jelliott
Member
I recently built a slight variation on Bill Bowden's "Automatic 12 Volt Lamp Fader" and have a few questions that I was hoping the experts on this forum might be able to help me with.
Background: My brother's electric car lacked any kind of visual indication that it's plugged in and charging. And I've always admired the way Apple computers of the last 15 years fade the power light in and out at ~0.2 Hz when they're in sleep mode. So I thought it'd be neat if the electric car would do the same with its parking lights when it's on the charger. However, the parking lights are wired with +12 V switched, rather than switched ground, so I needed a circuit that would accommodate the load being downstream of the transistor. I emailed the proprietor of the web site where I found the schematic, and got a prompt response indicating that the circuit (in its BJT incarnation, anyway) would work in that configuration without any further modifications. Mr. Bowden's response is as follows:
Questions:
1) Something must be wrong with the circuit as I built it because the transistor gets VERY hot; not just hot enough to burn a finger, but hot enough to discolor the metal case of the transistor, even when it's providing a paltry 85 mA to a single light bulb. When installed in the car it doesn't provide enough current to provide even a hint of illumination. Tested on the bench, the (open circuit) voltage ramps up and down as intended, and it works fine (heat dissipation notwithstanding) with a 1 W load, but with a 5 W bulb it only ramps up to ~5 V. Is there something wrong with the circuit design? Is it likely that I got something wrong in the course of assembling it that could lead to this behavior? Could it be a faulty transistor? Are there other transistors I could use to avoid this? (And if I'm going to drive multiple transistors in parallel, e.g. to account for the license plate lamps, can the LM324 handle that? Do I need to halve the resistance between the op-amp and the transistor base to maintain the same base current in a second transistor?)
2) The circuit works fine with load limited to 1 W, but only if it's powered by a 12 V battery, but not on the available rectified +12 V power. I thought this would be easy because the car's charger has a disused +12 V output, but when my circuit is connected to that power source, the load just just comes on at a constant partial brightness and doesn't fade at all. I assume this must be related to AC ripple on the rectified +12 V supply, but I don't know what to do about it (I'm a mechanical engineer by training, after all). Can I stick a capacitor somewhere to resolve this issue?
3) As designed, the Bill Bowden circuit switches the load completely off for a portion of the cycle. Ideally, I'd prefer that it never fade below a ~5 V level. Is this as simple as adding a resistor between +12 V and the transistor base? Or does it need to be a voltage divider sort of arrangement?
Thanks,
Joe Elliott
Background: My brother's electric car lacked any kind of visual indication that it's plugged in and charging. And I've always admired the way Apple computers of the last 15 years fade the power light in and out at ~0.2 Hz when they're in sleep mode. So I thought it'd be neat if the electric car would do the same with its parking lights when it's on the charger. However, the parking lights are wired with +12 V switched, rather than switched ground, so I needed a circuit that would accommodate the load being downstream of the transistor. I emailed the proprietor of the web site where I found the schematic, and got a prompt response indicating that the circuit (in its BJT incarnation, anyway) would work in that configuration without any further modifications. Mr. Bowden's response is as follows:
The four (LED) parking lights draw a total of 100 mA and instrument panel illumination that's on the same circuit draws maybe 350 mA, so with the USA-spec corner markers disconnected, I thought the 2N3053 would be adequate (I was forgetting 800 mA for the license plate illumination).
Questions:
1) Something must be wrong with the circuit as I built it because the transistor gets VERY hot; not just hot enough to burn a finger, but hot enough to discolor the metal case of the transistor, even when it's providing a paltry 85 mA to a single light bulb. When installed in the car it doesn't provide enough current to provide even a hint of illumination. Tested on the bench, the (open circuit) voltage ramps up and down as intended, and it works fine (heat dissipation notwithstanding) with a 1 W load, but with a 5 W bulb it only ramps up to ~5 V. Is there something wrong with the circuit design? Is it likely that I got something wrong in the course of assembling it that could lead to this behavior? Could it be a faulty transistor? Are there other transistors I could use to avoid this? (And if I'm going to drive multiple transistors in parallel, e.g. to account for the license plate lamps, can the LM324 handle that? Do I need to halve the resistance between the op-amp and the transistor base to maintain the same base current in a second transistor?)
2) The circuit works fine with load limited to 1 W, but only if it's powered by a 12 V battery, but not on the available rectified +12 V power. I thought this would be easy because the car's charger has a disused +12 V output, but when my circuit is connected to that power source, the load just just comes on at a constant partial brightness and doesn't fade at all. I assume this must be related to AC ripple on the rectified +12 V supply, but I don't know what to do about it (I'm a mechanical engineer by training, after all). Can I stick a capacitor somewhere to resolve this issue?
3) As designed, the Bill Bowden circuit switches the load completely off for a portion of the cycle. Ideally, I'd prefer that it never fade below a ~5 V level. Is this as simple as adding a resistor between +12 V and the transistor base? Or does it need to be a voltage divider sort of arrangement?
Thanks,
Joe Elliott
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