Rotary switch for enhancement to HP current source - Page 1
Forum section: electronics / projects

author: motocoder
author_rank: Unknown
timestamp: August 02, 2015, 06:33:49 pm
content: I have an HP 6177C current source that I really like. It has 3 current ranges, 5, 50, and 500mA. These are selected via a simple rotary switch that switches in the appropriate shunt resistors. The resistors are cascaded as you select the higher ranges, so the resistor values are 2, 18, and 180 ohms, giving a resistance value of 2 (500mA range), 20 (50mA range), and 200 (5mA range).I would like to add one or (ideally) two additional current ranges to this supply. My thought was that I could just source a rotary switch that has two additional positions, and then wire in an 1800 and an 18K resistor for 500uA and 50uA ranges, respectively. However, I am having zero luck locating a rotary switch with the appropriate characteristics. I am hoping some of the folks here might be able to help with that.The switch needs to have the following characteristics:Two pole5 positions/throws (I would settle for 4 if that's all I can find)Make before break (MFB)Maximum current through the switch would be 500mAThe maximum voltage across the shunt resistor will be 48VDC. However, obviously the max voltage across the switch would be a tiny fraction of that, so I am not sure what that translates to in terms of the switches voltage rating.The OEM switch is the standard 1/4" round shaft, and the shaft is about 3/4" long. I have some flexibility in the shaft length, and maybe I could use the switch even f the shaft is not round (i.e. flat/D shape).Switch body needs to be one inch or less in width

author: sync
author_rank: Frequent Contributor
timestamp: August 02, 2015, 09:25:06 pm
content: I done this with my 6181B (250mA version) for an additional 250uA range. I use a break before make switch without problems (http://www.tme.eu/en/details/ck1051/rotary-switches/lorlin/). This switch is also available as make before break.A 50uA range maybe problematic due the base current of Q19A. When base current changes (temperature) the output current will change by the same amount. So lower ranges will have less stability.

author: motocoder
author_rank: Unknown
timestamp: August 02, 2015, 10:56:23 pm
content: Quote from: sync on August 02, 2015, 09:25:06 pmThanks! I'll have to see if that is available from a US distributor.Quote from: sync on August 02, 2015, 09:25:06 pmYes, I figured this base current and any leakage current would have some impact, but the range might still be useful. But maybe I will just do the one extra range.

author: motocoder
author_rank: Unknown
timestamp: August 02, 2015, 11:07:45 pm
content: Sync - By the way, if I can get this working, I will likely print up a vinyl template for the faceplate - to show the extra range. If you're interested I can also make one for your 6181C. I think the only difference would be the numbers for the current ranges.

author: sync
author_rank: Frequent Contributor
timestamp: August 02, 2015, 11:15:22 pm
content: Thanks for your offer. But I have a 6181B. I don't know if it will fit.

author: motocoder
author_rank: Unknown
timestamp: August 02, 2015, 11:38:22 pm
content: Quote from: sync on August 02, 2015, 11:15:22 pmAh, ok. I didn't even know there was such a thing.BTW - I found a US source for the MBB version of the switch to the one you posted. They have it at Mouser.com. Thanks for the tip.

author: motocoder
author_rank: Unknown
timestamp: August 15, 2015, 09:30:19 pm
content: This morning I finished adding the extra range to the 6177C. Details including the part number for the switch and a wiring diagram are available in this post:https://www.eevblog.com/forum/repair/hp-6177c-dc-current-source-repair-and-restoration-(complete)/msg732099/#msg732099