Circuit Notebook 78 - Using Coax Relays with High Voltage Coils

Useful coax relays appear from time to time at radio rallies, but if the coil is marked 24 V or higher, rather than the more useful 12 V, then the tendency is to think again or haggle harder over the price.

With mains operated equipment in the home shack, the higher voltage coil is not a problem; all that is required is a suitable separate supply. With portable gear, operating from 12 V supplies, the problem is not so easily solved. If the coil is easily removable then reducing the number of turns or rewinding for the lower voltage is possible. Some experimentation may be needed to arrive at the optimum wire gauge and the right number of turns.

When a relay coil is connected to its correct voltage, the relay contacts will operate smartly and stay fully closed until the supply is removed. Relays can be operated from a reduced voltage, but the operation may be slower and the contacts may not be fully closed - causing poor electrical contact.

I have measured the mechanical performance of three typical coax relays, shown in Fig.1, with the results given in the table below:

	Relay type	Coil Volts	Coil Resistance	V operate	V release
(a)	Amphenol 'BNC'
Coil 28115-51	26 V D.C.	265 ohms
(98 mA)
(2.5 W)	14 V	5 V
(b)	Transco 'SMA'
317-10242	28 V D.C.	290 ohms
(96 mA)
(2.7 W)	14 V	7 V
(c)	Amphenol 'N'
317-10242-4	48 V D.C.	1215 ohms
(40 mA)
(1.9 W)	32 V	12 V

At the 'V operate' condition relays (a) and (b) closed with a 'click' whereas relay (c) closed electrically but did not seem to be fully 'home'.

A circuit by K1KP, shown in Fig.2, appeared recently in G3SEK's 'In Practice' column in 'RadCom' [1]. Its purpose is to speed up relay operation by momentarily doubling the supply voltage. The circuit would prove useful for relays (a) and (b) as it would provide about 24 volts to close the relay after which the 12 V supply would hold it closed.

The following description of how the circuit works is taken from G3SEK's article. Initially the 'control' line is ungrounded and C1 charges up to almost the full supply voltage of 12 V via the relay RL1, D1 and D2. TR1 has no forward base voltage at this time and does not conduct.

Activating (grounding) the 'control' line grounds the positive terminal of C1, so that the negative terminal of C1 takes the emitter of TR1 down below ground potential almost to - 12 V. This causes base current to flow into TR1, which turns fully on so that its collector is also very close to - 12 V. At this moment the relay sees + 12 V on one terminal and - 12 V on the other, a total of twice the supply rail voltage, so it pulls in very sharply.

The golden moment doesn't last, of course, because the relay current will discharge C1 within a few milliseconds. D1 and D2 were both reverse biased while C1 was pulsing the relay, but when C1 discharges D1 starts to conduct again and holds the relay in at its reduced voltage for as long as the 'transmit' line is grounded. Note that R1 is essential to allow the base of TR1 to follow the emitter down towards - 12 V.

When the 'control' line is released, C1 recharges through RL1, D1 and D2, so the circuit is ready for repeat operation. A bonus is that when you release the 'control' line, the back EMF, generated by the energy stored in the relay's magnetic field goes into recharging C1, no other diode is required. Further information is given in G3SEK's article. Typical component values are shown in Fig. 2.

Relays with even higher voltage coils

Where the above method is impractical, as in the case of relay (c) then an additional supply is required. This can be easily generated by using a small DC-DC converter and either using the output directly or by added it to the existing 12 V supply.

A suitable circuit is shown in Fig. 3. The DC-DC converter (NMH 1215S, Farnell 200-153) provides +15 V/ 0 / -15 V (total 30 V) from a 12 V input and this added to the 12 V supply to provide about 55 V under no-load conditions and about 42 V under load. Although this is below the 48 V rating, the relay pulls in with a click and is held fully 'home'. A conventional catching diode D1 is required across the relay.


[1] Relay Speed-up Circuit - K1KP 'In practice' Ian White G3SEK 'RadCom' Magazine, April 2002, p. 55.

Circuit Notebook 77, CQ-TV 198, up-date

The PAL waveform, shown in Fig. 1, was taken from Reference [5]. The timings shown differ slightly from the 'PAL System I', which is used in the UK. I wish to thank Peter Vince G8ZZR for providing the correct information, as follows:

More detailed information is given in the 1984 DTI document "Specification of Television Standards for 625-Line System I Transmissions in the United Kingdom"