My oscillators are a little unstable since I added a couple more modules, so I’d like to check out how much current I’m pulling. I’m using one of Christian’s simple DIY power supplies split via some home made buss boards into 3 VCOs, an LFO and an Electric Druid noise circuit I made from the data sheet circuit.
I’d like to see how much current I’m using generally but also I’m wondering if I’ve got a short somewhere in the noise module as the output is quite low - mic level (it’s buffered using an LM072)
(I will of course unplug the noise module and see if the instability in the oscillators goes away as a starting investigation).
I was wondering if a clamp amp meter around ribbon cables would work - I wasn’t sure of the effects of having a bipolar supply would cancel the reading out??
TLDR: can you use a clamp amp meter on the ribbon cables carrying the bipolar supply to synth modules?
As for measuring current draw, you can measure the voltage drop across a low value series resistor and use Ohm’s Law. If there are 10R resistors in the module you can use those, then for instance if you measure a 120 mV voltage drop that means 12 mA current on that rail for that module. If there aren’t, or if it’s inconvenient to reach them in the case, you can do what I did and make a board with headers and 10R resistors on it:
Wikipedia:
A clamp meter measures the vector sum of the currents flowing in all the conductors passing through the probe, which depends on the phase relationship of the currents. Only one conductor is normally passed through the probe. In particular if the clamp is closed around a two-conductor cable carrying power to equipment, the same current flows down one conductor and up the other; the meter correctly reads a net current of zero.
and
A relatively recent development is a multi-conductor clamp meter with several sensor coils around the jaws of the clamp. This could be clamped around standard two- or three-conductor single-phase cables to provide a readout of the current flowing through the load,[3] with no need to separate the conductors.
But I don’t think one of these can cope with bipolar supply on a ribbon cable.
And that’s assuming 100% efficient voltage regulators in your PSU, whereas 50% is closer to realistic. So then it’s 250 mA on both rails, running flat out; 125 mA per rail. More realistically probably under 100 mA per rail before hitting problems.
I have an 36 18 VA adapter but I’ve seen more like 80 VA recommended for 1.5 A per rail.
Thank you, this is very thorough and I appreciate it
Nice idea to make a junction to give a test point for current (via ohms law!).
I figured the vector sum of the current would be the result - not especially useful for opposite currents!
I’m waaaay over 100mA :D. I hadn’t really thought about inefficiencies in the system. I did struggle to find a 12VAC wallwart but I’ll have another look for something more appropriate. I can always repurpose the existing one for a testing rig where I’ll only be running one module at a time most likely.
From my experience, the current a midule draws should be very small - below 10mA. The most current in these modules is typically drawn by leds. I would expect if you had more than 10 modules that problems with piwer may occur. Maybe check module by module where the most currrent flows. You might have 4 modules consume 10mA in total, and another one with a leak consume the other 90
That’s… not typical. If you look at the post linked above by @Dud only one LMNC module draws less than 10 mA on the +12 V rail and only three on -12 V. Similarly on ModularGrid, the few modules that actually have current draw listed anyway. And similarly with the modules in my synth as I’ve measured them.
Typical supply current per amplifier for a TL07x is 1.4 mA, says the datasheet, so two TL074s right there is more than 10 mA.