strange use of 78** regulators or not ?

I do some repair work for my friends now and then and have a Furman Ar-

Original issue was the Triac and opto being shorted for the main primary side so it wouldn't shut the outputs down on extrame voltages. Anyways, while poking around and generating a reversed schematic for it I found it strange that it has a small low current control transformer supplying the volt sensing circuit which also supplies the 7805 reg, this same supply also supports a 7815 and 7915 to supplies a TLO84 chip to be used as a comparator for over volts from what I can see..

What is strange is there is only 10-12DC at the bridge and yes the caps are ok. this isn't enough to property operate the the regs.

After looking up the part number of the transformer it appears that it has a 230V primary but the unit is slated for 120V service? So since this transformer looks like it was made to order I thought maybe someone put the wrong one in and its been runing this way? I took a temp transformer that would produce the required output for the 7815 and 7915 post regs.

The strange part of this was that, although the regs were then ok, the unit went into a EVS mode (extreme shut down mode) which it does for a short time when it normally powers up, but this time never came out of it. It looks like it was designed to actually operate below the post regs ratings in normal input voltages and using the regs as a limit not a fixed output. Is this propery use of these regs ?

Amazing catch. Thanks.

it looks like the LT1057/LT1058 goes into oscillation at the negative peak. I'm not so sure that is a good idea.

They use a 230VAC transformer at the 120 VAC operating voltage to eliminate linearity and possibly saturation problems at line fault overvoltages. Put it back. Last time I checked, the 7800/7900 maintain their fault protectio n features at anything above 6.3V input. The power supply for the TL opamp does not require regulation, there is such a thing as PSRR, so no problem w ith the regulators not regulating so well.

There you go.

Yes.

the datasheet defines 7812 dropout voltage as 2.5V max. Opamps tend to not care much about mains derived ripple.

They're rated for anything upto 3 or 4v within psu rails. Many apps just won't take them into 'inversion.'

I don't think rejigging the circuit is a good idea until one understands what's being done already. I have encountered commercial products that were just not designed to work properly, but not often.

NT

You must mean the 7805. He posted "same supply also supports a 7815 and

Until there is "fuzz" on that from nearby dimmers and such, that made it past the caps because of their ESR. Then they often also use the rails to bias something and that's where it really becomes ugly.

If an overvoltage curb was needed the proper way to do it would be, for example, a depletion mode FET.

Quote page 9 "Most industry standard JFET input single, dual and quad op

I have seen really gross ones. The topper was a strip chart printer where the reset function consisted on shorting out the 5V rail. This was nowhere to be found in the datasheet. We planned to install them in large ultrasound machines where the 5V rail featured a sustained 100 amps ... *KABLAMMO* ... internal meltdown.

That manufacturer landed on our blacklist, for gross incompetence.

Gross incompetence knows no boundaries. Here's what I've seen in the power circuit of a set of desktop speakers. Note how the switch sits between 2 capacitors:

T ---- SW + )|( | | | | AC )|( | | = C1 C2 = DC )|( | | | | - ----

The caps C1 and C2 were on the order of 1000 uF (at 16 V or so) each, and were of identical type (pretty much maximizing the spark energy).

The switch had a sliding foil-like piece of brass. It did not last.

In your example, ( 1000uF & 16V ) about 0.1 Joules.

A famous brand tube amp I saw had its standby toggle switch wired so that it paralleled connected a charged 100uF electro cap with an uncharged one when operated. The voltage level was over 450V DC. Equates to 10 Joules.

The switch made a loud bang, every time this was done.

..... Phil

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nate linearity and possibly saturation problems at line fault overvoltages. Put it back. Last time I checked, the 7800/7900 maintain their fault prote ction features at anything above 6.3V input. The power supply for the TL op amp does not require regulation, there is such a thing as PSRR, so no probl em with the regulators not regulating so well.

"...maintain their fault protection features at anything above 6.3V input.. ", that means stuff like current limit and overtemperature foldback. You ca n't seriously think anyone would claim the 7815 maintains 15V regulation at 6.3V input???

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