I look at a relay like this one and see the markings on top but am unsure what refers to the rating for the main circuit and what refers to the voltage needed to throw the latch. Can someone explain it to me?

UPDATE: I'm looking at a datasheet here, and I have two specific questions:

  1. There are lots of entries in the 'coil data chart'. What do they mean, and why isn't there only one?
  2. The 'performance' table indicates a max operation time of 15ms. Does that mean that the relay will burn out (or reset the latch) if I try to run an appliance on it for longer than 15ms? (My intention is to permanently activate an appliance (see below).)
  3. So what voltage is required to make the latch switch?
  4. At what voltage/current is the main rated?
  5. At what voltage/current is the coil rated?

TMI: The reason I ask is that I'd like to put a relay in a cassette player's circuit (which I expect to be between 6 and 12 volts (haven't procured one yet)) and activate it with a contact trigger (which I intend to throw the latch). It's for a treasure hunt.

  • 1
    \$\begingroup\$ The "this one" link was dead so I scrounged up the link from archive.org : web.archive.org/web/20121102050540/http://… \$\endgroup\$
    – bgoodr
    Commented May 25, 2015 at 3:09
  • 1
    \$\begingroup\$ @bgoodr, updated the 'this one' link \$\endgroup\$
    – Jellicle
    Commented May 26, 2015 at 14:58

2 Answers 2


It is useful to provide the component name in the question if known (here = TRU-12VDC-FB-CL) and a datasheet reference - if Olin had had that when you first asked he would have been able to explain better.

After reading the following you should, in future, be able to find and read a data sheet of this sort and be able to find and understand all the information I have provided below.

Manufacturer is Taiwanese Tai-Shing Electronics Components Corp http://www.tai-shing.com.tw

A better copy of the datasheet can be found here - this is on the manufacturer's site and does not have the annoying frames and ads which try and kidnap you at every turn. I foiund this by reading the manufacturer's website address from the captive and annoying data sheet.

The meaning of the relay patrt numbver is clearly explained under "ordering information" at the top of the data sheet.


  • TRU General family

    12 - nominal coil voltage

    DC - DC operated

    F - Flux free type

    B - AgCdO contacts - ie NOT ROHS compliant (which they note)

    C - Form C contact layout - diagram supplied

    L - high sensitivity

The data sheet shows that at 12 VDC

  • Nominal operate current is 30 mA

  • Pull in (operate) voltage = 75% (=9V)

  • Drop out voltage is 5% minimum (MAY be as low as 0.6V !!!)

  • Coil resistance is 400 ohms.

  • Maximum allowable voltage is 130% (15.6V) As Olin said, the ratings on the label are the contact current ratings at various AC and DC voltages. THese are listed about hald way down "page 52" (2nd provided page) of the daty sheet.

As Olin notes, there are TWO 28 V DC contact ratings.
The data sheet explains why on page 52.

  • 28 VDC 12 Amp is the "UL" rating

  • 28 VDC 15A is "TUV" rating

UL is a US approvals authority.
TUV is a European (German) approvals authority.

The reverse image stylised "UR" logo on the relay is the symbol of the UL organisation (Underwriters Laboratory).

TTI on the relay is the manufacturers brand name.


The switch contacts are rated 10A at 250 VAC, 10A at 120 VAC. There appear to be two specs for 28 VDC, 15A and 12A. It's not clear which one applies to what cases just from the markings on the relay. Look up the part number to find the datasheet. That should tell you for sure.

The "12VDC" in the part number is almost certainly a indication that the coil is intended to be run from 12 VDC. But again, look at the datasheet for the real info. You can't expect all possible specs to be printed on the case.

Added now that datasheet is available

It is very common for relay manufacturers to design the mechanism once, then offer a family of parts with different coil characteristics. Different customers want different coil voltages, so they make a variety. The same space can hold lots of turns of a thin wire or fewer turns of a thick wire. The current required to activate the relay is inversely proportional to the number of turns (cause the same magnetic field). Longer thinner wire has more resistance, so requires more voltage to operate the relay but at less current. Typically the power required to activate the relay is roughly the same accross such a family.

The coil data chart shows exactly this for two sub-families, which they call the TRU High Sensitivity and TRU Standard. The high sensitivity probably has a slightly different mechanical structure so that less magnetism is require to operate the relay. That may trade off with other specs, like vibration resistance and possibly operating speed.

Each line of that chart therefore refers to a different specific model of the TRU relay product line. Since your relay is of the high sensitivity variant, the row for 12V in the top section applies. From that it can be seen that the nominal coil current will be 30 mA, the coil resistance 400 Ω, the coil power to operate the relay is 360 mW, the contacts will close at 75% of the rated coil voltage (9.0 V), once engaged at least a 5% drop in coil voltage is required to release, and you shouldn't apply more than 130% (15.6 V) to the coil.

The operation time of 15 ms means that's how long it takes worst case for the contacts to close after you have applied the specified coil voltage. You can apply this coil voltage and keep the contacts closed indefinitely. The release time of 5 ms is how long it takes worst case for the contacts to open after the coil is switched off.

  • \$\begingroup\$ Thanks. I found the datasheet (linked in my update above). Can you help me interpret a few things? (See update.) \$\endgroup\$
    – Jellicle
    Commented Oct 9, 2011 at 22:48
  • \$\begingroup\$ What does release time assume about the circuit used to interrupt the coil current? A 5ms release time would seem be pretty fast for a relay with a flyback diode. \$\endgroup\$
    – supercat
    Commented May 15, 2014 at 20:46

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