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DC input power filtering

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Hey all,

Newb here... I am building an LED dimmer/test circuit. This is for aircraft useage. I have my basic design, but want to put some finishing touches on it. It is basically a bunch of diodes for the push-to-test feature and a MC34060A chip driving some FETS that in turn drive the LED annunciators. The reason I made it is that some of the LED's need a reduced ground state and some need a reduced power state during night operation. I also want the night-time brightness level to be adjustable for all the LED's, thus the chip I chose. Here is my question... since this will be used in aircraft with either 14 VDC or 28 VDC input (with alternator running) what should I do in the way of protecting the power input to the FETS/chip/LED's? I was thinking of a 33V zener on DC input to the circuit. Is there a need to use capacitors to filter the DC input? If so, why and what size in mf? I also have a concern with the frequency of the IC (set to 600Hz) magically appearing in the aircrafts audio system. I have built a similar circuit in the past and it is audible in the intercom system, though very quiet. If you pull the circuit breaker to the dimmer the hum goes away. I did not filter or protect it in any way and it is still working after two years.

One more thing...anyone know of a web site that has a large selection of photocells? I am thinking of using a photocell so the pilot never has to select night operation; the annunciators will just dim auotmatically.


Depending on the voltage and current that is necessary for your circuit, you may want to consider using a voltage regulator, which will, you guessed it, regulate the voltage in to your circuit. The voltage regulators are designed to filter out voltage spikes which may be caused by the alternator. For instance, the LM7805 IC will accept a max input voltage of 35VDC and output at 5VDC at 1 amp. Similarly, a 78L12 (or Lm7812) provides an output of 12VDC. There are also voltage regulators designed for the VDC negative side, if you wish to filter the negative input as well. Also the LM317 is designed to be an adjustable voltage regulator, which you can set the output voltage that your circuit requires, and the circuitry to setup this adjustment capability is very minimal (1 resistor, a 5K trimmer and a couple of capacitors.) Please be aware that the voltage regulators will emit heat for the amount of voltage that you are filtering from the supply and the amount of current you are drawing from the regulator circuit. Thus, proper heat sinking is required in order to keep your circuit from overheating, though, the heat sinks required are generally very small, unless you are running a high VDC in (28 volts or so) and regulating that to a low VDC out (1 volt or so).

I'm not sure how to filter the supply only using capacitors. I have heard that voltage spikes from automotive alternators can reach 60Volts or so. Although, I am assuming that aircraft alternators are regulated more so than an automotive alternator would be. With that said, I'm not sure what type or size of capacitor will do here.

Regarding the photocells, have you tried

Finally, here is a link to a site that uses the LM317 as a power supply IC for ground operations, but the methods can be applied to your situation.

Here is the link for the datasheet for the LM317. Similarly, you can find the appropriate link for the datasheets for the other voltage regulators on this site.
**broken link removed**

Good luck!
Thanks Johnson,

I should have mentioned that there is a voltage regulator associated with the alternator. What made me think of using capacitors is that every aircraft radio I have worked on has had at least one or two capcitors going to ground from the power input. Usually the caps are 100uf/50V with a 100uh choke preceeding the caps. Sometimes there is a zener to ground in parallel with the caps, though not always. I assumed this was for noise filtering and spike protection. Set me straight.


You should read more carefully. I wrote in the past tense. I haven't touched a radio in over three years. Actually I was very good at repairing them. You do not need to be an engineer to fix radios, just to build them. I don't need to know why every component is present in a design in order to repair a device. I just need to know what will result if a component fails. e.g., if a cap on the power input shorts then the fuse will blow. It isn't rocket science nor is it hard to troubleshoot. In seven years I never had a radio come back with an associated failure or for the same failure relating to a repair I made.

One more thing... if you don't have anything constructive to add then I was always taught not to say anything.

The last I figured, the radio didn't really have anything to do with the flight characteristics of an aircraft. Plus I'm sure and VFR small to mid scale strip would welcome the attention from an unidentified / un-announced aircraft landing. LOL :shock:


Yes sir, you are correct with the filter caps and the zener being installed for noise and spike control.

Could you try sheilding your IC with some metallic plate over the IC, or encapsulating your entire core circuit? This may reduce or eliminate the unwanted noise in your headsets. Also, running power wires next to ground wires could provide an issue as well. Some manufacturers run a couple of small Ferrite cores onto their wiring to reduce the noise when power and ground wires are close together.

As far as figuring which type and size of cap you'll need, I'm not sure. Let me do some research and get back with you.
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