Super capacitors- any advantages for relay use?

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boji

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Jan 6, 2010
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Never used 'em before. Other than having high values for their size, don't know much about them. Just curious about your opinions.

Thanks!

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Yes, thank you gentlemen!
Someone had some on the cheap, but I did some reading. Super caps are in the wrong ballpark.

Basically I'm trying to reduce a potential dip in the PSU where 12 relays would simultaneously energize (TQ2-12).  Dunno how to calculate the minimum uF to offset the draw (course I googled it, but I don't think I'm using the right search terms)  I know!  Super basic stuff here...  ::)
So, fudging it, settled on a 2200uF, 50v lytic.

If you think this is overdoing it or is not sufficient, by all means I appreciate your input!

Thanks again, PRR and JR.

BTW, what's the proper term for when a PSU sees a current draw and dips a bit?  Ripple? Or is that just a term concerning rectification.
 
I think the equation is  C=I*t/U.

TQ2-12 draws 11.7mA.  * 12 relays =  .14 Amps

So,  0.14 A * .5 seconds / 12v = .00583 or 5800uF

I figured half a second is more than enough time for a relay coil to energize so 2200uF would be ok.

 
boji said:
I think the equation is  C=I*t/U.

TQ2-12 draws 11.7mA.  * 12 relays =  .14 Amps

So,  0.14 A * .5 seconds / 12v = .00583 or 5800uF

I figured half a second is more than enough time for a relay coil to energize so 2200uF would be ok.
11.7 mA is the steady-state current. Kick-in current is the combination of the inductor kick-in current, which is zero, and the magnetization current, that is unspecified.
Trial-and-error is de rigueur. I believe you need a smaller cap.
 
11.7 mA is the steady-state current.
Quite right, thanks for pointing that out.
Kick-in current is the combination of the inductor...and magnetization current.. Trial-and-error is de rigueur.
Nice.  I've been at the soldering station in the evenings for what feels like three months straight. Now I have an excuse to learn this little 2ch LeCroy scope my brother gave me. 

Thank you so very much Abby!  ;D ;D
 
Be sure to read the specs carefully, specifically the equivalent series resistance and the max current you can get out of it. I recall the original ones from decades ago had a few hundred ohms ESR and basically the only application was to maintain low-current CMOS RAM for a few minutes or hours. Modern supercaps are surely much better, but I'd double-check to be sure.
 
Be sure to read [super capacitor] specs carefully, specifically the equivalent series resistance and the max current you can get out of it.
Yes, thanks Benb.  I've decided to use standard polar lytics for buffering. They should do the do.
 
> specifically the equivalent series resistance and the max current

They used to be pretty poor. Recent products have shockingly low ESR and high peak current. They may be synergistic with scooter/car electric batteries for taking surges that stress batteries.

I had an old design which got better the larger the emitter cap got. I dredged it up and stuck a 50F (not uF!) cap in a sim. While it cured the subsonic quirk, the start-up time could be an hour.... and it proved that "compensating" a HIGH-gain amplifier is a long-long Bode road, even if the fall-off is on the bass side instead of treble. Poked it a bit more and got a simple scheme using only small cheap e-Caps.
 
What is the psu / voltage source for the relay coil voltage ?
Hello NM,
Dedicated 12v out from an old 600w Wheatstone broadcast desk PSU, or possibly a nos Amek big44 SMPS.

Thanks
 
boji said:
Hello NM,
Dedicated 12v out from an old 600w Wheatstone broadcast desk PSU, or possibly a nos Amek big44 SMPS.

Okay. So they are regulated supplies with enough current on tap. The change on the 12V should be minimal (ideally zero but subject to 'Load Regulation' and 'Transient Response'.
In general larger values of output capacitance and low ESR will improve transient response but you don't need the huge values you have mentioned. Sounds like you are aiming to provide all the switching current from the caps. You don't need to do that - just need to 'help' out the psu and you probably don't really need to do that unless you have analogue stuff running on the same 12V supply and associated 0V.
See link for definitions - it's focussed on LDO regulators but the same principles apply.
http://www.ti.com/lit/an/slva079/slva079.pdf
 
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