Protection diode question - Page 1
Forum section: electronics / beginners

author: bobbydazzler
author_rank: Regular Contributor
timestamp: May 19, 2021, 10:26:12 pm
content: Here we have a standard 7805 with the recommended diode between output to input, my question is if you have the diode in series with the input is the protection diode between output-input needed?

author: pqass
author_rank: Super Contributor
timestamp: May 19, 2021, 11:50:52 pm
content: The purpose of the diode that straddles the regulator is to protect it in case a power source of higher voltage is connected to the output. Also, a reverse biased diode on the output (with ground) is also commonly fitted. However, both are not strickly needed for the circuit to work.A diode in series with the regulator input pin is only really needed if there is a possibility that someone could reverse the input/ground pair. Such as the case where the input to the regulator is connected to a barrel jack which accepts any dc wall wart.https://electronics.stackexchange.com/questions/401460/protection-diodes

author: ledtester
author_rank: Super Contributor
timestamp: May 20, 2021, 12:04:42 am
content: I think the question relates to this scenario where there is a reverse polarity diode in series with Vin:Is a protection diode around the regulator needed if Vin is shorted to ground in this case?

author: bob91343
author_rank: Super Contributor
timestamp: May 20, 2021, 12:09:25 am
content: I suspect it's a good idea. If the input power is removed, the input capacitor is still charged and, if it's large enough, may be held charged by reverse current through the regulator, which could be too much for it. This is speculation and I may be very wrong.

author: penfold
author_rank: Frequent Contributor
timestamp: May 20, 2021, 12:11:14 am
content: Quote from: pqass on May 19, 2021, 11:50:52 pmAlso not necessarily "connected" to the output, but that condition can also result when there is a significant capacitance on the output. When the input power is disconnected, the voltage at the input could decay faster than that on the output side.EDIT: I think the "reverse diode on the output" remark may to be prevent conditions that would result in a negative voltage being forced on the output (can happen when there's a few voltage rails which all rise at different rates)

author: pqass
author_rank: Super Contributor
timestamp: May 20, 2021, 12:17:37 am
content: Quote from: ledtester on May 20, 2021, 12:04:42 amI just noticed that switch. I'm not sure why that would be there. It looks to me that it's the common pin that is unconnected.What I was getting across was that each diode has an independent, unrelated purpose to be where they are.

author: ledtester
author_rank: Super Contributor
timestamp: May 20, 2021, 12:25:30 am
content: QuoteThe switch is included in the TI diagram just to indicate where the short would happen -- it's not meant to be part of an actual application circuit.

author: pqass
author_rank: Super Contributor
timestamp: May 20, 2021, 12:39:08 am
content: Quote from: ledtester on May 20, 2021, 12:25:30 amOkay. A sudden short on the regulator input should not cause any damage to downstream components with or without a series diode on its input. However, a series fuse will be necessary to protect upstream components.The existence of the stradling diode also won't matter to this scenario.Correction, having the stradling diode will make sure the regulator won't be damaged when the output capacitor tries to discharge through the regulator when its input is shorted. If there was a series diode at the regulator input AFTER the short, then the input cap would just decay through the regulator; output would not have an opportunity to be higher than its input and no need for a stradling diode.So WRT the original question, for the purpose of saving the regulator in the case of an input short, one can provide a series diode OR stradling diode.

author: bobbydazzler
author_rank: Regular Contributor
timestamp: May 20, 2021, 12:57:33 am
content: Quote from: ledtester on May 20, 2021, 12:04:42 amYes I was thinking with the diode in this configuration the diode between output to input might not be needed. If there were say a 10000uf output capacitor then with the series diode the current wouldn't be able to flow through the regulator to input, but in the series diode circuit the current could still flow through the regulator to ground I think without the diode between output to input.I just tested a 7806 I had and measured 6.8kohms resistance between the output and ground pins so current could flow through there if input were shorted to ground with a series input diode. I don't know if that could/would damage the regulator though.

author: ledtester
author_rank: Super Contributor
timestamp: May 20, 2021, 01:33:05 am
content: Well, a 10,000 uF output cap is extraordinarily large for a 78xx regulator. What's your application? Usually a 100nF and/or a 10uF cap is all that is needed on the output.The other reason the datasheet says a straddling diode is needed is if the output voltage could ever be raised above the input voltage (i.e. due to the load).Diodes are cheap enough. I think I would include the diode if the output cap was > 10uF (per the datasheet) or if there was a chance the output voltage could rise above the input voltage. If you're designing a PCB just leave a footprint for it so it can be added if necessary.

author: bobbydazzler
author_rank: Regular Contributor
timestamp: May 20, 2021, 02:08:11 am
content: Quote from: ledtester on May 20, 2021, 01:33:05 amNo application in mind I'm just trying to understand the way the diodes protect the regulator, 10000uf is just an arbitrarily large number I picked. I know the regulator doesn't need anywhere near that much capacitance.

author: Guy Shemesh
author_rank: Contributor
timestamp: May 20, 2021, 02:38:20 am
content: Hi bobbydazzler,Please note big capacitance is usually not good either. For two reasons -1. regulators in general both DC DC and linear are designed with a reference capacitance in mind. 1uF-22uF is typical, sometimes even higher like 47uF. The designer of the asic (the regulator) often try to compensate for delays between input and outputs so that 'load regulation' is improved. But by doing so may also introduce instability at certain capacitive loads - it becomes susceptible to oscillation. So when you add crazy big capacitors, that surely not what was designed and unfortunately here at ePiccolo Engineering we also had a pcb board that was burned right on initial power up. It was a DCDC that had 330uF (if I remember correctly) at the output, and on power up we got an overshooting Vout that exceeded the absolute maximum ratings and burnt the DSP. When we significantly lowered the output capacitance the problem was solved (although the DSP IC had to be replaced)2. The protection diode you asked about is specifically there because of situations of large output capacitance. On power down, input voltage is removed from the circuit and all regulated output voltages fall at different (often unpredictable) rates. because the pass element in the LDO may need to be protected from this situation a protection diode is recommended. Please note, its more of an issue with older ICs, modern regulators rarely need this. But here is the chicken and the egg: If you had put low output capacitance, chances are that a. your Vout would fall faster and b. even if some back current to Vin flows during power down, because of your low output capacitance it is not much and chances are it will not hurt your design. So you add a protection diode just to protect against large output capacitance that may have not been needed at the first place.It's a little bit deep at beginner stage, still I hope this clears issues somewhat.Guy

author: David Hess
author_rank: Super Contributor
timestamp: May 20, 2021, 11:35:45 am
content: In the past the common reason for having the anti-parallel diode from input to output was to protect the regulator if the crowbar located at the input was activated. Just a power failure which allows input capacitor leakage to pull the output down is not a problem.