Re: Thank you for Advice on ELECTRONIC LOADS


 

Hi Ed,

Thanks for pointing that out. It will be a quick way to increase the voltage to the range I need. 400V makes me nervous ever since I got a nasty shock from a 400V transformer when I was in college 50 years ago. Ever since that incident I become extremely cautious around circuits where there is that much voltage and I take plenty of precautions. The irony is that at about the same age I was given a 15KV Neon Sign transformer which I played with extensively without ever getting a shock. To this day I have more fear of 400V than I do of 15KV.

Dennis Tillman W7PF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Ed Breya via Groups.Io
Sent: Tuesday, January 30, 2018 10:02 PM
Subject: Re: [TekScopes] Thank you for Advice on ELECTRONIC LOADS

If you need to use an electronic load at higher voltage than it's made for, you can easily rig up a a high power Zener diode equivalent circuit in series to drop the voltage, and dissipate the excess power. For example, for a B+ supply in the 300-400V range, you can make an N-channel power MOSFET act as a shunt regulator in conjunction with say, 200-300V-worth of Zener diodes drop from D-G, allowing a 200V-rated load to handle it. A clamp circuit would be needed to make sure the electronic load's voltage can't get too high when its current is set very low. It will have some limitations, and need some figuring out of the external conditions and range requirements. The external shunt regulator will have to dissipate the extra power involved, of course.

Another, slightly more complicated option is to form a cascode stage with an external MOSFET, with gate biased (and G-S clamped and current-limited) to a fixed voltage somewhat less than the electronic load's rating. Virtually all of the current would then flow through the electronic load, for any test voltage above the gate bias, up to the MOSFET's Vds rating, while limiting the load's voltage to the gate bias level.

In any scenario, the load's output should be OVP'd, in case things go wrong. It's also good to add fuse protection at just over the maximum expected current, if it's not already provided. Reverse-polarity protection is intrinsic with most MOSFETs, as long as there's a fuse to clear/limit the fault current.

Ed



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Dennis Tillman W7PF
TekScopes Moderator

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