Help with Understanding Relay Driver's specs

Thread Starter

128ITSH

Joined Jul 20, 2017
101
Hello everyone!
In my project I want to drive two G2RL-1-H DC5 relays (datasheet) (Coil rating is 5V@50mA).
In order to save board space I want the most integrated solution, so I found the MDC3105DMT1G dual relay driver (datasheet).
In terms of voltage and current there is no problem whatsoever, the voltages of the coil and the driver match, and the max current sink of the driver is significantly higher than the coil current.
The thing is that the driver's datasheet states another limit:
"Repetitive Pulse Zener Energy Limit"
I am not an EE, just an hobbyist, so for me this specification is cryptic. what does it mean? Right now I understand it refers to the zener's ability to handle the EMF induced in the coil when the transistor turns off, but I have no idea how to determine if it matches the relay I want to drive or not.
The application notes ("Designing with this Data Sheet") is very confusing as well.
Thanks in advance.
 

MaxHeadRoom

Joined Jul 18, 2013
19,682
Another thing to consider when using sealed relays that have a fairly high switching current, especially DC, is when they are used sealed as the PDF shows the two versions sealed for fluxing,cleaning etc, some have a small vent pip that can be removed in use in order that the relay is made vented, with relatively high current relays, the air inside the relay can become ionized with repetitive use/switching for e.g. causing arc-over.
Therefor it is often best to either use a vented relay or one that can be made so, if used in a dry environment etc.
Max.
 

crutschow

Joined Mar 14, 2008
24,062
"Repetitive Pulse Zener Energy Limit"
I am not an EE, just an hobbyist, so for me this specification is cryptic. what does it mean?
It's the energy the built-in Zener can absorb from the relay coil inductive energy, which is 1/2 LI².
The pulse rating is 50mJ maximum for very low duty-cycle operation (≤ 0.01%).

The coil current is 80mA (not 50mA as you stated) but the coil inductance is not given in the relay data sheet.
Typical values for such a 5V relay are around 0.1H.

For 0.1H of inductance, the inductive energy to be dissipated would then be 1/2 * 0.1 * .08² = 0.32mJ, which is over a factor of a hundred below the 50mJ rating.

So you should be fine even if the relay is operated frequently.
What is the maximum frequency of relay operation?
 

Thread Starter

128ITSH

Joined Jul 20, 2017
101
It's the energy the built-in Zener can absorb from the relay coil inductive energy, which is 1/2 LI².
The pulse rating is 50mJ maximum for very low duty-cycle operation (≤ 0.01%).

The coil current is 80mA (not 50mA as you stated) but the coil inductance is not given in the relay data sheet.
Typical values for such a 5V relay are around 0.1H.

For 0.1H of inductance, the inductive energy to be dissipated would then be 1/2 * 0.1 * .08² = 0.32mJ, which is over a factor of a hundred below the 50mJ rating.

So you should be fine even if the relay is operated frequently.
What is the maximum frequency of relay operation?
Thanks for the clarification.
The relays are (indirectly) controlled by human interface (buttons), so they may open and close as fast as the human presses the buttons.
Should I make a software limit for the frequency?
 

Thread Starter

128ITSH

Joined Jul 20, 2017
101
That's probably not a problem.
So how long are they likely to repeatedly press the button?
To be more clear this project is a remote control for a rolling shutter, so there are some options for how frequently the relays open and close.
This is one of the simplest operations of the system:
The rolling shutter is closed --> human activates relays (single button push)--> after 30 seconds the rolling shutter is fully open and the relays automatically stop. This means only 2 changes with 30 seconds in between.
The thing is when someone will try this remote for the first time they might try "playing" with the system and repeatedly press the up and down buttons a few times.
I Believe this wont go for more than 10 presses - Should I worry about this?
 
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