Discussion in 'Electronic Basics' started by Captain Dondo, Oct 6, 2003.

1. ### Captain DondoGuest

This is a very basic Q - but it's been a while since high school DC
circuits (and I'm not sure I could have answered it then....)

I have a car radio that I will be switching between two batteries. I
don't want to lose the radio memory while the switch is between contacts
(it's a non-shorting switch.)

So I figure a capacitor would do the trick - but I have no idea how big to
make the thing. I've flipped through the few books I have on the topic,
but no clear answer jumped out at me.

Here's the conditions (purely a guess on my part):

12VDC, 100ma current, time 1/10 second; I don't want to drop the voltage
below say 11 volts.

I know the radio might have enough internal backup, but this has my
curiosity up.

Thanks,

-Dondo

2. ### John FieldsGuest

---

C = I*dt/dV, where

C = the capacitance in farads
I = the current in amperes
dt = the time the radio is diconnected from the battery in seconds
dV = the drop in supply voltage you can tolerate in volts, so:

C = 0.1A*0.1s/1V = 0.01F = 10000µF.

For a capacitor with a tolerance of -20 +80%, you'd have to specify
12,500µF to assure getting a minimum of 10000µF.

3. ### JeffMGuest

If you make sure that the radio is switched off
or you find the (unswitched) MEMORY_BACKUP wire
--You did know that it is seperate, didn't you?--and keep only it powered,
the current draw of the memory should be so tiny, 100uF could easily be enough.

4. ### Terry PinnellGuest

I'd have thought the radio would be OFF at time of switch-over? If so,
I'm puzzled by your 100 mA guess. I'd have guessed 1mA or less?

5. ### Captain DondoGuest

I have no idea what the idle draw should be. I just picked 100mA as a
safe guess - ISTR that 100mA was the max acceptable idle draw on a car
battery. If the real idle draw is much less, I'll revise accordingly.

If it's 1mA, that scales down the capacitor quite a bit....

-Dondo

6. ### Terry PinnellGuest

Well, why not measure it first? With radio OFF, place a DVM set to a
low current range across (i.e. in parallel with) a suitable connector
or fuse. Then remove that connector or fuse and see what current is
flowing. Remember to reconnect it, of course, before removing the DVM
(or switching it off), or you could cause the very problem you're
trying to avoid!