Sanity check on power supply backfeeding - Page 1
Forum section: electronics / general

author: Chesthair Ascot
author_rank: Newbie
timestamp: August 26, 2025, 04:34:26 pm
content: Hi all-Is backfeeding a bench power supply kosher? Eg, putting a voltage on the output of a bench power supply from an external source while the bench power supply is either powered off, or has its output disabled. (Assuming the applied voltage is within the output range of the bench power supply.)I'm generally used to working with HP/Agilent bench power supplies, and have never had issues with backfeeding them, but when I backfed a Sorensen DCS 60-20E with 35V (it's rated for 60v) while trying to charge a battery, the Sorensen power supply let out the magic smoke immediately. On the other hand, I can do the same thing with an Agilent 6634B or 6030A, or Keithley 2231A-30-3 all day long without issueI asked several other EE's and they all expected bench power supplies to handle this. The technician at the company I purchased the power supply from said that no bench power supplies can handle back feeding, and that it's always necessary to use a series diode to prevent backfeeding.I also checked the manual of the Sorensen and didn't see anything about restrictions on backfeeding except when driving a inductive load/motor, where it's easy to backfeed above the power supply's output range.A sanity check on this would be much appreciated =)

author: mtwieg
author_rank: Frequent Contributor
timestamp: August 27, 2025, 11:47:01 am
content: I don't recall ever actually damaging a bench supply in such a way (so long as Vout is within its limits). But I definitely have seen some behave erratically or going into an alarm state.All power supplies should be capable of some degree of backfeeding. Consider what happens if the PSU has a large output capacitor, and you remove its power source while its output is enabled. You immediately have a condition equivalent to backfeeding. But this is a common occurrence, which no decent PSU should struggle with.However, if the rise in Vout is very fast, that could be a different story. I've worked with several power supplies (both switchmode and linear) where a large and fast positive transient Vout (or a very fast drop in Vin) would cause large currents to flow through low-power circuitry, causing damage.

author: Siwastaja
author_rank: Super Contributor
timestamp: August 27, 2025, 11:55:13 am
content: Absolutely necessary. Only absolute total crap self-destruct when there is voltage (within specified output range) present with power off or output disabled. The same crap which self-destruct anyway.This is because lab supplies are commonly used to power up DUTs with a lot of capacitance, charge batteries, or run brushed DC motors (with inertia).Now, how it shouldexactlybehave is a hard question. Some show some kind of alarm; some sink current; some even have a (fused) crowbar circuit which shorts the external voltage source. As long as it does not break all of this is OK, what is really needed depends on case.Of course, protecting against large overvoltages (that of exceeding what the supply would be capable of producing on its own) would be a premium feature and I would not expect that from a cheap unit.

author: Seekonk
author_rank: Super Contributor
timestamp: August 27, 2025, 02:07:53 pm
content: Switching power supplies should be able to handle back feed. There are some power supplies which are designed to sink and dissipate power from a load. Can't assume anything without knowing internals. Funny, I've seen a number of people on solar forums try to discharge a battery by reversing the polarity and putting supply in constant current mode. Constant current is just what they got.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 29, 2025, 05:22:26 pm
content: Any lab power supply should be able to handle backfeed within it's operating range. The whole reason we pay for and use "laboratory" power supplies is that they are flexible and bulletproof tools to use for a wide variety of applications.Adding an external blocking diode makes no sense. It completely destroys your output regulation.

author: David Hess
author_rank: Super Contributor
timestamp: August 30, 2025, 12:32:02 pm
content: A power diode could be added in series between the power pass element and the output capacitor and feedback point to block reverse current. I guess a 2 or 4 quadrant supply would have to disconnect its output to protect itself.On a Tektronix PS503A, backfeeding will go into the operational amplifier inputs and outputs, usually destroying them. I think it can also destroy the drive transistor for the power pass transistor. Maybe adding a power diode from the output to the supply rail would prevent this by limiting the backfeed potential to one diode voltage drop.

author: Chesthair Ascot
author_rank: Newbie
timestamp: September 12, 2025, 04:45:56 pm
content: Thanks to all that replied. It looks like the only consensus is 'get to know your test equipment'. I think I'll take the advice on adding a power diode to be safe.I finally got the power supply mailed out to me and found out why it died. The Sorensen DCS 60-20E has a two-stage discharge resistor. It has a 1.6k ohm 5W always across the output (2.5w at 60V), and another 5.6 ohm across the output through a fet. The lower value appears to kick in around 3.75V to quickly discharge down to 0v. The aggressive discharge resistor appears to be in circuit while the power supply is off, so when the battery was plugged in, it put like 250w across that 5W resistor.All that said, it was a treat opening up that Sorensen. The components inside are all top-shelf.