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Ok here's the problem. I have a power antenna in my car. The antenna goes up when a voltage is supplied to the voltage sensor in the antenna and goes down when the voltage is removed. I used this fact to install a switch allowing me to listen to CD's without deploying the antenna.

The problem is: Whenever I start the car when the radio is on, the voltage drop is enough so the sensor registers a loss of signal and begins to withdraw the antenna into the car only to redeploy three seconds later when the car is running and the voltage returns to normal. This is hell on the gears and the torque switch. My plan is to install a capacitor in the signal circuit so that the voltage sensor no longer registers a temporary loss of signal when the car is started but will still withdraw the antenna (after a bleed time ) when the car is off or the switch is turned off.

Two questions: Since I need about a 10 second bleed time how big a cap do I need? And I install the cap in parallel across the signal leads, correct?

\$Update\$

Got it done tested, soldered, boxed and installed it works great!!! Thanks to @Davetweed AND @RedGrittybrick for your help and insight.

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  • \$\begingroup\$ How old is the battery? It might be time to replace it, before it leaves you stranded somewhere. \$\endgroup\$
    – Dave Tweed
    Jan 24 '14 at 14:42
  • \$\begingroup\$ @DaveTweed I've been in several vehicles that exhibited similar behavior - that some electronics will power off during starting. \$\endgroup\$
    – W5VO
    Jan 24 '14 at 14:54
  • \$\begingroup\$ The battery is good. The voltage is just turned off during starting. I just want to keep the antenna from cycling during this time \$\endgroup\$
    – Chris
    Jan 24 '14 at 16:47
  • \$\begingroup\$ @W5VO: I have, too. I replaced the 6-year-old battery in my Jeep, and everything went back to normal. \$\endgroup\$
    – Dave Tweed
    Jan 24 '14 at 16:55
  • \$\begingroup\$ @Chris: You did say, "the voltage drop is enough so the sensor registers a loss of signal", not that the signal was deliberately turned off. I drew the obvious conclusion. \$\endgroup\$
    – Dave Tweed
    Jan 24 '14 at 17:21
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In order to pick a capacitor for this application, you're going to need to know two things:

  • How much current does the antenna mechanism draw from the control signal when it is active? Call this I.

  • How low can the voltage drop on the control signal before the antenna starts to retract? Call this Vmin.

You have already stated that the hold time required is 10 seconds. Call this t.

We'll call the nominal voltage of the battery (when not cranking) Vsupply.

Therefore, the charge required on the capacitor is

$$\Delta Q = I\cdot t$$

The amount of voltage drop we can tolerate is

$$\Delta V = V_{supply} - V_{min}$$

Therefore, the minimum capacitance required is

$$C = \frac{\Delta Q}{\Delta V}$$

You should probably go with 2× this minimum value, in order to allow for variations in Vsupply and I under different operating conditions.

Finally, you're going to want to put a diode between the radio and the capacitor in order to keep the capacitor from discharging back through the radio.

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  • \$\begingroup\$ The current draw is easy ( think) . It should be negligible, as the motor draws its power from the battery. So then just to throw in a draw lets say 2 amps ( the fuse rating for the radio) The voltage drop I'll fudge at ${\Delta V = 2 (5-3)}$ (Half the battery voltage based on the fact that the signal lead comes out of the back of the radio If it were a 12 volt sensor it would be run off the power lead.) So then by the equation ${20 \mu f}$ \$\endgroup\$
    – Chris
    Jan 24 '14 at 18:47
  • \$\begingroup\$ Measure the control current with a multimeter. It'll be a lot less than 2A. More like a few hundred mA if it's driving a relay, or just a few mA if there's an electronic controller of some sort. Either way, you'll get a much smaller capacitance value. Also, keep in mind that the units in the equations above are Volts, Amps, seconds and Farads (not microFarads). \$\endgroup\$
    – Dave Tweed
    Jan 24 '14 at 18:49
  • \$\begingroup\$ The reason the contol signal comes from the radio has nothing to do with the voltage (which is probably equal to the supply voltage), but rather with the logic required: You want the antenna to go down if the radio is switched off, even if the car's accessory bus is still on. \$\endgroup\$
    – Dave Tweed
    Jan 24 '14 at 18:50
  • \$\begingroup\$ Would it be better IYO to hardwire the cap and diode or "hard wire" them using taps? \$\endgroup\$
    – Chris
    Jan 24 '14 at 18:51
  • \$\begingroup\$ No, the radio will send a signal to the sensor anytime it is on. So when the bus is on ( as long as the radio is also on) the antenna stays up. That is why I put in the switch, to disrupt the signal when I am playing CD's and restore it when I am ready for the radio again. When the radio is off the antenna withdraws. This is great when I want the radio off, but when I am listening to the radio at the gas pump, not so much. When I start the car the radio shuts off, and the problem arrises. \$\endgroup\$
    – Chris
    Jan 24 '14 at 19:00
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Why not just wire the antenna's motor power connector into your ignition system like the accessories, so that while you are cranking the car, the antenna does not receive power either, whch will prohibit it from attempting to retract.

I.e. while holding the key switch in the START position, power is cut off to many accessories. Use this same power bus to power your antenna's up/down motor. So when cranking the engine, the antenna's motor cannot get power, and can't move up or down. Then when you release the start switch, the antenna's motor is again receiving power as well as the other accessories.

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  • \$\begingroup\$ Have you ever pulled apart a dash? Too much work. Besides, that wire is hot at all times and I am not going to fish those wires again. Thirdly, that would cut the power when I shut off the car leaving the antenna extended when the car is off. Placing the cap in the sensor circuit let me put the box in the wheel well with the antenna, close to the connectors, and it will allow the antenna to retract when I turn the car off. \$\endgroup\$
    – Chris
    Jan 28 '14 at 19:29
  • \$\begingroup\$ @Chris yes I have pulled a dash - NOT fun. However, I am sure those lines are available under the hood too. But you have a good point about retracting after shutting off the engine... I should have thought of that. :P \$\endgroup\$ Jan 29 '14 at 2:41
  • \$\begingroup\$ Yeah but then I would have to run my wire through the firewall. The hot lead is at the antenna but tapping it would just leave the antenna up all the time. \$\endgroup\$
    – Chris
    Jan 29 '14 at 3:15

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