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I have hacked my old rc car like in this tutorial here

I have measured manually the h bridge logic voltage (the voltage that the transceiver send to the h bridge) , which is 3.7volt

  • in the tutorial, the guy is sending 5v (instead of 3.7) from the output pin of the arduino to the hbridge, isn't that dangerous for the h bridge?

  • would it be possible to send pwm to control the motor speed, even if the h bridge wasn't constructed for this? (the original car only have 1 speed) (I'm afraid of breaking my h-bridge or my motor if I send pwm...)

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  • \$\begingroup\$ Can you read the part-nr on the h-bridge chip? \$\endgroup\$ – Gerben Jan 12 '16 at 15:56
  • \$\begingroup\$ For future reference. Please don't cross-post. StackExchange policy is against cross-posting. You can make a flag to the moderators to migrate this question to another stack, if you so choose. \$\endgroup\$ – Nick Alexeev Jan 15 '16 at 21:19
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That tutorial shows a Tyco R/C vehicle, which uses an RX2C decoder IC with outputs that can source or sink up to 6mA to drive H-bridge transistors. Typically the signals are coupled to the transistors through current-limiting resistors. Raising the drive voltage to 5V would just increase the Base current a bit and shouldn't hurt the transistors.

Here is a partial circuit traced from a Tyco 'Canned Heat' r/c car, showing the RX2C and H-bridge motor driver. Each decoder output is coupled via a 1k resistor, which would limit Base current to less than 4mA at 5V.

enter image description here

To verify that the higher voltage won't affect any components on your board you should trace out the circuit and and understand what each part does. Provided that they are all still within spec at the higher voltage it should be OK.

If you want to apply PWM then each output transistor in the H-bridge should have a reverse-biased Schottky diode from Collector to Emitter, to soak up back-emf spikes and recirculate current through the motor. If your your circuit uses FETs then you won't need diodes because they are built in to the transistors.

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  • \$\begingroup\$ okay thanks. In fact, for every h-bridge, you need a diode to soak back emf spike, right? Even I you don't plan to use pwm, when you suddenly stop the motor, there is a back emf spike, so you need a diode (or a FET) , right? \$\endgroup\$ – n0tis Jan 12 '16 at 9:13
  • \$\begingroup\$ when looking at the code that he uploaded to his arduino here pastebin.com/Jc85KeWu on line 39 he his already using pwm, do you understand why? \$\endgroup\$ – n0tis Jan 12 '16 at 10:30
  • \$\begingroup\$ "for every h-bridge, you need a diode...?" without PWM you can get away without a diode because the transistor can handle an occasional spike, and it saves a few cents in component costs! \$\endgroup\$ – Bruce Abbott Jan 12 '16 at 17:27
  • \$\begingroup\$ "on line 39 he his already using pwm, do you understand why?" He is using a fixed PWM ratio of 100/255 = 39%, perhaps to slow the motors down? Or perhaps it's a bug - undocumented code is so much fun... \$\endgroup\$ – Bruce Abbott Jan 12 '16 at 17:43
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Integrated circuits have a range of usable voltages and in this case it is most likely driven at 3.7v because that is what the voltage of the battery (atleast 3.7v sounds like a lithium battery), and not because that is the optimal value to drive this particular IC at. But what the H-bridge can and cannot tolerate is something you have to look up in a datasheet if you want to be sure - so if you can read the partnumber just use Google.

So if you don't wanna try to control it with 5 volts you have a few options.

Make a voltage divider to drop the voltage to 3.7v it only requires some resistors.

buy a 5v to 3.3v level shifter.

Insert 2 diodes or a red LED in series - this will drop the voltage to 3,8V and 3V respectively. You can always salvage a red LED somewhere if you have no Electronic components at hand.

regarding PWM Yes you can use pwm to control the motor speed. PWM is just a way of turning the power all the way on and off repeatedly and by that way modulating how much power the motor receives on average, thereby changing the speed. Your H bridge might even have a dedicated PWM pin making it easier for you- I assume thats what you mean by build to PWM Again you need the datasheet to know this. Otherwise if no PWM pin is present doesn't mean you cannot run it at PWM, just that you have to give PWM commands to all your input pins with the analog write command.

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  • \$\begingroup\$ the car is powered by a 6v NiCd battery and I think I won't find any datasheet because it's an rc car, the public isn't supposed to open it ;) Anyways, thanks for your response ! :D \$\endgroup\$ – n0tis Jan 12 '16 at 17:24
  • \$\begingroup\$ Hi, It is unlikely that they made their own H-bridge, it is almost certainly a generic H-bridge bought from another producer. Just try googling the text atop the chip + datasheet \$\endgroup\$ – Mikael Jan 12 '16 at 20:43
  • \$\begingroup\$ HI :) I did but I haven't found anything (i'm still a beginner in electronic), maybe you can find an interesting number in this album that I made ? imgur.com/a/7muQl thanks :D \$\endgroup\$ – n0tis Jan 12 '16 at 20:58
  • \$\begingroup\$ Hi Again Just looked at your Pictures - just to be precise, you have 2 H-bridges, one in each corner made from 4 beefy transistors each screwed to heatzinks. These in turn are regulated by a couple of smaller transistorsI assume these are the one you wish to take control over with the arduino. They will have a part number \$\endgroup\$ – Mikael Jan 12 '16 at 22:04
  • \$\begingroup\$ okay,thanks, so I need to check the smaller transistor spec, right ? Do you understand why there are smaller transistor before the big one ? \$\endgroup\$ – n0tis Jan 12 '16 at 22:09

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