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How is this IC powered?

Started by Bob Engelhardt March 5, 2012
I brought home a fence charger from the dump, hoping that it was there 
due to some simple malfunction that I could fix.  On opening it up, I 
found no blown fuse or other obvious problem.  I found the circuitry 
interesting: it uses a 7-bit ripple counter IC to count 60 Hz pulses, 
with a 64-count triggering an SCR to dump a capacitor into a 
high-voltage xformer.  I.e., the fence is pulsed once every 64/60 
seconds.  The ckt is here:
http://home.comcast.net/~bobengelhardt/ChargerCkt.jpg

The question is: where does the Vdd power come from?  There is a 22u cap 
from the Vdd pin to gnd, but that's it.  The trigger signal is a 5v 60 
Hz square wave & is the only non-gnd connection to the chip. So, does 
this signal somehow, magically, charge the Vdd cap?

And ... it doesn't work because the chip does not count.  The 22u cap 
passes a VOM test, but I didn't scope the Vdd pin.  I concluded that the 
chip is bad.

Thanks,
Bob
On Mon, 05 Mar 2012 21:43:51 -0500, Bob Engelhardt
<bobengelhardt@comcast.net> wrote:

>I brought home a fence charger from the dump, hoping that it was there >due to some simple malfunction that I could fix. On opening it up, I >found no blown fuse or other obvious problem. I found the circuitry >interesting: it uses a 7-bit ripple counter IC to count 60 Hz pulses, >with a 64-count triggering an SCR to dump a capacitor into a >high-voltage xformer. I.e., the fence is pulsed once every 64/60 >seconds. The ckt is here: >http://home.comcast.net/~bobengelhardt/ChargerCkt.jpg > >The question is: where does the Vdd power come from?
From the T input, through the ESD diodes inside the chip. Which leads to this zero-crossing detector circuit: http://dl.dropbox.com/u/53724080/Circuits/ZCD_x.JPG -- John Larkin, President Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
On Mar 5, 10:43=A0pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
> On Mon, 05 Mar 2012 21:43:51 -0500, Bob Engelhardt > > <bobengelha...@comcast.net> wrote: > >I brought home a fence charger from the dump, hoping that it was there > >due to some simple malfunction that I could fix. =A0On opening it up, I > >found no blown fuse or other obvious problem. =A0I found the circuitry > >interesting: it uses a 7-bit ripple counter IC to count 60 Hz pulses, > >with a 64-count triggering an SCR to dump a capacitor into a > >high-voltage xformer. =A0I.e., the fence is pulsed once every 64/60 > >seconds. =A0The ckt is here: > >http://home.comcast.net/~bobengelhardt/ChargerCkt.jpg > > >The question is: where does the Vdd power come from? > > From the T input, through the ESD diodes inside the chip.
Nasty. An external diode from T to Vdd would make that a lot safer. James
On Mon, 5 Mar 2012 19:50:18 -0800 (PST), dagmargoodboat@yahoo.com
wrote:

>On Mar 5, 10:43&nbsp;pm, John Larkin ><jjlar...@highNOTlandTHIStechnologyPART.com> wrote: >> On Mon, 05 Mar 2012 21:43:51 -0500, Bob Engelhardt >> >> <bobengelha...@comcast.net> wrote: >> >I brought home a fence charger from the dump, hoping that it was there >> >due to some simple malfunction that I could fix. &nbsp;On opening it up, I >> >found no blown fuse or other obvious problem. &nbsp;I found the circuitry >> >interesting: it uses a 7-bit ripple counter IC to count 60 Hz pulses, >> >with a 64-count triggering an SCR to dump a capacitor into a >> >high-voltage xformer. &nbsp;I.e., the fence is pulsed once every 64/60 >> >seconds. &nbsp;The ckt is here: >> >http://home.comcast.net/~bobengelhardt/ChargerCkt.jpg >> >> >The question is: where does the Vdd power come from? >> >> From the T input, through the ESD diodes inside the chip. > >Nasty. An external diode from T to Vdd would make that a lot safer. > >James
There's not a lot to admire about that circuit. An analog PUT-type pulser, or one of those trigac things, would be cheaper and more reliable. -- John Larkin, President Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
On Mon, 05 Mar 2012 21:43:51 -0500, Bob Engelhardt wrote:

> I brought home a fence charger from the dump, hoping that it was there > due to some simple malfunction that I could fix. On opening it up, I > found no blown fuse or other obvious problem. I found the circuitry > interesting: it uses a 7-bit ripple counter IC to count 60 Hz pulses, > with a 64-count triggering an SCR to dump a capacitor into a > high-voltage xformer. I.e., the fence is pulsed once every 64/60 > seconds. The ckt is here: > http://home.comcast.net/~bobengelhardt/ChargerCkt.jpg > > The question is: where does the Vdd power come from? There is a 22u cap > from the Vdd pin to gnd, but that's it. The trigger signal is a 5v 60 > Hz square wave & is the only non-gnd connection to the chip. So, does > this signal somehow, magically, charge the Vdd cap? > > And ... it doesn't work because the chip does not count. The 22u cap > passes a VOM test, but I didn't scope the Vdd pin. I concluded that the > chip is bad.
Well, that's inspired. Earl Muntz lives! The square wave on the clock gets rectified by the chip's on-board protection diode, which charges the cap, which powers the chip. It probably also fries the chip over time -- but hey, the thing only needs to work long enough for someone to buy it, right? -- Tim Wescott Control system and signal processing consulting www.wescottdesign.com
Tim Wescott wrote:


> Well, that's inspired. Earl Muntz lives!
You beat me to it!
> > The square wave on the clock gets rectified by the chip's on-board > protection diode, which charges the cap, which powers the chip. > > It probably also fries the chip over time -- but hey, the thing only > needs to work long enough for someone to buy it, right?
No, it probably is just fine, under normal conditions. But.... Fence chargers are hooked up outside the house, and the first time a lightning bolt strikes within half a mile, it probably induces a huge jolt into the line input, and poof! So, I think some protective circuitry would be a good idea. Jon
Tim Wescott wrote:

> On Mon, 05 Mar 2012 21:43:51 -0500, Bob Engelhardt wrote: > > >>I brought home a fence charger from the dump, hoping that it was there >>due to some simple malfunction that I could fix. On opening it up, I >>found no blown fuse or other obvious problem. I found the circuitry >>interesting: it uses a 7-bit ripple counter IC to count 60 Hz pulses, >>with a 64-count triggering an SCR to dump a capacitor into a >>high-voltage xformer. I.e., the fence is pulsed once every 64/60 >>seconds. The ckt is here: >>http://home.comcast.net/~bobengelhardt/ChargerCkt.jpg >> >>The question is: where does the Vdd power come from? There is a 22u cap >>from the Vdd pin to gnd, but that's it. The trigger signal is a 5v 60 >>Hz square wave & is the only non-gnd connection to the chip. So, does >>this signal somehow, magically, charge the Vdd cap? >> >>And ... it doesn't work because the chip does not count. The 22u cap >>passes a VOM test, but I didn't scope the Vdd pin. I concluded that the >>chip is bad. > > > Well, that's inspired. Earl Muntz lives! > > The square wave on the clock gets rectified by the chip's on-board > protection diode, which charges the cap, which powers the chip. > > It probably also fries the chip over time -- but hey, the thing only > needs to work long enough for someone to buy it, right? >
And so it worked :) Jamie
Bob Engelhardt wrote:
> ... fence charger ... > The ckt is here: > http://home.comcast.net/~bobengelhardt/ChargerCkt.jpg > ...
Thanks. I knew there must be an internal path, but had never seen this arrangement and didn't suspect that it was so direct. I must say that even as a very naive circuit-eer it looks really hacky to me. Another question, about component selection. The 100u charge cap is charged through 2 75R 5w resistors. Giving a time constant of 1 ms. But the cap is discharged only every second or so & a much larger time constant would work. Say 100ms, using 7.5k resistors. Then the resistors wouldn't have to be 5 watt-ers. Am I missing something? Thanks again, Bob
On 3/6/2012 6:46 PM, Bob Engelhardt wrote:
> Bob Engelhardt wrote: >> ... fence charger ... >> The ckt is here: >> http://home.comcast.net/~bobengelhardt/ChargerCkt.jpg >> ... > > Thanks. I knew there must be an internal path, but had never seen this > arrangement and didn't suspect that it was so direct. I must say that > even as a very naive circuit-eer it looks really hacky to me. > > Another question, about component selection. The 100u charge cap is > charged through 2 75R 5w resistors. Giving a time constant of 1 ms. But > the cap is discharged only every second or so & a much larger time > constant would work. Say 100ms, using 7.5k resistors. Then the resistors > wouldn't have to be 5 watt-ers. Am I missing something? > > Thanks again, > Bob
My thought is, the voltage on the anode of the SCR must go to 0 volts for it to shut off. Mikek
amdx wrote:

> On 3/6/2012 6:46 PM, Bob Engelhardt wrote: > >> Bob Engelhardt wrote: >> >>> ... fence charger ... >>> The ckt is here: >>> http://home.comcast.net/~bobengelhardt/ChargerCkt.jpg >>> ... >> >> >> Thanks. I knew there must be an internal path, but had never seen this >> arrangement and didn't suspect that it was so direct. I must say that >> even as a very naive circuit-eer it looks really hacky to me. >> >> Another question, about component selection. The 100u charge cap is >> charged through 2 75R 5w resistors. Giving a time constant of 1 ms. But >> the cap is discharged only every second or so & a much larger time >> constant would work. Say 100ms, using 7.5k resistors. Then the resistors >> wouldn't have to be 5 watt-ers. Am I missing something? >> >> Thanks again, >> Bob > > > My thought is, the voltage on the anode of the SCR must go to 0 volts > for it to shut off. > Mikek
yes, that is exactly what it does. The primary coil will saturate and magnetize, mean while, the charge cap in the supply depletes itself to the point where it can not sustain the minimum holding current. The SCR then opens and the main supply then slowly charges up the main cap via the line R. Years ago, I used to live in a lot of farm country, I repaired many fence pulsers. I have also built many for people using uC and serial networks so that one can detect when a line got touched and report back to the server PC. This would alert the farm hand or who ever was incharge of watching over the animals to take a look or at least get a record of reports. Some of the farms around me had a lot of small sectioned areas, each having their own controller. Mother nature really loves those controllers! Jamie