Re: Design check for a telephone ringer
From: R C (yeah.right_at_example.com)
Date: 02/16/05
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Date: Tue, 15 Feb 2005 21:53:26 -0500
<mroberds@worldnet.att.net> wrote in message
news:8jgQd.6726$zs.5926@okepread04...
>R C <yeah.right@example.com> wrote:
>><mroberds@worldnet.att.net> wrote in message
>>news:444Pd.486$zs.51@okepread04...
>>> inductive spike
>>
>>25 V max open circuit with the heavy supply; ~20 V open circuit with
>>standard supply. Less with a phone connected (22/17V) or shorted.
>
> How'd you measure this? Scope or meter? If scope, OK, but if meter,
> there might be higher voltages that the meter isn't catching...
Scope.
>>The transformer is not very beefy.. I'm not sure of the actual specs, but
>>it's less than half an amp[3].
>
> http://repairfaq.ece.drexel.edu/sam/aapsfaq.htm#adpsdpv has info on how
> to work it out, if you'd like.
Describes how to measure the current ratings.. but not the ratio. However,
there are other sites that do.. I'll look into it, but I don't have a variac
handy.
>>> What happens if the output (to phone) is shorted?
>>
>>It draws a little more current, but not much. Max draw while ringing is 2A
>>(roughly 50% of the time.. the other 50% is at 1 amp.)
>
> 1 A sounds like a lot for just a microprocessor sitting there, but then
> again I'm not that familiar with the Tiny26.
2/1 A is during ring cycle only. Total current during idle is roughly 20 mA
including the LED.. and I'm not using the sleep capabilities of the tiny26
at all. I suppose I could have rephrased that a bit.
The average draw while ringing is about 1.5 A, with a plateau at 2 and
valley at 1. The ring cycle is 2 seconds on, 4 seconds off, which gives
overall average of 500 mA.. which means I need to current limit the mosfets.
>>Voltage sags pretty far down during ringing.. all the way down to 7V with
>>the weaker supply. If I can dig up one or two more big caps it may help
>>smooth it a bit more.
>
> Look up the October 1989 "Hardware Hacker" column by Don Lancaster
> (available in PDF at his Web site, tinaja.com , the last time I looked)
> for a neat way to work out power supply filter cap sizes.
Thanks, I'll look into it. To be honest, I may be better off
current-limiting the mosfets. The supplies are different designs.. the
'Beefy' one is 12V, regulated, and rated for 1A continuous. The standard one
is 400 mA, unregulated.
Hmm.. doesn't look like an exact match as I'm feeding off an already fixed
wall supply, but I'll read the whole thing. But all very interesting.. I'll
have to read some more tonight.
>>Open circuit voltage is ~400 V PtP 12V beefy. 12V standard is 325 V. With
>>a 0.6 REN load it's about 350/200V PtP. Note that the 70-90 V nominal AC
>>is the RMS value.
>
> Hmm... 86 V RMS is about 86 x 1.414 or 121 V PtP for a sine wave or
> 86 V PtP for a square wave with 50% duty cycle. Going by your loaded
> value, you're delivering somewhere between 247 and 350 V RMS - this seems
> high to me.
It is a bit high.. esp. as I'm not testing it with the stage phone, which is
a lot higher draw (mechanical, not piezo). The 200V PtP value is the
important value, as it's the one with the weaker supply that I'm actually
using with the circuit.
Also, this is not a true square wave output.. it looks very much like a
square wave and a low-pass.
>>In the field, you also see actual voltages of about 50-150 V RMS.
>
> I haven't stuck a meter on that many phone lines, but the ones I have
> have been 90 V RMS or lower. But these were all CO-served lines; PBX
> extensions might be higher.
That's from my notes gathered from who knows where. It may be a bit dated.
>>([2] This is a random transformer I pulled from a broken cassette player;
>>it might be better off being replaced by a known 12-0-12/120 transformer.)
>
> Agreed, or at least test it per the link above to know what it can do.
It may be a 6-0-6 or 9-0-9. I may try running the circuit at 9V if I
current-limit the mosfets to preserve the headroom on the 5V regulator.
Hmm.. probably a single beefy resistor on the center tap, or two smaller
ones between the mosfets and the transformer. Have to check the parts bin.
R C
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