measuring current using 1ohm resistor. How?

[unclejed613]

31ma is correct, and the reg is using 2ma. does the voltage at the input to the regulator sag very much when you load the regulator, or does it stay almost the same?

 

[riotubes]

Thanks UncleJed for hanging in there with me. Yes, the voltage does sag depending on whether or not the reg is loaded:

No load
178 VDC on input of reg
70 VDC on output of reg

With Load
143 VDC on input of reg
58 VDC on output of reg

 

[AndrewT]

what type of regulator are you measuring, Series or Shunt?

 

[riotubes]

It's a series pass regulator. The schematic is in post #15 with a description linked underneath. Thanks!

 

[AndrewT]

try a variety of lower current loads.
Measure to see if regulation is good at 1mA, 2mA, 5mA, 10mA, 20mA of output.
You may find that the zero output current is not properly regulated.

 

[riotubes]

Thanks Andrew. So to measure at lower current loads I would use Ohm's Law to find the proper dummy load.

For instance, I'm using a 2K resistor to proxy 2.121K ohm (70V divided by 33mA = 2,121 ohm)

For 1mA dummy load, I would use 70V / .001 mA = 70K ohm power resistor?

So for 5mA dummy load, I would use a 70V / .005 mA = 14,000 ohm resistor?

I think I got it. Thanks

 

[AndrewT]

yes.
I think you may find that the regulator requires a minimum load to regulate and you must ensure that this minimum current is always taken by your load.
ClassA amplifiers, in general, do not draw constant current.

 

[riotubes]

I will vary the load and take measurements.

I guess what I don't understand is that I can easily dial the pot on the reg to precisely 70V (so it can take 178V on the input and reg to 70V) with NO load at all.

 

[riotubes]

Recall that I am trying to troubleshoot a discrete regulator. Actually, trying to determine if the problem lies with the reg or transformer. Per Andrew's suggestion in the post above, I measured the mV through a 10 ohm 1% resistor in order to calc Current both before the regulator and after it. Andrew suggested I vary the load (ohms) and take measurements and report back. For a proxy load I used various sized resistors. Recall that the real load is an active IV amplifer circuit that requires 70V and about 33mA running through it.

Because I took measurements with various resistors that result in mA from 1mA all the way up to about 30mA, I've created a table attached as the PDF. Note that I can dial in 70V only when using >3900 ohm resistor. When going to lower ohms, I can't get enough voltage.

I welcome and appreciate your thoughts on what to do next, and whether the problem is the reg or traffo (triad n68x 50VA)

 

[unclejed613]

your input voltage is sagging. with your "reference" voltage being derived as a percentage of the input voltage rather than a fixed constant, this causes your output voltage to sag as well. you need a fixed reference not a floating one. you should have a zener in the source circuit of Q5 to maintain a fixed reference voltage for Q5 to compare against. it would be best if you eliminate the two resistors altogether and replace them with a zener that is about 5 volts lower than the voltage you want at the output, and bypass the zener with a 10uf cap to keep zener noise out of your supply rail. if you have difficulty finding a 65V zener, you can put zeners in series to get the desired voltage (2x 33V zeners will give you 66V) make sure the zener wattage is high enough that they don't self-destruct. the current through that leg is about 4mA, so a 1/2W or 1W zener will suffice (go with the 1W for long term reliability.