so the less you can generate the better. It looks like the beaglebone can use up to 0.5A steady state (depending on peripherals), so it would not be a bad idea to switch to a switching converter for that too - the beaglebone has internal regulators so noise level on the power line is probably less critical (although still something to watch for) \$\endgroup\$
    – BeB00
    Commented Jul 7, 2022 at 21:27
  • \$\begingroup\$ Thanks for mentioning this. This is a proof of concept at this point. I fully expect A LOT of reliability, cost, and quality engineering to follow this stage. \$\endgroup\$
    – WalterCEden
    Commented Jul 8, 2022 at 19:16

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