# How is this IC powered?

Started by March 5, 2012
```amdx wrote:
> On 3/6/2012 6:46 PM, Bob Engelhardt wrote:
>> 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?

> My thought is, the voltage on the anode of the SCR must go to 0 volts
> for it to shut off.

I see what you're saying about the SCR turning off, but I don't see how
the use of the small (75R) resistors helps that.  Wouldn't _larger_
input resistors help the SCR turn off (reduce source current)?

Bob

BTW - if there's a newsgroup more suited for my naive questions, I'll
```
```On Wednesday, March 7, 2012 12:34:08 AM UTC-5, Bob Engelhardt wrote:
> amdx wrote:
> > On 3/6/2012 6:46 PM, Bob Engelhardt wrote:
> >> Another question, about component selection. The 100u charge cap is
> >> charged through 2 75R 5w resistors. Giving a time constant of 1 ms. Bu=
t
> >> the cap is discharged only every second or so & a much larger time
> >> constant would work. Say 100ms, using 7.5k resistors. Then the resisto=
rs
> >> wouldn't have to be 5 watt-ers. Am I missing something?
>=20
> > My thought is, the voltage on the anode of the SCR must go to 0 volts
> > for it to shut off.
>=20
> I see what you're saying about the SCR turning off, but I don't see how=
=20
> the use of the small (75R) resistors helps that.  Wouldn't _larger_=20
> input resistors help the SCR turn off (reduce source current)?
>=20
> Bob
>=20
> BTW - if there's a newsgroup more suited for my naive questions, I'll=20

The output transformer in conjunction with the series line resistors looks =
like a parallel RLC to the SCR. If there is no resistive load on the fence,=
i.e. a body of some kind to discharge through and damp the reactance, the =
circuit will ring in such a way that the transformer magnetizing inductance=
current will keep the SCR turned on. In that case, smaller line resistors =
will damp out the oscillations faster than large ones. There are UL, CSA, a=
nd other international standards regulating the output of electric fence co=
ntrollers, and they do set limits on the allowable duration of transients c=
oupled onto the fence. This is why they're using smallish 75R line resistor=
s.
```
```bloggs.fredbloggs.fred@gmail.com wrote:

> The output transformer in conjunction with the series line resistors looks like a parallel RLC to the SCR.
...
This is why they're using smallish 75R line resistors.

Thanks.  I knew they wouldn't be using 5W resistors without a reason.

Bob
```
```On Tue, 06 Mar 2012 19:46:27 -0500, Bob Engelhardt
<bobengelhardt@comcast.net> 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=20
>arrangement and didn't suspect that it was so direct.  I must say that=20
>even as a very naive circuit-eer it looks really hacky to me.
>
>Another question, about component selection.  The 100u charge cap is=20
>charged through 2 75R 5w resistors.  Giving a time constant of 1 ms.=20
>But the cap is discharged only every second or so & a much larger time=20
>constant would work.  Say 100ms, using 7.5k resistors.  Then the=20
>resistors wouldn't have to be 5 watt-ers.  Am I missing something?
>
>Thanks again,
>Bob

The 75 ohm resistors are there for the MOV.  That way it won't blow up on
the first surge that hits the unit.

?-)
```
```On Saturday, March 10, 2012 10:30:09 AM UTC-5, josephkk wrote:
> On Tue, 06 Mar 2012 19:46:27 -0500, Bob Engelhardt
> <bobengelhardt@comcast.net> 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
>
> The 75 ohm resistors are there for the MOV.  That way it won't blow up on
> the first surge that hits the unit.
>
> ?-)

The resistors don't do anything to protect the MOV against the 8x20u current wave transient which is the most common test for protection against lightning induced surge. The differential 150R looks more like a line match from that perspective.
```