Are my LM 723 shorted

[seanchan00]

Hi Guys,

I have 2 shunt regulators made to Chris Greacen's design from homepower ref below:

http://www.homepower.com/magazine/downloads_homebrews.cfm

A month ago while hooking up I accidently reversed the polarity of the regulator and since then it has not worked. When I hook up correct both LEDs come on no matter how high I set the Voltage for shunting. Yesterday while trying to find where the bug is by comparing my 2 sets together hooked to the same battery I seemed to have also shorted my good regulator by accidently touching pin 9 and 10 with my multimeter probe while the power is on. Have I shorted both the LM 723 ? When I accidently shorted pin 9 & 10 the second led comes on for a second to 2 then goes off again. Now both sets seems wonky and I can't set the Voltage to shunt as in the spoilt one I was trying to debug.

SeanChan.

[Nando]

The only solution change the LM723.

Do you have a voltmeter ?.

tell what type of IC is the LM, 14 pin dip or round.

I could give you some pin voltages to read.

Nando

[Flux]

If you reversed the supply you will almost certainly have killed the 555 as well.

I would change the 723 and 555 and try again. Very few IC's will stand a supply reversal and even if they seem to be working they may do strange things after such an accident.

Flux

[alancorey]

I think the 1N4001 off the switch is there to protect the chips if the regulator's hooked up backwards. I don't know if any current would get through the FET or not.

Sounds like new chips are in order. If they're in DIP packages put in some sockets for them, if you didn't when you built. Maybe keep some spare chips on hand? They cost a lot less than shipping on the order will.

  Alan

[seanchan00]

Thanks Guys,

Lm 723 is pin dip like a 14 legged spider. I was trying to measure voltages of the pins 1 to 14 when I shorted between pin 9 and 10 then the shunt came on for a second. After that the good one also seems to go haywire.

I have ordered the two LM 723 ICs and have to wait a week for them to arrive. I have spare 555s. Will change them both and post the results.

I was hoping the IN4001 diode at the positve end of the circuit could protect a reverse polarity accident as the current should only flow one way. Still can't see how a reverse polarity could burn the ICs as there is no completed circuit going the opposite way.

SeanChan.

[Ungrounded Lightning Rod]

I was hoping the IN4001 diode at the positve end of the circuit could protect a reverse polarity accident as the current should only flow one way. Still can't see how a reverse polarity could burn the ICs as there is no completed circuit going the opposite way.

Yeah, that's what it's for.

But there is very little load on it:  Looks like about 7k.  Diodes do leak, and with no load to eat the leakage there may have been enough reverse voltage on the power rail to drive the chips into conduction.  (Also:  There might be a path through the FET - like a protective diode to the gate or inadequate gate-drain voltage rating.  Check the specs.  Also: the 1N4001 may be defective or you may have a solder bridge on the board that shorted it out.  Ohm-meter it.)

I'd hang another diode pointed the other way between the post-protective-diode power rail and ground.  That may keep reverse leakage from frying anything.

And I'd ohm-out the FET between the gate terminal and the others, with the meter in both directions to hunt for diodes.

[Flux]

I don't see any reason for the failure on reversed supply if the 1N4001 diode is working properly. There shouldn't be enough reverse leakage to damage anything if it is from a reputable supplier ( not sure I trust some backyard suppliers).

The body diode of the mosfet and the zener clamp between them should have clamped reverse volts across the fet to under .8v and the gate clamps will be about 20v so nothing should feed back.

Double check that diode. Shorting between pins may have killed that IC ( I shall have to look and see what pin 10 does, that circuit only uses the reference and comparator of that chip).

If you add the diode that ULR suggests then you will need a low current fuse in the control circuit supply and it will blow if you reverse the supply.

Flux

[Ungrounded Lightning Rod]

If you add the diode that ULR suggests then you will need a low current fuse in the control circuit supply and it will blow if you reverse the supply.

A fuse is a good idea regardless.

But the place I suggested the clamp diode is after the existing 1N1004.  The fuse will only blow on reverse-polarity if the first diode is shorted or installed backward.

[Flux]

Yes you are right. I forgot that when I used that method of protection for amplifiers I wasn't using the series diode ( I didn't want the volt drop).

Flux

[seanchan00]

Hi U L Rod,

I'll appreciate further clarification of the following pointers you suggested.

"And I'd ohm-out the FET between the gate terminal and the others, with the meter in both directions to hunt for diodes."

I take it to mean test for conduction between gate to source and drain. Is that correct and there should be no conduction.

"I'd hang another diode pointed the other way between the post-protective-diode power rail and ground.  That may keep reverse leakage from frying anything."

I read it to mean connect a diode directly between the post IN4001 positive rail to ground rail pointed reverse to current IN4001 so any reverse polarity will flow in parallel to the circuit sparing the circuit?

SeanChan.

[seanchan00]

Hi U L Rod,

I'll appreciate further clarification of the following pointers you suggested.

"And I'd ohm-out the FET between the gate terminal and the others, with the meter in both directions to hunt for diodes."

I take it to mean test for conduction between gate to source and drain. Is that correct and there should be no conduction.

"I'd hang another diode pointed the other way between the post-protective-diode power rail and ground.  That may keep reverse leakage from frying anything."

I read it to mean connect a diode directly between the post IN4001 positive rail to ground rail pointed reverse to current IN4001 so any reverse polarity will flow in parallel to the circuit sparing the circuit?

SeanChan.