34401a switching between DC-I AC-I while under load - Page 1
Forum section: electronics / testgear

author: hp3310a
author_rank: Regular Contributor
timestamp: February 28, 2025, 03:27:06 pm
content: Hi everyone,stupid question due to stupid blackout (mentally) during current measurement: I was comparing the current settings in my electronic load to what was actually flowing in the wire using my 34401a. At 2A load I had the great idea to switch from AC-I to DC-I without turning the load off first. When I heard the relays click my mind started visualizing the huge arcs that had just incinerated those poor contacts.I took a look in the schematics but I couldn't find the shunt resistor for current measurement so I'm not sure how it really works, hence the question: Does the current load circuit get interrupted when switching between DC-I and AC-I or is it just measurement relays that one can hear?

author: bdunham7
author_rank: Super Contributor
timestamp: February 28, 2025, 03:50:40 pm
content: I looked but I'm not sure without testing one. I believe that it will switch down to the 5R shunt if you go from ACI to DCI because DCI starts off in the mA range whereas the ACI only has the high current range unless you add it in--and even then it will not default or autorange there. IDK about going the other way from DCI to ACI if you are already in the high current range as there would be no need to switch but K102 might drop out briefly anyway. There are two shunts, 5R and 0.1R, and K102 switches between them. You could put an ohmmeter or something across there and switch and see if you get a change in the input resistance. Here's an excerpt from the Function Switching diagram. The shunts are R121 and R120.I wouldn't worry too much about arcs though, the voltage across the contacts is limited to about 2V by CR100 (look on the Front/Rear Switching diagram)

author: hp3310a
author_rank: Regular Contributor
timestamp: February 28, 2025, 04:41:10 pm
content: Thanks for pointing me in the right direction (though I can't say I understand how this works, exactly)! Looks like I definitely managed to get contacts to open while under load, that's less than ideal.The Ohmmeter (Brymen 789) says 0.56Ω (it fluctuates) in AC-I mode and 5.6Ω in DC-I mode (where it draws 0.36mA).

author: bdunham7
author_rank: Super Contributor
timestamp: February 28, 2025, 05:06:31 pm
content: Quote from: hp3310a on February 28, 2025, 04:41:10 pmI cut off the part where the bottom wire is labeled "AMPS". That wire goes to pin 13 on K102 and then K102 switches that to either 9 or 11. If it is 9, then the current goes through both shunts to ground, if 11 then just through the lower shunt. That's what you're seeing with your Bryment and it shows you that the ACI range only uses the lower shunt (K102 engages) The voltage across whatever resistors are selected goes on to the input amps or TRMS converter and ADC.I doubt you've damaged anything and I don't know of any specific prohibition on switching the meter like that. Perhaps if it were doing it rapidly in an automated test procedure it would wear out eventualy, but a one-off switching of 2A at <2V probably isn't an issue. You can scour the manual if you like to see if they mention it.

author: hp3310a
author_rank: Regular Contributor
timestamp: February 28, 2025, 05:25:07 pm
content: Quote from: bdunham7 on February 28, 2025, 05:06:31 pmThe manual dealing with current measurement (below) doesn't mention prohibitions, on the contrary. I would interpret that page as "go for it".In the "Theory of Operation" section under "Function Switching" (also below) the ranging states are discussed. I would interpret this as K102 not having switched, because it was not in the DC mA range at the time. It dawns on me that you already explained that.My power supply was set to 5V at the time. I apologized to the 34401a and promised not to do that again.Thanks for your quick help!

author: Kleinstein
author_rank: Super Contributor
timestamp: February 28, 2025, 07:28:40 pm
content: The relay switching would be K104, just switching the sense path for ADC from voltage to current. Even if K102 would switch the limited voltage would also limit and damage from switching under load. The relay data-sheets usually still allow some 100000 switching events under load, even with a higher voltage.