# H Bridge Using Momentary Switches

I'm trying to get back into electronic projects and I'm now working on controlling the direction of a DC motor with 2 push button momentary switches. Motor is 6-8 amps, switches are rated for 18 amps. S1 is a latching push button switch which is currently working fine. S2 and S3 are momentary.

I'm having trouble translating diagrams to actual wiring and components. One bit that is causing me some confusion is the types of switches I'm using for this project. I have some push button switches with both NO and NC contacts. I've attempted to wire this up to no avail. I need some help. I've drawn what I think should be the diagram. Forgive me for the rudimentary notation. Seems to me that wiring the NC contacts of S2 to the NO contacts of S3 should give me one path When S3 is pushed, and the opposite would give me the opposite path. It seems my assumptions are wrong.

The intent is one button for a direction, the other button for the other direction. Full manual control; No limit switches, no relays, no protection diodes, no interlocking circuit, etc. Just the 3 basic components I have on hand. I'm tracking the short circuit condition if both are pressed at the same time. I'm not concerned with that for this experiment.

I could really use some assistance on this one. I'm a former 15Y (Avionics, Armament, and Electrical) and have worked on military aircraft so I'm not totally new to this. Just been out of the game for quite some time.

Thanks.

Updated diagram for reference. Now I just need to get an LED wired across the NO contacts for both switches. Suggestions appreciated.

simulate this circuit – Schematic created using CircuitLab

Figure 2. My circuit.

simulate this circuit

Figure 3. LED options. D1 indicates power on. D2 indicates motor running one way and D3 the other.

The back-to-back arrangement of D2 and D3 ensures that either LED will only see about 2 V reverse voltage and prevent damage.*

D4 to 7 are snubber diodes to protect the LEDs and contacts against high voltages generated by switching the inductance of the motor. They should be rated to carry the same current as the motor.

simulate this circuit

Figure 4. Motor current from left to right before and after SW1 opens.

• Remember that inductors "dislike" changes in current and will do their best to maintain current flow in the direction it is going.
• Just before SW1 is opened current is flowing from left to right.
• In the time it takes SW1 to change over current will continue to flow and the inductor's (your motor) polarity will reverse in an attempt to keep it going. Current wil flow through SW4 and back around through D9 as it decays.
• When SW3 has fully changed over current will just flow through SW3 and SW4 and none of the diodes will conduct.

So:

• D5 conducts when motor is running forward (L to R) and SW1 is switching from + to -.
• D7 conducts when motor is running backwards ans SW2 is switching from + to -.
• D4 conducts when motor is running backwards and SW1 is switching from - to +.
• D6 conducts when motor is running forwards and SW2 is switching from - to +.
• Thank you for your updates to my drawing. Works as expected. I was surprised that the short circuit across the power supply condition that comes with an H bridge when all switches are connected, does not exist with my configuration. Is this not a true H bridge? Nov 8, 2021 at 0:55
• Also, after we shorted pairs of NC and NO contacts, I don't see anywhere to connect the LED indicator. There doesn't seem to be any point where the connection to the positive rail is live only when the switch is depressed. Where do you suggest I connect an LED across? Nov 8, 2021 at 0:58
• @DavidNewton There's no physical way to close all switches in your design, since closing one pair of contacts necessarily opens the other. Nov 8, 2021 at 1:19
• What is the LED supposed to indicate? Power on? Forward? Reverse? Nov 8, 2021 at 6:52
• Yep. Just as you said, power, forward, reverse. My switches have built in LEDs and I assume they have resistors built in. The wiring diagrams that come with them don't show any additional resistors required. I've inquired with the manufacturer for more info. Nov 9, 2021 at 0:55