Farnell LB30 2
Posted: Thu Jun 05, 2025 9:14 pm
Another item from the DDRC boot sale, this is the usual Farnell 30V 2A power supply in an unusual IEEE488 equipped form. I like the Farnell PSUs and it was £5 and have a use for another programmable supply so I bought it.
When I got home I discovered that the internet had never heard of this PSU, so perhaps it wasn't very popular. This also meant that the documentation for the GPIB interface would not be easy to find so after establishing that it worked in local mode I removed the lid to see if I could dump an EPROM and find the commands.
Here is what I saw… The big chip is a Philips GPIB bus interface. On the other side of an opto-isolated bridge there are two Analog Devices 3.5 digit solid state potentiometers, one for current and one for voltage. The datasheet was available and I quickly spotted that the most significant digit was hardwired to zero, meaning that each potentiometer had three BCD digits, which would likely mean six latches. The six 4042s are quad D-types, each with a separate clock fed by some logic contraption that includes a 4017 counter and a big 1 of 16 decoder chip. I connected up the bus and fed it some letters and numbers, numbers were advancing the 4017 count and alphabetical characters seemed to be firing its reset line. I started to draw a schematic but there were so many wires and it seemed that given certain assumptions I'd be able to figure out what to feed it. I worked out which characters performed each operation by monitoring the 1 of 16 decoder and the 4017.
Vddd programs voltage
Iddd programs current where ddd is 000 to 999, with 999 representing the maximum voltage and current of 30V and 2A.
P resets the state machine without appearing to have another function
C or S switches on the output to the programmed voltage
B or R switches off the output
Both the on and off switching is fake, it just changes the voltage setting to zero as far as I can tell. There's a separate manual switch for isolating the output on the front panel.
Noise level seems very good. I've tested it at full current. In terms of programmability, "000" or 0V measured 17mV and 999 measures 30.088V and current was similarly fine, so it seems the only thing I really need to do is find this power supply four replacement plastic feet that aren't broken.
Here is what I saw… The big chip is a Philips GPIB bus interface. On the other side of an opto-isolated bridge there are two Analog Devices 3.5 digit solid state potentiometers, one for current and one for voltage. The datasheet was available and I quickly spotted that the most significant digit was hardwired to zero, meaning that each potentiometer had three BCD digits, which would likely mean six latches. The six 4042s are quad D-types, each with a separate clock fed by some logic contraption that includes a 4017 counter and a big 1 of 16 decoder chip. I connected up the bus and fed it some letters and numbers, numbers were advancing the 4017 count and alphabetical characters seemed to be firing its reset line. I started to draw a schematic but there were so many wires and it seemed that given certain assumptions I'd be able to figure out what to feed it. I worked out which characters performed each operation by monitoring the 1 of 16 decoder and the 4017.
Vddd programs voltage
Iddd programs current where ddd is 000 to 999, with 999 representing the maximum voltage and current of 30V and 2A.
P resets the state machine without appearing to have another function
C or S switches on the output to the programmed voltage
B or R switches off the output
Both the on and off switching is fake, it just changes the voltage setting to zero as far as I can tell. There's a separate manual switch for isolating the output on the front panel.
Noise level seems very good. I've tested it at full current. In terms of programmability, "000" or 0V measured 17mV and 999 measures 30.088V and current was similarly fine, so it seems the only thing I really need to do is find this power supply four replacement plastic feet that aren't broken.