Modified Heising at high power for solid state RF decks

[steve_qix]

If one had say a 1KVA, 120VAC 1:1 input (Isolation) power transformer, it appears one could dispense with U801, T800, D802 through D805, and the separate grounds?

Great circuit BTW.

Phil - AC0OB

Right!  If you want to use a power transformer, this circuit is more suitable.  This is what most people use for pulse width modulation.

[steve_qix]

By the way, the circuit above shows 5 MOSFETs in the modulator.  For 45V @ 10A, only 3 MOSFETs and damper diodes are required.  The circuit as shown is fully populated, and will modulate up to a 1kw carrier.

[ka1tdq]

Here's the audio amp I'm planning on getting:

http://www.sweetwater.com/store/detail/XLI3500

Just to be sure, because I don't want to fry the amplifier... I can connect the audio output directly to the Heising capacitor once I power on the drain voltage first and let the capacitor charge, correct?

Jon

[w4bfs]

hi Jon .... that Crown amp is a switcher so the usual caveats about rf radiation and susceptability apply .... consider the end of xmission positive going spike generated by the modulation reactor and how to deal with it

[ka1tdq]

Good point.  I'll sequence both the input and the output of the amp.  Here's the events:

Event #1 - Antenna relay
Event #2 - Drain voltage
Event #3 - Audio amp output connection
Event #4 - Audio amp input connection

Jon

[WD5JKO]

Jon,

   That Crown looks nice. Since your load is around 4 ohms, does 1 channel of the Crown have enough scrote to fully modulate your class E final? Is there a way to parallel the two channels? The Spec sheet says the output is class AB, so the audio channels are linear. The power supply could very well be a switcher.

   I have an old Crown M600. It weighs around 100 pounds, and is rated at 600W / 8 ohms, 1 channel. If you want it, come and get it. (free, as-is)..It is too heavy for me to move.

The Crown M600 Audio Amplifier is a commercial quality amplifier. Frequency range of DC-45kHz with 600 watts (8 ohms) or DC-40kHz with 1000 watts (4 ohms). 0.05%THD.

Jim
Wd5JKO

[AB2EZ]

Jon

At the input of the Heising capacitor (where the output of the audio amplifier will connect), you should add a 25 ohm resistor to ground (wirewound is okay).

This resistor will consume some of the audio amplifier's audio output power .... i.e. about 100% x [1/25]/[(1/25) + (1/4)] = 13.8% of the amplifier's output power.

However, it is needed to provide a path to ground for the Heising capacitor to charge up through... when the audio amplifier is not connected to the Heising capacitor. Without it, there will be a voltage spike (equal to the DC drain supply voltage) across the audio amplifier's output when the switch between the Heising capacitor and the audio amplifier's output is closed.

Stu

[ka1tdq]

Thanks for the offer Jim, but the output per channel on this amp is over a KW.  It should be able to handle it.

And thanks for the tip Stu about the 25 ohm resistor.  I'm not an electronics genius (I copy a lot), but I was thinking about that voltage that would be felt at the amp's output terminals.  This makes sense.

I should be ordering all the big stuff in a couple weeks, so I'm hoping to have this on the air in about a month. 

Jon

[AB2EZ]

With respect to the rate of change in the current through the Heising reactor when the drain voltage DC supply is disconnected from the bottom of the Heising reactor:

A diode to ground, capable of handling a spike in forward current whose peak value is equal to the average drain current... e.g. a 1N5408... can be placed between the bottom of the Heising reactor (DC voltage input side) and ground.

With the DC supply connected, this diode will be reverse biased. When the DC supply is disconnected from the bottom of the Heising reactor... this diode will become (briefly) forward biased... and it will discharge the energy in magnetic field of the Heising reactor ...without producing an arc across the switch that disconnects the DC supply from the Heising reactor.

Stu

[ka1tdq]

Let me propose something different...

For my particular setup, I was planning on disconnecting the drain voltage after the Heising choke.  This would leave the Heising choke and capacitor energized all the time.  Any changes in modulation would be cut off before the drain voltage disconnects via a sequencer disconnecting the audio amp's input and output.

For this situation, wouldn't I then put the 1N5408 reverse biased on the output of the Heising choke (nearest the drain) to ground?  This would also quench an arc drawn on the relay disconnects.

During initial testing, I was using a standard household 15 amp house switch to disconnect the 48 volts.  It was in a regular blue plastic electrical box.  Every time I disconnected the voltage at 400 watts, there'd be a blue sizzle-lean like arc being drawn in the box.  Although, my hand making the disconnect is a lot slower than a relay.  

I ordered a hefty relay for this (regular, not a vacuum one).  

Jon

[AB2EZ]

Jon

The approach you describe would not prevent a discontinuity in the current through the Heising reactor. The diode would always be reverse biased (never conducting). Opening the switch between the Heising reactor/Heising capacitor connection point and the FET's drain (assuming the output of the audio amplifier has been disconnected) would cause a large voltage surge between that connection point and ground. I.e. a 10A x 25 ohm peak voltage surge across the 25 ohm resistor that is located on the audio amplifier side of the Heising capacitor and ground (assuming that the 25 ohm resistor is present). If the 25 ohm resistor is not present, opening the switch would produce destructive reverse breakdown of the reverse-biased diode and/or an arc across the switch.

Stu

[ka1tdq]

Ok, will do.

I'm glad we had this discussion because I just ran a pair of 10 guage DC cables from the closet to the transmitter. One for the drain supply and the other for the IXDD bus. I was just about to cut the drain supply line near the desk to insert the relay. Now, I'll incorporate it into the rack assembly in the closet and leave the cable runs untouched.

Jon