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Old 10th July 2013, 08:00 PM   #2301
Salas is offline Salas  Greece
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Quote:
Originally Posted by roger57 View Post
Salas,
Still a bit curious as to why the math doesn't work out here: I measured the exact drop across the green LED's (2.07V each)
Now the drop across the 4.7R is 2.51V


(2.07 x4) = 8.28; 8.28-2.51 = 5.77V
The curve indicates for Vgs at 5.7V with Ta=25C to be about 1A. (Clearly; more to the numbers that this calculation?)

It's interesting to note the spreadsheet indicates a total current of 650mA output using a 4.7R resistor, for 5.3V and 450mA load.

All circuit parts as recommended.

Can you provide some insight? How to account for this difference? I can understand variations of perhaps 10% due to parts, but it seems a bit much in this case.

Thanks.
If you could curve trace that individual MOSFET taking into account the VDS (equals vin-vout in this case) to test at, and the junction temperature Tj in your particular set up, it would surely show the deviation parameters that make sense.
Making the basic VRset/Rset verification first when assembly is over with a light load resistor, shows the truth without all the above.
Tj in particular can play a big part when targeting over 400mA for an IRF9610. Beyond batch deviations etc. See how it can vary for 0.6A between extreme temp cases at -6V VGS as an example.
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Old 11th July 2013, 01:03 AM   #2302
roger57 is offline roger57  Canada
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Quote:
Originally Posted by Salas View Post
If you could curve trace that individual MOSFET taking into account the VDS (equals vin-vout in this case) to test at, and the junction temperature Tj in your particular set up, it would surely show the deviation parameters that make sense.
Making the basic VRset/Rset verification first when assembly is over with a light load resistor, shows the truth without all the above.
Tj in particular can play a big part when targeting over 400mA for an IRF9610. Beyond batch deviations etc. See how it can vary for 0.6A between extreme temp cases at -6V VGS as an example.
Thanks Salas; appreciate the insight.
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Old 11th July 2013, 01:15 AM   #2303
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Bummer, no go. LEDs don't light up still, and I measure 1.5v on the outputs. I do notice some voltage drop on the inputs, from 15 down to ~9 or so.

I'm just going to build another board, as I started this one a way back and only recently got back to it so maybe I messed something up along the way.

R101: 12ohm
R103: 2Kohm
R105: 1K trimmer

Quote:
Originally Posted by yammy1688 View Post
I didn't realize the unpopulated LEDs needed to be jumpered.

What resistor value should I select for the dummy load? I'm looking to simulate 90ma, am not quite making sense of the calculation.

I end up with 1.3k, but that seems way too high compared others. I think 30something Ohm seems more appropriate?

I'll try that tonight. Thanks!
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Old 11th July 2013, 09:39 AM   #2304
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Quote:
Originally Posted by yammy1688 View Post
Bummer, no go. LEDs don't light up still, and I measure 1.5v on the outputs. I do notice some voltage drop on the inputs, from 15 down to ~9 or so.

I'm just going to build another board, as I started this one a way back and only recently got back to it so maybe I messed something up along the way.

R101: 12ohm
R103: 2Kohm
R105: 1K trimmer
If you start a new reg first to complete the pcb populate & check only the CCS, if OK continue to complete the rest of the board.
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Old 11th July 2013, 11:49 AM   #2305
Salas is offline Salas  Greece
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It can only be a wrong orientation for an LED or a dead one. I would check that each one can light up in the PCB noted direction with a 9V battery or a capable DMM in diode mode.
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Old 11th July 2013, 05:19 PM   #2306
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Hi, hopefully someone can point me in the right direction. Ive built 3 salas shunts v1.1, one BJT based for buffalo dac which works perfectly. The other two IRF base shunts not working as desired: looking for circa 290ma for legato IV inc. 100ma shunt @ 12-15v. Transformer is 2x15v 30va. Measured dc after shunt rectifier as ~20.5v] Both IRF have same problem - With 75R dummy load; V can be set to 12-15v seems ok but with real load; legato 3.1 IV stage, no red leds lit, output V drop to near 2v. Across R201/301 (8r2 /3w) measures 2.5v. So. Measuring the negative shunt current out to dummy load (75r) I get 144ma at 13v (too low to power the legato and hence no leds and V drop?) and also this rises with increase in V to 169ma at 14.5v. Can anything be concluded from this? I have also tested with CCS separated, shorted current out is 312ma 2r load 310ma 8r2 load 302ma 75r load 133ma
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Old 11th July 2013, 07:54 PM   #2307
Salas is offline Salas  Greece
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2.5V/8R2=305mA. That is their CCS. 312mA shorted measurement ties with that by only 2.3% experimental error. 20-30mA from that should go to the reg itself as a minimum.
If legato IV draws about 290mA then the regs are almost choked. Use 5.6R 3W-5W. Set Vo so to have 7V DCVin-Vout minimum. It helps the CCS rigidity.
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Old 11th July 2013, 08:38 PM   #2308
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Thanks for the reply Salas. Apologies - to be clearer in description: my legato requires +190ma and -205ma (based on twisted pear placid BP HD measurements). So 290ma output would allow 85-100ma of healthy shunt. Ive tried with 12v 13v 14v etc for output, it seems to not make much difference, except for increasing current with V !? With the load over a certain amount - I guess somewhere nearer to 75r, the current drops off below functioning output. 12V out should allow decent V drop from 20.5vdc in? but from measuring, it shows the lowest current (75r load 133ma).

Last edited by lucaslucas; 11th July 2013 at 08:41 PM.
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Old 12th July 2013, 02:37 AM   #2309
Salas is offline Salas  Greece
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That's weird then. What value are the gate stoppers? Can you confirm that?
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Old 12th July 2013, 04:37 AM   #2310
nezbleu is offline nezbleu  Canada
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Your dummy load uses, depending on voltage, 160-200mA per rail, and you say the regulator behaves properly with that load. The actual load is spec'ed at 190mA pos and 205mA neg, which is slightly unusual for the negative rail to draw more current. Have you tried dummy loads that more accurately mimic that expected actual load? That is, can the regulator drive a resistive load that draws >200mA? Can it do so at your target voltage range? How are mosfet temperatures when doing so? I would be especially interested in the negative supply, as I found I needed a much lower current setting resistor on the negative rail to get the same current, certainly lower than 8R2. To have a -15V regulated output at about 280mA I needed a current setting resistor close to 2R5.

If the regulator can drive a resistive load in excess of the expected load current, then you need to look at the circuit you are supplying. Could it be causing the regulator to oscillate? Again, what happens to device temperatures? Does the circuit you are driving actually draw that current? Can you supply it from a simple 3-terminal regulator to measure the current draw?

When you say "Measuring the negative shunt current out to dummy load (75r) I get 144ma at 13v", what current are you measuring exactly? (13 volts across 75 ohms means 173mA, so where are you measuring 144mA?)

I'm just trying to understand.
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