I need help finishing/fixing a circuit design. It isn't/shouldn't be terribly complex, and I'm not certain whether it's my relative lack of circuit design experience or simply myopia that's getting in the way of finishing it. It's an automotive lighting control circuit (relay-fed low and high beam headlamps with a daytime-running mode).
Not sure if this is the right group in which to be making this request for help. If not, I'd appreciate a pointer to a more appropriate forum.
The following operational modes are needed:
Engine running, handbrake applied, parking lamps "off": No lights on.
Engine running, handbrake released, parking lamps "off": Low beams and high beams in series with one another.
Parking lamps "on": No headlamps energized in any way
Headlamps "on": Headlamps function normally, can be switched between low and high beam, etc.
The difficulty is that I want to control the headlamps via relays to eliminate voltage drop endemic in many vehicles' factory headlamp circuitry, per e.g.
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.
What we've got so far is at
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, but it's only half-baked yet (where does R1 power come from with headlamp switch turned off/in DRL mode? How do we discern between "parking lamps on" and "headlamps on" switch positions so as to kill the DRLs and low and high beams when the parking lights are on and/or the handbrake warning lamp switch is closed?)
Any assistance appreciated. I'd rather not get into discussion of alternative DRL modes; there is a specific reason why I need the circuit to put the lows and highs in series with one another.
If I understand you correctly- this should do it. You will have to use diode logic on the relay control circuits to save a bunch of parts. R2 puts BATT on the high beam lamp, R1 puts BATT on the low beam lamp when activated and GNDs the filament when not activated. R5 is the DRL mode control relay, in the deactivated state, it will allow IGN to activate R2 through the diode, and this throws BATT onto the two headlamps in series to GND through R1, R3 is open in DRL mode. The diode logic activates R5 whenever any one of HLSW, PLSW, HBSW are on, preventing DRL mode, and prevents backflow of DRL control into the HLSW circuit. The PLSW activates R4 which opens the HLSW circuit. The HLSW circuit activates R3 which GNDs the headlamp common terminal-allowing normal operation. In summary then, the parking and headlamp circuits operate off the BATT circuit and work normally, with the parking circuit shutting off the headlamp circuit. DRL mode is enabled only when the ignition circuit is on, controlled by the handbrake switch, and disabled by either the parking lamp or headlamp switch. Use diodes with 1000V PIV and minimum 5x the relay coil current rating. It is important to note that the Bosch 87a terminals are 1/2 the current rating of the 87 terminals, and this is why the headlamp power circuit relays are arranged so that the 87 terminals take the surge.
Thanks! I will need to study this and mock it up on the test bench, but it certainly looks well enough conceived to work. Under what name shall I credit you? (send me an e-mail if you like, dastern at torque dot net).
Another (private) response to my "Halp!" squawk is this:
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with explicatory notes thus:
Each side of the car will need a DPDT relay, and you will need a heavy duty SPST for the low beam power.
Operation:
Car off - relays are in de-energized state. Headlights and high beams work as normal.
Ignition on, hand brake off. Relays are energized and configure the lamps to have both filaments in series. The high beam filament is grounded, and the low beam has +12V. The ground terminal is floating.
Turn on the lights (parking lights or better) and the relays are de-energized so the headlights and high beams operate normally.
Ignition on, hand brake on, same as lights on -- relays de-energized.
You will notice the HB+ and HB- inputs. Those are for the hand brake. You will use only one, depending on how the hand brake switches. If it switches +12V, the you connect to HB+. If it switches to ground, you connect to HB-. You only use one.
How's this strike you?
Thanks again,
DS
-- Being born on a Class-M planet is like winning the lottery.
The circuit is ordinary enough to attribute it to standard relay logic practice. One detail not contained on the schematic is "quenching" the relay coil driver circuits- this means suppressing the tendency of the coil inductance to produce transiently large voltages in the circuit when its current is abruptly interrupted at turn off. The only relays in the circuit for which this could be a problem are R4 and R5. Both of these relays will be quenched by the parallel lamps PL and HBL. As long as the lamp is rated for greater current than the relay coil, this will present no problem. The PL is most likely rated for much larger current than R4 coil- so no quenching is necessary, but the HBL is most likely a miniature thing and could be damaged by the quenching action. To be on the safe side, add a diode in parallel with R5 85/86 terminals with anode to GND- so that makes a 6-diode terminal strip. From what I can see, most of these Bosch power relays draw under 200mA coil current so that the diodes can all be the inexpensive 1N4007 type- usually a 3 cent part.
The logic is correct except that the PARK input should mean parking light switch OR headlamp switch is on. Three drawbacks to this are 1) Bosch does not seem to offer a DPDT in this automotive relay line- at least not any that are readily available from the re-sellers- they are all single pole, and 2) unprotected transistors are too fragile to survive the potential transients in the automotive enivironment- so that is just one more headache to deal with, and 3) from what I can gather from the literature, most of the heavy duty relays have higher current ratings, like a factor of 2x, for the contacts that make when the coil is energized, and this is because the contact pressure is much greater than the de-energized contact makes where the pressure is applied by a weaker spring force- therefore it is better, not only for current handling but also vibration resistance, to arrange things so that the normal headlamp operation uses the relays in the energized state-not the de-energized state. I notice that you like to eliminate power line loss feed to the headlamps, so that you will want to duplicate R1-R2-R3 on each side of the car- everything else can go in a small enclosure near the switches- break out a pair of 3-wire harnesses running to each control relay cluster at the headlamps- one wire to each pin 85 input- you can GND the pins 86 locally. The under dash control schematic would look something like this, diodes double off the HLSEL and DRL drive to reduce current rating and allow possibility that HLSEL is make-before-break:
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