Are these voltage measurements feasible?? - Page 1
Forum section: electronics / beginners

author: Chris Wilson
author_rank: Super Contributor
timestamp: September 25, 2014, 02:11:19 pm
content: Maybe I am doing something wrong, or misinterpreting my readings, would someone with more experience please look at this OP AMP circuit and explain my results please? Localised image of the circuit, and full schematic hopefully attached, many thanks. The localised image is from the first page of the full schematic linked to below, IC3 area, AC balance control.http://www.chriswilson.tv/schematic.pdf

author: jlmoon
author_rank: Unknown
timestamp: September 25, 2014, 03:24:16 pm
content: Need more information: What do you read on the Anode side of D15?

author: Kevin.D
author_rank: Frequent Contributor
timestamp: September 25, 2014, 03:28:55 pm
content: yeah your readings are feasable .The voltages shown above on the schematic (ac +13.5 dc -15) look like they are supposed to refer to the anode side of D15 ,since this is derived from IC3 on page 2 of the schematic (this is a simple latching switch with hysteresis). Your readings seem suggest no ac/dc switching signal from IC3 (it's output is probably stuck low) ,so if you've got a fault that is preventing you from switching ac/dc start here and work back toward cntrl input.Regards

author: Chris Wilson
author_rank: Super Contributor
timestamp: September 25, 2014, 03:50:13 pm
content: I'll have put the box back in the welder and check the anode voltage, I'll report back. The issue is the AC balance seems skewed, it welds fine on DC, but on AC the AC balance control knob needs to be fully clockwise to even get any sort of sensible weld. I had the choke rewound after an internal short. Other devices blew, I am trying to be certain it's *NOT* the rewound choke that's the issue, and it's still a control box issue. It could be the control of the power thyristors in AC mode is skewed. Would the 30% to 70% control voltages still be correct and controllable if the input to pin 1 was *IN*correct? Thanks for looking at this for me!

author: Kevin.D
author_rank: Frequent Contributor
timestamp: September 25, 2014, 04:26:23 pm
content: Quote from: Chris Wilson on September 25, 2014, 03:50:13 pmYes that makes sense if the anode of D15 is stuck at -15 V (the dc positition )then your not going to be able to switch into ac and have any cntrl .Quote from: Chris Wilson on September 25, 2014, 03:50:13 pmYou mean the output of opamp IC3 (A) .No I don't think it would , looking at this balance opamp circuit it looks like when it's in dc (input to pin 3 ~gnd)then it's output is also going to be close to gnd . (the only current through the input's is going to be In bias current) .So I am surprised if it's so as I would have said the 30% and 70% output cntrl voltages refered to on the schematic are only meant to be present in ac mode and this balance would have no effect in dc mode.

author: Chris Wilson
author_rank: Super Contributor
timestamp: September 25, 2014, 04:56:26 pm
content: The 30 to 70% AC balance voltages cited on the schematic change as expected when turning the balance knob. They do not change in DC mode. I am still perplexed as to why the pin 1 voltage is unchanging in AC or DC mode, it stays in the mV range, unmoving at all. I'll check the anode voltage on D15 tomorrow, thanks Kevin and everyone.

author: Chris Wilson
author_rank: Super Contributor
timestamp: September 27, 2014, 12:21:05 pm
content: I tagged test leads onto both the anode and cathode of D15, and measuring to the ground point shown in schematic excerpt I see, in AC welding mode -1.2mV at the anode. The AC balance control has no effect on this voltage. I see the same voltage in DC mode. The balance control also has no effect.,On the cathode of D15 to my ground point I see +56.3mV in both AC and DC modes. The balance control again has no effect on these voltages. The schematic shows there should be either +13.5V in AC mode, and -15V in DC mode.....On IC3 pin 1, I still see -3.8mV in DC mode, unchanged by the AC balance control position.In AC mode pin 1 of IC3 shows a voltage swing of +4V to -14.4V depending on the AC balance knob position. That is correct according to the schematic.Comments very welcome, thanks for reading!

author: Kevin.D
author_rank: Frequent Contributor
timestamp: September 27, 2014, 06:02:59 pm
content: What your saying here seems contradictory .If as you say the input's to IC3 A (the cathode of D15) are not changing when you select ac or dc mode (u measure a constant 56 mV ) .But the output of this opamp (pin 1) still adjust in ac mode correctly . (and so it would in that event function normally anyway ).Then when you switch into dc mode (still 52mV at that D15 cathode node)then pin 1 will no longer adjust using balance cntrl (which is also what we would expect for normal function in dc.). So in effect your saying this cntrl's output voltages seems to be working fine but the input's to it are broken . Thats a bit contradictory reallyWhat also makes it odd is that given the schematic you posted that output behaviour doesn't seem to be possible with that D15 node stuck at 56mV .When balance is adjusted to give output (pin 1) -15V in ac mode what is the V at pin 2,3 and the balance cntrl output pin ? What Does the bottom of this balance cntrl connect to ?(I was thinking the small sloping line symbol here refered to a local case gnd ?) .Check OP IC3B pins 6,5,7 ,given what it is , ic3B is meant to be either full on or full off ,it goes back to M1 connector what goes into this conector ? is this supposed to feed a simple dc level then a continuous square wave in ac mode ?. Is there a part of the schematics missing with your dc mode switching..etc on it ?Given the output's from this cntrl seem to be in normal spec what would we hoping to achieve ? except to solve a puzzle .Regards

author: Chris Wilson
author_rank: Super Contributor
timestamp: September 27, 2014, 06:33:50 pm
content: I hold my hands up , with apologies for wasting your time. The pcb is faded and it's hard to see the screen print. I have been measuring from the wrong diode!! I wasn't on D15! Now I see where D15 REALLY is I get these readings. To the ground point indicated (earthy end of R35) to the anode of D15 I see, in AC mode, +13.5V , and in DC mode -15VIn AC mode from ground to the cathode I see +0.2mV in DC mode, and +12.8V in AC mode. The schematic seems to indicate the voltages should be measured at the cathode though..., in which case they are incorrect. Measured from the anode they appear correct. Sorry for my error ID'ing D15 beforeThe schematic for the rest of machine is athttp://www.chriswilson.tv/schematic.jpgif it is too big to show below.There's a further document explaining functionality, if that too is not appended please seehttp://www.chriswilson.tv/function-description.jpg