How to read DIAC datasheet? Diac On resistance? - Page 1
Forum section: electronics / beginners

author: TiTanerCZE
author_rank: Contributor
timestamp: July 25, 2021, 11:57:32 am
content: Hi, I have very noob questiion about DIAC DB3, datasheet in the description.I am not sure how to read datasheet.I will put 40 volts DC on a DIAC directly and it will be in negative resistance mode, because 32V is breakover voltage.But how much current will flow true the circuit?- what is the ACTUAL resistance of the diac? (for the given voltage)

author: Zero999
author_rank: Super Contributor
timestamp: July 25, 2021, 12:26:02 pm
content: DIACs have a very low Ohmic resistance. Once on, they're electrically similar to a diode. The current will exceed the maximum rating of the DIAC, in your circuit, causing it to blow up.

author: TiTanerCZE
author_rank: Contributor
timestamp: July 25, 2021, 01:59:15 pm
content: Thanks for the info, but it is still not clear to me, how do you know that. I can take it as general knowlage, but i cant make any calculation with that...I know, that it will propably be low, but a dont know it it will be 1m ohm, 1 ohm, 20 ohm, 500 ohm, or 5k ohm "low". I want to know that and with specified tolerance. How can i even know that it will break, when a dont know how much current will flow?This guy make a video with DB3:https://www.youtube.com/watch?v=eWOiUKAehmo.Hemessuret 23.86 V AC on his diac -> 33,74 V peak, and it is still working, it will blow up on 40V peak? From what line in the datasheet you know that? Or is it a general rule?

author: SeanB
author_rank: Super Contributor
timestamp: July 25, 2021, 04:39:35 pm
content: Once the voltage across the DIAC exceeds 32V, it will turn on, basically acting like a regular forward biased diode, with the current limited by the resistance in the circuit that delivers power. If the power source can only supply a limited current, the DIAC will stay on till the current drops below the holding current, and then turn off again. Thus you typically use a DIAC where you charge a capacitor up to a higher voltage than the breakdown voltage, and discharge it through the DIAC, with the current that flows through the DIAC and the load being limited to some current pulse below the maximum rated current. The resistance that charges the capacitor must be high enough that the current flow when the capacitor is fully discharged is below the holding current, and thus the DIAC turns off, and the capacitor starts to charge again.In AC use the same, just that you typically want to get the current pulse some time after the mains zero crossing, so the resistor allows the DIAC to charge up on each half of the mains cycle, and then discharge the capacitor typically to turn on a bigger thyristor or TRIC, which then keeps the voltage across the DIAC low for the rest of the mains cycle, so it only generates a single delayed pulse per half cycle of the AC voltage. next half cycle the thyristor or TRIAC turns off at the zero crossing point of the current, and then the voltage across the resistor limiting the charge current into the capacitor allows it to slowly charge to the breakdown voltage of the DIAC again, turning it on, and thus the thyristor or TRIAC, for the remainder of that half cycle.

author: bdunham7
author_rank: Super Contributor
timestamp: July 25, 2021, 05:59:56 pm
content: Quote from: TiTanerCZE on July 25, 2021, 11:57:32 amThe specific answer you have already gotten--a 40 volt non-current-limited voltage will vaporize the DIAC. And it doesn't have a 'resistance', as it is strongly non-ohmic. It has a general V-I curve that is shown in the datasheet (Diagram 1) that indicates the voltage will remain fairly constant as the current increases above the 10mA point, but the lack of data beyond that is a strong clue as to how the device is intended to be used.The device is intended as a trigger, such as in a classic lamp dimmer circuit. The datasheet only characterizes the device in respect to its intended use, which is typical for specialized components. What you are proposing is something else and if you wanted some sort of extended or more precise V-I curve, you would have to characterize it yourself--and there might be quite a bit of variance between specimens. And unless you use a sophisticated short-pulse test technique, they will all be destroyed in the process.

author: perieanuo
author_rank: Frequent Contributor
timestamp: July 26, 2021, 06:57:25 am
content: it's right in the face, page 2 DIAGRAM1, IF goes ti infinite if your device is trigerred by goind with V higher than VBO.IF infinite means equivalent resistance close to zero, that's as simple as it gets, like a diode pushed with high voltage.you have also noticed on mains 220V they added 10kohms with 20 ohms, that should notify you after diac treggers, the current rises (low resistance!!!)