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I have a basic circuit that uses 555 timer to generate PWM signal and the PWM signal turns the high side switch, SI1865DDL, on and off at certain frequency and duty cycle. The output from the high side switch goes to a camera which then takes photo. Both the high side switch and the 555 timer, are powered by a single power supply that outputs 5V, 1Amp max.

Power Supply:

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Here is a block diagram enter image description here I will use a IN4148 diode for reverse protection. I would also like to mention that I have turned the system on for an hours, and it worked fine. What sort of protection can I use on the output of the power supply to maintain at 5V 1Amp?

So, I was able to find a tvs diode that I believe fits my need: uClamp0871P. Can anyone please verify if this is a good tvs diode for a 5V power supply? I understand that it clamps at 15V which I am fine with it as it has low steady state power rating under 8V.

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  • \$\begingroup\$ Are you asking about protection on mains side (230V)? \$\endgroup\$
    – user208862
    Apr 23, 2021 at 22:46
  • \$\begingroup\$ @MichalPodmanický Protection on the output of the power supply so that it can remain at 5V and not exceed 1Amp. \$\endgroup\$
    – Sam
    Apr 23, 2021 at 23:15
  • \$\begingroup\$ Are you trying to protect your circuit in case the power supply does something bad? If so, have you thought about a transzorb (zener diode on steroids) and a polyswitch (resettable fuse). \$\endgroup\$
    – qrk
    Apr 24, 2021 at 2:03
  • \$\begingroup\$ @qrk Yes, I want to protect my circuit in case the power supply goes bad. I have thought of adding a fuse but not sure what capacity of fuse should I buy. Selecting transorb is another monster that I haven't learned to do. \$\endgroup\$
    – Sam
    Apr 24, 2021 at 2:09
  • \$\begingroup\$ The 555, from memory, can tolerate > 18 V. The SI1865DDL works at up to 12 V and can tolerate 8 V on the logic input. It's not clear to me what your concern is. "... to maintain at 5 V, 1 amp ..." You can control the voltage or the current. You can't control both. Since your circuit needs 5 V it will determine the current drawn. The power supply is rated at 1000 mA. It will probably supply more but at some point the voltage will start to droop below 5 V. \$\endgroup\$
    – Transistor
    Apr 24, 2021 at 11:30

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In order to protect from overvoltages, a zener is a common solution. They have a 'zener voltage', for 5V buses, 5.6V zener voltage is commonly used.

For avoiding overcurrents, you could use simple fuses, in order to size it you could measure what the current is under normal operation and then use one with a rating, say 30% above that current value. Bad thing about fuses is that once they trip you have to replace them. If you want to avoid that you could use PPTC which are similar to fuses, but once the short circuit is repaired, they turn back to normal, so you don't need to replace them.

Also, you said that you use a diode to avoid reverse conduction. That is definitely a good idea, although the voltage drop in a diode could be significant (up to 1V in some cases, 0.4V if you are using a shottky), in case that this diode created an issuee, a PMOS could be a good idea (with the body diode connected in the same polarity as you have your current diode).

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  • \$\begingroup\$ Zener diode requires a resistor in series which limits the current; I will be using close to 1Amp. I will use fuse, and a diode to avoid reverse conduction. What are your thoughts on tvs diode? I have looked into some, and they tend to consume higher power at normal operation. \$\endgroup\$
    – Sam
    Apr 26, 2021 at 19:15
  • \$\begingroup\$ Note. The mentioned 1N4148 cannot carry the 1A : either something bigger should be used, or 1A is definitely overestimated. \$\endgroup\$
    – andrea
    Apr 26, 2021 at 23:00
  • \$\begingroup\$ They do require a resistor in series for the case of a lasting overvoltage, else they rely on their thermal capacitance to dissipate power while the overvoltage lasts. TVS are not really that different. It all really depends on how serious you want to be protected against overvoltages. If just casually, add a zener or TVS. If you want to be 100% certain, then you should add a safety switch, like a MOSFET which can be disabled by a simple overvoltage detection circuit, which could be done with a comparator. \$\endgroup\$
    – N. Murgui
    Apr 27, 2021 at 14:14

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