# How to solve for relay specifications?

I'm working with an unknown brand of relay and I need to know its specifications - most important is the minimum On current at approximately 42vDC, other values would be appreciated if possible.

Does anybody recognise the relay itself and have access to a specs sheet? Otherwise, can someone show me how to find the correct values from first principles, given limited benchtop equipment? The photo shows three relays on a 1cm grid; the pins are spaced for typical breadboard/veroboard.

I've tested the relay with a model train power supply (variable up to 14.6vDC):

• Von = 12.54V, R=564ohm, Ion = V/R = 12.54/564*1000 = 22.24mA to switch on
• Voff = 2.50V, R=564ohm, Ion = V/R = 2.50/564*1000 = 4.43mA to switch off

It is a 6PDT relay with pins spaced for veroboard and a clear shell, with raised text "820W" over the switch and "46091634" over the coil. I have been told that it is designed for 24vDC and has a 540Ω coil, but I have no evidence to confirm that, and it doesn't match the 12.54vDC on and measured 564Ω coil resistance.

The relay is presently used in a circuit of power > pushbutton > 1KΩ/1W > coil > ground, with the power supply being nominally 42-48vDC.

I need to branch from that circuit between the resistor and the coil, to: a) 1KΩ/1W > 1N4001 > 39KΩ/½W > base of BC549 NPN transistor, and b) 1N4001 > [resistor] > 1N4001 > +100uF- capacitor > ground. Note I have also added a diode in the lead to the 6PDT coil; this is necessary to separate part of the latching system not shown.

The capacitor will be charged at the same time that the 6PDT relay is powered by the push button, then it will discharge via a separate circuit to power a HK19F relay (12v, 720Ω coil) for about a second. I was considering a 68Ω resistor in the lead to the capacitor to minimise charge time to within the length of the push button press, but that arrangement leaves only 7mA for the 6PDT coil, so I need a higher resistor that provides enough power to the 6PDT while also ensuring that the capacitor charges within the time of the push button being pressed. How do I balance those figures, other than trial and error?

• I would say, for a relay coil, that 564 Ohms is close enough to 540 Ohms to be considered "the same". A relay specified for 24 volt operation could possilby pull in at 12.5 volts. Commented Dec 21, 2022 at 17:15
• Why must you use these unmarked relays? Commented Dec 22, 2022 at 14:11
• @Lundin - These are the relays already in situ, and I'm adding additional functions to the system. Commented Dec 22, 2022 at 18:45

and it doesn't match the 12.54vDC on and measured 564Ω coil resistance.

Well it does, actually.

You can expect coil resistance to vary by $$\\pm\$$10 or 20%.

When a manufacturer designs a relay to operate at a specific nominal voltage (such as 24V), then they design it so that it will reliable pull in over it's entire rated temperature range, at the low end of it's rated voltage range (which may be as low as 20V, or lower), after it's been in the field, wearing out, for a good long while.

So the fact that it pulls in at 12.5V in a nice comfortable lab is entirely consistent with it having a 24V rating.

I'm working with an unknown brand of relay

It's an ITT -- it's molded into the case, visible in the photo. It's probably an ITT part number 46091634, but I did a search on that and didn't find anything. That probably means its old.

I have been told that it is designed for 24vDC and has a 540Ω coil, but I have no evidence to confirm that

Yes, that's a problem. That's a common form factor for a relay -- if I really needed to know I'd go shopping for similar-sized relays, to see what correlation there is between coil resistance and rated voltage. If you found it in equipment that runs off of 42V, that's a good indication that it's a 48V relay, or that someone was playing games.

I was considering a 68Ω resistor in the lead to the capacitor to minimise charge time to within the length of the push button press, but that arrangement leaves only 7mA for the 6PDT coil,

You're misunderstanding the circuit. The PNP transistor there is an emitter-follower that's actually driving the relay -- although there may still be insufficient gain.

If your goal is to have a time delay, it would be much more reliable to have a delay circuit with a logic-level output (i.e., something with a comparator, or a properly set-up 555 circuit). Then adequately amplify that logic-level output to drive your 12V relay.

• Thanks Tim. I'd guess that the relay is at least 20 years old, but I can't prove that; a lot of the spare equipment stored nearby was about 50 years old. In situ, the circuit is (approx) 42V > pushbutton > 1KΩ > coil > ground, so it must be less than 50V to operate; that plus your note above effectively confirms 24V nominal operating requirement. In the circuit above both transistors are NPN, not PNP. The green button works both relays, the bottom left NPN, and also charges the capacitor. When released, the capacitor briefly works the 720Ω relay (unless the red button is pressed). Commented Dec 22, 2022 at 8:42
• I'm not aiming for a specific time delay, just enough to simulate a sequence of push buttons being pressed, with time for external relays (a mix of 500 through 2K5Ω coils) to click over between each press, activating and deactivating a sequence of latches. I do not have access to a 5V supply and the circuits are in an open-air environment surrounded by hundreds of other twisted and single wires, so I prefer bulkier components if possible. Commented Dec 22, 2022 at 9:11
• The Falstad simulator - falstad.com/circuit/circuitjs.html - says that the 100uF capacitor will take ~104ms to charge, after which the 540Ω relay then 720Ω relay will switch on. How long is a typical push button press considered to be "on", and how much margin for error should I incorporate? If I increase from 68Ω to 680Ω, the time fpr the 540Ω coil decreases to 97ms, and at 6k8Ω, to 9ms, but the capacitor takes a lot longer to charge and probably doesn't store enough power to work the 720Ω coil. Commented Dec 22, 2022 at 9:39
• As an alternative solution to the capacitor vs relay problem, I think I can add an NPN between the pushbutton (base) and the capacitor (emitter), with the nominal 48V source on the collector. That reduces the capacitor charge time to 7ms (based on the Falstad simulator), without slowing the operation of the 540Ω coil. Commented Dec 22, 2022 at 17:30