sjb@dalek.UUCP (Seth J. Bradley) (05/24/89)
I have a VCR which I'd like to modify by greatly increasing
the logic supply filter capacitance. The purpose being to
increase the time the memory will be retained during power
outages. The question is, are the new ultra-high caps
(i.e. 3.3F at 5.5 V) which are designed for power backup
suitable for use as power supply filter capacitors?
Thanks in advance!
--
Seth J. Bradley UUCP: uunet!{lll-winken|ubvax}!dalek!sjb
Internet: lll-winken.llnl.gov!dalek!sjbhenry@utzoo.uucp (Henry Spencer) (05/24/89)
In article <484@dalek.UUCP> sjb@dalek.UUCP (Seth J. Bradley) writes: >... The question is, are the new ultra-high caps >(i.e. 3.3F at 5.5 V) which are designed for power backup >suitable for use as power supply filter capacitors? In general, no -- their internal resistance is too high. Substantial currents flow in and out of filter capacitors during the AC cycle. -- Van Allen, adj: pertaining to | Henry Spencer at U of Toronto Zoology deadly hazards to spaceflight. | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
hoang@rex.cs.tulane.edu (Dzung Hoang) (05/26/89)
In article <1989May24.154332.29113@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes: >In article <484@dalek.UUCP> sjb@dalek.UUCP (Seth J. Bradley) writes: >>... The question is, are the new ultra-high caps >>(i.e. 3.3F at 5.5 V) which are designed for power backup >>suitable for use as power supply filter capacitors? > >In general, no -- their internal resistance is too high. Substantial >currents flow in and out of filter capacitors during the AC cycle. >-- >Van Allen, adj: pertaining to | Henry Spencer at U of Toronto Zoology >deadly hazards to spaceflight. | uunet!attcan!utzoo!henry henry@zoo.toronto.edu To: henry@utzoo.uucp Subject: Re: Substituting Caps oin Logic Supplies Newsgroups: sci.electronics In-Reply-To: <1989May24.154332.29113@utzoo.uucp> References: <484@dalek.UUCP> Organization: Computer Science Dept., Tulane Univ., New Orleans, LA Cc: Bcc: In article <1989May24.154332.29113@utzoo.uucp> you write: >In article <484@dalek.UUCP> sjb@dalek.UUCP (Seth J. Bradley) writes: >>... The question is, are the new ultra-high caps >>(i.e. 3.3F at 5.5 V) which are designed for power backup >>suitable for use as power supply filter capacitors? > >In general, no -- their internal resistance is too high. Substantial >currents flow in and out of filter capacitors during the AC cycle. >-- >Van Allen, adj: pertaining to | Henry Spencer at U of Toronto Zoology >deadly hazards to spaceflight. | uunet!attcan!utzoo!henry henry@zoo.toronto.edu Uh, high internal resistance is desirable in capacitors. I really doubt that much AC current will flow through the capacitor if it is used as a power supply filter. You should have no problem if the DC voltage is less than 5V, although 3V or so would be better. These caps can be easily damaged by apply too- high voltages. Try it and see. Dzung Hoang hoang@rex.cs.tulane.edu
jeffw@midas.STS.TEK.COM (Jeff Winslow) (05/27/89)
In article <814@rex.cs.tulane.edu> hoang@rex.UUCP (Dzung Hoang) writes: > Uh, high internal resistance is desirable in capacitors. I really doubt >that much AC current will flow through the capacitor if it is used as a power >supply filter. The first statement is true for parallel resistance, which is virtually irrelevant in power supplies. What Henry was talking about, and he is absolutely right, is internal series resistance, which is most definitely UNdesirable in a capacitor. (Unless you are relying on the ESR of the capacitor to stabilize your regulator feedback loop or damp your decoupling circuit - but those are subtleties.) As for your second statement, I implore you to become a believer. If there's not much AC current flowing through a power supply filter capacitor, somebody wasted a capacitor. The whole point of such capacitors is to prevent AC currents from affecting the power supply voltage. And internal series resistance will of course conflict with that purpose. Jeff Winslow
henry@utzoo.uucp (Henry Spencer) (05/27/89)
In article <814@rex.cs.tulane.edu> hoang@rex.UUCP (Dzung Hoang) writes: >>>... The question is, are the new ultra-high caps >>>(i.e. 3.3F at 5.5 V) which are designed for power backup >>>suitable for use as power supply filter capacitors? >> >>In general, no -- their internal resistance is too high. Substantial >>currents flow in and out of filter capacitors during the AC cycle. > > Uh, high internal resistance is desirable in capacitors. Cough choke splutter. Unless everything I learned is wrong, high internal resistance is exactly what you *don't* want in capacitors. The purpose of capacitors is to store charge and dispense it as desired. *Any* internal resistance interferes with that. Can you justify this remarkable statement? >I really doubt >that much AC current will flow through the capacitor if it is used as a power >supply filter. Try computing it sometime. It's a lot more than you think. During a fair part of the AC cycle, the voltage out of the diodes will be less than the desired DC voltage, so the capacitor is what's supplying the entire power drain of your circuit. During the remainder, the voltage is higher than the desired, so it's recharging the capacitor. This period will generally be a relatively small portion of the cycle, meaning that currents then have to be even higher to recharge the capacitor adequately. > You should have no problem if the DC voltage is less than 5V, although >3V or so would be better. These caps can be easily damaged by apply too- >high voltages. Try it and see. Do remember that the voltage out of a transformer-diode front end will be *considerably* higher than the final output voltage. First, because it's pulsating DC with peaks rather higher than the average. Second, because there is voltage drop in the regulators, often quite a bit. A supply which delivers +5 will often have peak voltages of +10 or more at the filter caps. -- Van Allen, adj: pertaining to | Henry Spencer at U of Toronto Zoology deadly hazards to spaceflight. | uunet!attcan!utzoo!henry henry@zoo.toronto.edu
sukenick@ccnysci.UUCP (SYG) (06/01/89)
In article <1989May24.154332.29113@utzoo.uucp> henry@utzoo.uucp (Henry Spencer) writes: >In article <484@dalek.UUCP> sjb@dalek.UUCP (Seth J. Bradley) writes: >>... The question is, are the new ultra-high caps >>(i.e. 3.3F at 5.5 V) which are designed for power backup >>suitable for use as power supply filter capacitors? >In general, no -- their internal resistance is too high. Substantial >currents flow in and out of filter capacitors during the AC cycle. Since Seth wants to use these caps for power backup, pehaps keep the original filter caps in place and put the 3.3F after the filter caps or (if that will cause problems with the power supply, then perhaps) in parallel, if the voltage specs at that point are acceptable: 5.5V might cut it close, although for less cap., you could use two of them in series. The filter caps will filter when power is ok; when AC power goes down, the 3.3F will supply reserve power. (ie: the PS caps are the 60 Hz filters; the 3.3F does the low low frequency (power outage) filtering :-) )
barry@hprmokg.HP.COM (Barry Fowler) (06/03/89)
Howzabout using a rechargeable battery with an isolation diode and forgetting about all of that capacitor stuff? They use 'em in computers all the time and also in my cheepo Radio Shaft scanner.