Electronic load for split/bipolar supply testing - Page 1
Forum section: electronics / projects

author: Beakman
author_rank: Newbie
timestamp: August 09, 2020, 04:20:32 pm
content: Hello All,Researching here and on the wider web I've seen dozens of designs for programmable electronic loads, but none! for testing both positive and negative currentssimultaneouslyas necessary for testing split or bipolar power supplies.Can someone point me towards an existing design or comment on how to modify a normal "positive" load for use in with negative voltages. This assumes that the positive and negative voltages share a common ground.Thanks!

author: OM222O
author_rank: Frequent Contributor
timestamp: August 09, 2020, 05:10:37 pm
content: It's literally called a class A amplifier (or AB). Instead of using 1 fet connected to a shunt which goes to ground, you use 2 fets in common source configuration (AB) or pre bias the fet (A). I personally recommend AB since it's easier to implement.Do some research on that topic and let me know if you have any questions regarding that.

author: Howardlong
author_rank: Super Contributor
timestamp: August 09, 2020, 05:19:10 pm
content: Quote from: Beakman on August 09, 2020, 04:20:32 pmWhat current and voltage ranges?

author: Beakman
author_rank: Newbie
timestamp: August 09, 2020, 05:27:41 pm
content: Funny, I had just begun contemplating the similarities of what I want to do to the way power amplifiers draw current from their power supplies.In my work power supplies are normally +/-15 to 18V, up to 10A per rail. If I was going to build something I would want it to handle +/-24V at 10A.Programmability is a requirement, voltage and current readouts a nice luxury.

author: Beakman
author_rank: Newbie
timestamp: August 09, 2020, 05:33:56 pm
content: These are cheap and have nice heatsinking:https://www.ebay.com/itm/250W-Electronic-Load-Battery-Capacity-Tester-Testing-Module-Discharge-Board-E6V9/254649147899?hash=item3b4a45a1fb:g:3vIAAOSwKrJfBwbXUsing two of these loads in tandem would be quick and easy but I don't know if it would be possible to get a schematic for them with an eye to modifying one for use on the negative rail.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: August 09, 2020, 05:34:14 pm
content: Hi,Here are two options:1) Using two 'positive' loads.Assuming the load terminals are floating, you can connect two loads in series, and connect the center tap of the loads to the center tap of the power supplies. Each load is connected with the correct polarity.2) Make a 'negative' load:This is the normal load circuit. The N. Channel MOSFET has been replaced by a P Channel MOSFET. The reference polarity is inverted.This has the advantage that the current monitor and reference are with respect to ground.Jay_Diddy_B

author: David Hess
author_rank: Super Contributor
timestamp: August 09, 2020, 11:31:24 pm
content: Electronic loads with multiple channels are available.

author: Beakman
author_rank: Newbie
timestamp: August 09, 2020, 11:48:09 pm
content: Thanks, Jay. It looks like it should be simple enough to graft your suggestion onto your "standard" uni-polar load.

author: eblc1388
author_rank: Frequent Contributor
timestamp: August 10, 2020, 04:59:51 am
content: If the current rating of both the power supplies are equal, then you can test them as a single supply. Just connect the electronic load to the +pos and -neg terminals, ignoring the common terminal.e.g. a +12V 3A and -5V 3A supply can be tested to full load as a 17V 3A supply.

author: Beakman
author_rank: Newbie
timestamp: August 11, 2020, 02:28:46 pm
content: That's a good idea in a lot of situations if everything is operating as specified, but if one rail is not up to snuff, say, sagging under a heavy load before the other rail, I need to be able to see that. Voltage and current monitoring for each rail are necessary for me.

author: 2N3055
author_rank: Super Contributor
timestamp: August 11, 2020, 02:44:49 pm
content: For single output bipolar supply you can use unipolar load, diode bridge and auxiliary 3-4 V 10A supply to achieve zero dropout for load...

author: eevcandies
author_rank: Regular Contributor
timestamp: August 15, 2020, 07:19:34 am
content: you could get one of these cheephttps://www.ebay.com/itm/Electronic-Measurements-Inc-The-BOS-S-Bipolar-Operational-Source-Sink/184372807091?hash=item2aed79c5b3:g:m7EAAOSwODZfGD07detailshttps://us.lambda.tdk.com/products/legacy-products/pdf/bos-s.pdf

author: Howardlong
author_rank: Super Contributor
timestamp: August 15, 2020, 08:26:31 am
content: Quote from: eevcandies on August 15, 2020, 07:19:34 amThat’s a pretty useful bit of test gear, four quadrant 20A/20V for $95. Sadly for me not shipping to the UK.