Virtual ground and shielding.

[rgawron]

I want to build a device (amplifier for a photodiode), that would need dual power supply +3V (for itself) and -20V (bias voltage to polarise the diode), current drawn would be very small, couple of mA for the amplifier, almost nothing for a bias voltage. The output will be connected by the BNC to the scope. There will be a grounded metal enclosure to reduce noise electromagnetic interference.

I don't want to use dual power supply (because I don't have one), batteries, or DC/DC converter (I want to avoid potential sources of a noise, because the signal is small).

I came with this:
- I will use power supply with not grounded output
- set its output to roughly 25V, my device will have one socket to plug power in, inside the device, I will connect in parallel two linear stabilisers:  one set to output voltage 20V, one set to output voltage 23V.
- I will use 20V rail as a mass, 0V as a -20V rail, and 23V rail as a 3V rail. Then according to my new mass I will have +3V and -20V.
- the metal enclosure will be connected to the new mass as well as the mass of the BNC for the osciloscope.
- differences between input and output voltages will be small so the stabiliseers won't heat up


I know that if some day I will use grounded  output from a power supply and connect a scope, it will be bad, but as long as I won't, should it be ok? In addition, the voltages are too low to be dangerous (metal enclosure that can be touched) I think so with being careful I think it is acceptable.

I think this should work, or did I make a mistake somewhere?

I won't use neither LM317 nor 80xx family here.

Thanks for sharing your advices.

[bson]

inside the device, I will connect in parallel two linear stabilisers:  one set to output voltage 20V, one set to output voltage 23V.
- I will use 20V rail as a mass, 0V as a -20V rail, and 23V rail as a 3V rail.

The 20V rail won't be able to sink current if it's the output of some kind of regulator, so won't function as a ground.  Ground needs to be able to both source and sink.  In fact, hooking it up to a rail 3V higher than its output is likely to destroy a linear regulator.

I'd just start with a transformer with suitable outputs, say 18VAC and 4.5VAC and rectify and regulate those into whatever I need.

[Kleinstein]

Using a 20 and 23 V regulator will not work well, as the 20 V regulator may not be able to sink much current. One could add a minimum load though - still a strange way. Better, and only slightly different would be using on regulator for -3 V and one for -23 V, as there is very little current at the 20 V. The 20 V between the -23 and -3 would need extra filtering anyway.