Hi all,

I'm a bit of a newbie so I need assistance please. I have created the following in my car. The idea is that the lamp will stay lit for a small period of time after the ignition is shut off. I have 6 x 2.7v 50F Super Capacitors in series and a 1k Resistor in Parallel. However, as you can see, the output voltage (after the Capacitors) while the ignition is turned on, is only 4.4v rather than just over 12v.

What have I done wrong ? Or what am I missing or how to I change things to get the desired result ?

I was under the impression that the voltage would only dropoff AFTER the power source was turned off.

 

The capacitors should be in parallel with the lamp not in series.
You will need balancing resistors across each capacitor to equalise the voltage as the capacitances will not be exactly equal.
You will probably also need a resistor to limit the charging current when you switch on, else it may blow the fuse.
It should look something like this:

 

What value resistor do I need to equalise each capacitor please ? And what value would R6 be in your diagram ?

I went with 6 caps since 5 x 2.7v would be slightly too few for when the car is running on the alternator

 

I need some numbers to be able to answer that.
Which capacitors are you using (so I can look up their specs)?
What is the wattage of the lamp?
What is the fuse rating feeding this circuit?

 

I went with 6 caps since 5 x 2.7v would be slightly too few for when the car is running on the alternator

Yes, quite right, I am not suggesting 5, I just can't count to 6!

 

the super capacitors are like big capacity rechargeable batteries there is limit how fast you can charge them also for discharge ... i guess
https://www.tecategroup.com/ultracapacitors-supercapacitors/ultracapacitor-FAQ.php
http://www.ewh.ieee.org/r6/scv/pses/ieee_scv_pses_jan05.pdf
Googled

 

Which capacitors are you using (so I can look up their specs)?
2.7v - 50F
What is the wattage of the lamp?
Say 50w
What is the fuse rating feeding this circuit?
10A - running off the Car Radio feed

It actually not a lamp but a CarPC. But it's still under 50w

 

Which capacitors are you using (so I can look up their specs)?
2.7v - 50F

This doesn't give me enough information.
Capacitance tolerance, leakage current, maximum charge/discharge currents.
Can you give me the actual capacitor model or a link to their datasheet?

 

Sorry, http://www.maplin.co.uk/p/50f-27v-super-capacitor-n51qq

http://www.nesscap.com/common/download.jsp?dir=product&sfn=FLMOEHFXJCHGAZU.pdf

 

Just an update on the power consumption. I used my digital meter to measure the amps and it was pretty much bang on 1amp @ 21v which it therefore seems that the PC draws under 25watts (it's a Lenovo M53 Thin Client) - so sizing for 50watts is more than enough

 

Just an update on the power consumption. I used my digital meter to measure the amps and it was pretty much bang on 1amp @ 21v which it therefore seems that the PC draws under 25watts (it's a Lenovo M53 Thin Client) - so sizing for 50watts is more than enough

Was that intended for a different thread?

 

Not at all. I just wanted to confirm the power requirement. I had been guessing before where as I have now measured it properly

 

R1-R5 should be 5.6Ω and R6 should be 2Ω. That will keep the currents and voltages within the limits for the capacitors.
R1-R5 should 2W.
The problem is R6. The peak power is about 110W and it is dissipating a significant part of that power for 25 seconds so even though the power is not continuous a resistor of significant size would be required. Also about half an amp is flowing those equalising resistors all the time the lamp is on.

I think it would make more sense to have a timer switching a MOSFET. This would require a direct connection to the 12V even when the lamp is switched off.

 

Not at all. I just wanted to confirm the power requirement. I had been guessing before where as I have now measured it properly

So the 12V lamp of the original question is now a 21V PC?
Is the 21V a typo?

 

21v is correct. It's powered by a car charger which upscaled the car's 12v to 21v. So I assume we still work off a 12v supply principle

 

The other problem using the capacitors is that the voltage will start falling immediately that switch is off and will continue gradually falling.
Is this circuit intended to run from the cigar lighter socket? That would clearly make a direct 12V supply problematic.

 

So for a 50w load, do the resistor sizes noted in post #13 above still apply ?

 

Yes. The only consideration for the load is that it should not be greater than 6A - the limit for the capacitors.

 

The issue I initially had was to run the CarPC for a few minutes after the car was turned off. I bought a Turbo Timer which is perfect, except for the fact that sometimes the power drops the PC when the ignition turns off and the Timer kicks in. So I reckoned that Caps could do a decent job and bridge the possible 0.5 sec gap between switching from the ignition to the Timer.

 

A 1F capacitor charged to 14V will drop to 12V in about 1 second with a load of 1.75A (1A @ 21V = 1.75A @ 12V), so that would be a much more manageable circuit.