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 Post subject: Tantalum Capacitors
PostPosted: Feb Tue 01, 2011 1:40 am 
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Hi Guys, I have access to tons of old circuit boards. I want to build a DC to AC inverter. What kind of boards use Tantalum Caps?
I need two 68uf 25V. I seen they are quite expencive, like 5 bucks a piece. To much for me!
Thanks for any help,
Gary


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 Post subject:
PostPosted: Feb Tue 01, 2011 3:29 am 
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Most Tantalum Caps have extremely low ESR and leakage . You will find them on processor boards and in smaller switching power supplies. What you are wanting to build is a form of switching power supply. The question is.....are the tantalums required for the build? Do you have a Schematic?


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 Post subject:
PostPosted: Feb Tue 01, 2011 3:44 am 
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They're likely to be used as bypasses on power-supply rails, but 68 is an odd value for that, so you may not find any. Without knowing what type of circuit boards, it's hard to be more specific.


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 Post subject:
PostPosted: Feb Tue 01, 2011 2:13 pm 
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Here is the plans..
[img]http://www.aaroncake.net/circuits/inverter.asp
Gary[/img]


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 Post subject:
PostPosted: Feb Tue 01, 2011 2:29 pm 
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Gary,

You had your signature as part of the file URL. Here is the corrected link

_________________
Pics, Bold & underlined text, are usually links.

https://mikeharrison.smugmug.com/

Mike


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 Post subject:
PostPosted: Feb Tue 01, 2011 2:41 pm 
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That circuit should be fine with just about any kind of electrolytic cap. I've seen that value in surplus stores and such.

If it matters, note that the circuit may not be exactly 60 Hz---but you can adjust the cap values and/or R3 and R4 to get it there.


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 Post subject:
PostPosted: Feb Tue 01, 2011 3:12 pm 
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Thanks Mike for fixing that..The person who designed this says do not use electrolytic caps. I am not an engineer, just an old retired
electronics bench tech. with cabin fever!! :lol:
When I'm not restoring old radios, I set around and try to build something out of the spare parts in my junk box.
Thanks guys, Gary


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 Post subject:
PostPosted: Feb Tue 01, 2011 6:06 pm 
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As I look at this web page, I get the feeling I'm in a time warp, reading Popular Electronics in 1965. 2N3055s??? Directly-coupled multivibrators??? There have to be far better devices and circuits by now, though I suppose most people either buy an inverter or use an old UPS. Those are often thrown out when the original battery dies.

I think the original builder is optimistic about power output and frequency stability.


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 Post subject:
PostPosted: Feb Tue 01, 2011 7:44 pm 
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Don't build that circuit. It's bad for the following reasons:

1) When a transistor doesn't conduct, its base goes negative and beyond the emitter-base voltage rating due the negative charge on the capacitor side connected to it.

2) Both tantalum and polarized electrolytic capacitors will get hot and explode because there are half cycles where they are charged backward against their polarity.


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 Post subject:
PostPosted: Feb Tue 01, 2011 9:14 pm 
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Gary T. Lane wrote:
Thanks Mike for fixing that..The person who designed this says do not use electrolytic caps.

Which is why there is a polarity on the schematic and "Tantalum" specified in the parts list? :? :?
Anyone seen 68 mF caps that were NOT electrolytics? (I'm sure such a thing exists, but is not common.)


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 Post subject:
PostPosted: Feb Tue 01, 2011 9:31 pm 
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bb.odin wrote:
2) Both tantalum and polarized electrolytic capacitors will get hot and explode because there are half cycles where they are charged backward against their polarity.


Not sure about that...Here is how I read one cycle (for C2):

Start with Q2 on. + end of C2 at ~0.2 volts, - end at ~-12

C2 charges until - end (base of Q1 hits ~0.5 volts, Q1 switches on

when Q1 switches on, Q2 switches off. + end goes to +12 (carging thru the series resistance of the trans. primary.

Q2 switches on, + end of C2 goes back to 0.2 volts, - end goes negative, and we're back to step 1.


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 Post subject:
PostPosted: Feb Tue 01, 2011 10:33 pm 
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Quote:
Start with Q2 on. + end of C2 at ~0.2 volts, - end at ~-12

C2 charges until - end (base of Q1 hits ~0.5 volts, Q1 switches on


When Q2 turns on, the negative end of C2 which starts at a negative voltage near -12V charges positively through R3 until it reaches about 0.6V and causes Q1 to turn on. During this half part of the cycle, C2 is connected backward and behaves more like a diode.


Last edited by bb.odin on Feb Tue 01, 2011 10:50 pm, edited 1 time in total.

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 Post subject:
PostPosted: Feb Tue 01, 2011 10:50 pm 
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+ end is at 0.2 volts (Q2 on) - end charges from -12 up to +0.6. C2 does in fact get reverse-biased (0.4 volts), but only for a small fraction of a second. As soon as Q1 turns on,, Q2 turns off, and the + end of C2 goes to +12.

On a scope, you might never see C2 with reverse voltage.


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 Post subject:
PostPosted: Feb Tue 01, 2011 10:51 pm 
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PS;
Do electrolytics have a spec on allowable reverse voltage? (Never thought about this....)


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 Post subject:
PostPosted: Feb Tue 01, 2011 11:35 pm 
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I've seen reverse voltage specs for electrolytic caps.

I agree that the intervals during which the caps are reverse-biased are small. It may not cause the caps to blow up. Still, this circuit is not reliable.

I think the real culprit that causes the caps to overheat is the ripple current through the caps and the bases of the transistors. It's limited only by the resistance of the transformer winding. The heat generated in a cap is Irms^2 * ESR, where Irms is the RMS value of the ripple current and ESR is the equivalent series resistance of the cap. To minimize the heat generated in the cap, the ESR needs to be very small.


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 Post subject:
PostPosted: Feb Wed 02, 2011 12:17 am 
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I think that inserting two resistors, one between the - cap end and the base of each transistor, would resolve the issues mentioned earlier. Their value would be chosen so that the reverse bias voltages on the bases don't exceed the limit and the caps are never reverse biased. These resistors also significantly reduce the ripple current through the caps and eliminate the overheat. The caps need to have a lower value to maintain the same frequency and their ESR is less important.


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 Post subject:
PostPosted: Feb Wed 02, 2011 6:07 am 
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pixellany wrote:
Gary T. Lane wrote:
Thanks Mike for fixing that..The person who designed this says do not use electrolytic caps.

Which is why there is a polarity on the schematic and "Tantalum" specified in the parts list? :? :?
Anyone seen 68 mF caps that were NOT electrolytics? (I'm sure such a thing exists, but is not common.)


What......you mean these things? Don't apply reverse power or
cablamo!

Image


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 Post subject:
PostPosted: Feb Wed 02, 2011 2:55 pm 
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Quote:
cablamo
That's a technical term I had not heard before......;)

Quote:
Don't apply reverse power
Perhaps this is the key---in many situations, the damage is done by heat---heat in turn is the result of excessive POWER--regardless of the voltage or current.

Applying this logic to the discussion of this circuit, the reverse voltage on the cap is small and for a very short time. Assuming that there is leakage at the low voltage, the net energy dissapated will be very low.


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 Post subject:
PostPosted: Feb Wed 02, 2011 5:26 pm 
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Kyocera recommends no more than 1 volt reverse on a tantalum cap.

http://www.kyocera.co.jp/prdct/electro/pdf/technical/revtant.pdf

Rich


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