Heater Delay Timer

See updated version 2 of the Heater Delay Timer.

This article describes a flexible timer for delaying PTT operation of a valve amplifier until sufficient time for the cathode to reach operating temperature.

Design criteria

The timer sits between the low voltage positive DC source used to control TR relays and the relays themselves. Note that if the same DC source is used for other purposes (eg bias, meter lamps, ALC delay, bias etc), the supply to those functions must be tapped off before the timer.

Key design criteria were:

  1. operation from +12 to +20V DC supply;
  2. load current to 200mA;
  3. delayed switching of input supply to output based on timer operation;
  4. flexible, adjustable delay interval;
  5. reliable;
  6. simple in construction and adjustment;
  7. low power requirements; and
  8. capable of retrofit to existing amplifiers to ensure adequate cathode warm up time at initial power on.



Fig 1: Circuit diagram.

The heart of the timer is a PIC12CE519 microcontroller (MCU). The MCU starts a timer when it is initialised at power on, and after a predetermined delay (fetched from MCU EEPROM), asserts GP5 which causes the BC337 to conduct, which in turn causes the BD140 to conduct. The transistors are both saturated, so they operate as switches.

Fig 1 shows the circuit diagram.

As stated, the required delay is stored in EEPROM. The value is configured by starting the timer with L1 closed. The link is then opened for the desired time and reclosed. The period is measured, rounded and written to EEPROM. When the link is then opened, the circuit will restart with the newly configure delay time. Link L1 must be left open when configuration is complete, the timer will not operate normally with L1 closed.

Link L2 can be used to select between two timer ranges:

The timer can easily be programmed (and reprogrammed) in this way for say, 10s for 811A, 60s for 6146B, 180s for 4CX250B.

The MCU is used in internal RC oscillator mode, and the OSCCAL feature is used. Maximum error is about 10%, but usually much less. To some extent, the piece to piece variation is compensated by the method of measuring and storing the required delay time. The MCU goes in to SLEEP mode when the time expires, meaning that the oscillator ceases to run, reducing the risk of radiation of radio frequency energy.

Fig 2: Component side of Verobard prototype.


The MCU was socketed on the prototype to permit replacement for the purposes of firmware revision. Reliability is enhanced by soldering the chip directly to the board.


Fig 3: Copper side of Veroboard prototype.


Ameritron AL-811H

Fig 4: Prototype fitted to an Ameritron AL-811H.

Fig 4 shows the prototype fitted to an Ameritron AL-811H amplifier. The board is mounted on threaded pillars fixed to the chassis using countersunk screws.  The yellow wires were taken off the "XMT Opr" switch and connected to the timer input, and the timer output (white in the picture) connects to the "XMT Opr" switch in place of the yellow wires. The TR relay is operated from the output of the timer, and will not be enable until the timer expires. In this case, an additional green "READY" LED was fitted above the power switch and powered from the timer output (white) to provide a visual indication of when the amplifier is ready for transmission.

Interruptions to the power supply will reactivate the timer if the input voltage to the timer decays to about 2V. The low voltage DC supply in the AL-811H uses a full wave rectifier and filter capacitor of 2200μF with a load of 90mA for meter lamps, and 75mA for the TR relays when transmitting. The decay time in receive mode is about 0.5s, so supply interruptions greater than that will reactivate the timer. There is a trade-off in design of these power supplies, excessive filtering of the relay supply has the downside of sustaining TR relay operation too long after mains failure. The AL-811H might benefit from a smaller filter capacitor (eg 470μF), but I have not checked the impact of that, especially on speed of normal relay operation.

Frequently asked questions (FAQ)

Can the circuit be used at higher control circuit voltages?

The circuit was designed for nominal +12V control circuits, and will operate at up to +20V safely. To use it at higher voltages, the two transistors and their base resistors may need to be revised. For operation from +20V to +40V DC, the existing transistors are ok, the BC337 collector resistor and the 820Ω resistor should be doubled, and the 1k resistor in the collector of the BC337 should be increased to 3.3k.

Can a 16F84/84A MCU be used?

The code is written for a range of 12xxx family parts and will not run on a 16F84/84A. The 16F84/84A is larger, more expensive, and needs more external parts (lacking the Internal RC Oscillator capability) than the recommended MCUs.

Why does my timer never expire?

Is L1 closed? If the timer is started with L1 closed, it puts the MCU into configuration mode and will not expire until the configuration process is completed. If the configuration has been performed, remove power, open L1, and reapply power for normal operation.

Is the timer compatible with the Yaesu FL-2100Z?

Whilst it has not been tested, the timer should be compatible with the FL-2100Z. Use the timer to control the DC supply to the coil of Relay RL1 only.

Sourcing parts

This timer has been replaced by a new version based on an Atmel ATTiny25, see Heater Delay Timer V2. The chip must have the CLKDIV8 fuse turned OFF, and use the internal RC oscillator.

© Copyright: Owen Duffy 1995, 2021. All rights reserved. Disclaimer.