Positive And Negative Reactances

[Nowhere-Man]

There is something in my HAM book that sadly isn't clarified by actual examples. It was talking about how to total up inductive or capacitive reactances either in series or in parallel. The way to total capacitor reactances in series or just coils in series was simple to explain. It's just the same as resistors so there was no problem.

Where it got vague was the case when there is a series of capacitors and coils. In that case, the XC is said to be negative and the XL positive. If you were making a total for, say, resistors in parallel you'd just multiply the two values to get one sum, add the two values to get another sum and then divide the latter into the former for the overall total.

As it gave no actual example, I can only assume you're supposed to subtract the XC negative from XL positive. It's because the XL voltage leads the current and the XC current leads the voltage so any calculation has to take phase into account. Actually the sum is obviously very simple if you are already given values to work with in Ohms. I mean, if you have, say, 10 Ohms XL and 6 Ohms XC in parallel, do you just subtract? Sadly the book doesn't fully explain as I can always work it out if there's a working example.

[Refugee]

Have you got a scope?
It needs to have an external sync socket.

[ppppenguin]

Example of L and C in series.

If, at a particular frequency (remember that reactance varies with frequency) the L is +6 ohms and C is -6 ohms they just add giving zero. This is series resonance, when the overall reactance of the tuned circuit falls to zero. If the frequency rises the inductive reactance will rise while the capcitative reactance will fall (towards zero). Hence the overall reactance will rise.

If you try the same trick for a parallel LC circuit and use the usual product divided by sum method you'll end up dividing by zero at the resonant frequency. In other words for an ideal L and C a parallel tuned circuit has infinite reactance at resonance.

[Kalee20]

If you have XL = 10 ohms and XC = 6 ohms, as per your example, in parallel then yes just do as you say.

Product over sum.

So, taking signs into account, that's +10 times -6 (giving -60) divided by (+10 plus -6) or +4. Result is thus -15.

So the resulting reactance is 15 ohms, and being negative, it is capacitive reactance.

[Nowhere-Man]

That's now quite clear. I doubt I'll ever be working out series impedance for coils and capacitors but I just wanted to clear it up. Last night it was going on to explain the KVA aspect with respect to Watts. Just basically saying with capacitors voltage drop was energy being displaced and not the same as actual watts that would power a lamp.

Some people have experimented with capacitors as an alternative to the regular resistance dropper - they tried it with TV sets in the seventies.

[pwdrive]

One of the Thorn/BRC tv chassis used a capacitive dropper in the 1960's, I remember them well, opposite the tag end of the capacitor were a couple of tag strips that had the propensity to turn into a carbon arc, the usual fix was to snip them in halve at the stress points with a pair of side cutters.

Lawrence.

[Nowhere-Man]

One of the Thorn/BRC tv chassis used a capacitive dropper in the 1960's, I remember them well, opposite the tag end of the capacitor were a couple of tag strips that had the propensity to turn into a carbon arc, the usual fix was to snip them in halve at the stress points with a pair of side cutters.

Lawrence.

They were mentioned by Chas Miller in the Valve Radio And Audio Book. He seems to dislike the whole idea because he says one TV brand had to be recalled due to failure issues in the capacitor wattless droppers. Maybe it was the Thorn.

My HAM theory book is really pretty good for theory. It gives all the sums for calculating power factors and reactances.

[pwdrive]

I wasn't particularly fond of that chassis either, here's a link that shows a picture of the said tag strips that appear to have had the anti flash over mod, ie: cut in half:

http://www.vintage-radio.net/forum/showt...p?t=103917

Lawrence.

[Nowhere-Man]

If you have XL = 10 ohms and XC = 6 ohms, as per your example, in parallel then yes just do as you say.

Product over sum.

So, taking signs into account, that's +10 times -6 (giving -60) divided by (+10 plus -6) or +4. Result is thus -15.

So the resulting reactance is 15 ohms, and being negative, it is capacitive reactance.

I'm carrying on with this fairly late at night as my theory could be improved a bit. There's some information on power factors and resistors combined with reactances. It also tells you how to work out the resonant frequency - my calculator helps.