Hi Everyone

Some further comments....

I removed the in-rush resistor and it made a 2V difference.   Key up 28V  Key down 26V (as opposed to 24V key down before).   I plan to upgrade the wiring to 10 or 12 guage.

The filter capacitor didn't care about the lack of an in-rush resistor, so it's permanently gone.  

I ran my SCR-274N transmitters yesterday in the Classic Exchange on both 40 and 80M.  Power out was 40W on 40M and 50W on 80M.   That's more than I've gotten before, so even with 26V key down it's an improvement.   I cheated for the receivers.... I used my R-390A.   Running stock 274N receivers isn't fun on the ears....been there, done that,  have a LOT of the T-Shirts.....

Every now and again it's fun running with dynamotors, but I typically don't.   I just insert 550V and 28V into the plug on the BC-456 modulator and that works nicely.   Quieter too....

I plan to add a regulator to my 28V supply; just have to find some suitable NPN transistors that will handle the current.

Scott, I read your recent post on using a float power supply.  I did try, but even with fresh batteries I never got more than 24V key down from 2 batteries in series while getting close to 28V key up.   So, I'm still interested in this approach, but I'd need some detailed info on how you have yours set up.    I just have a float supply across 2 tractor batteries in series.

73 Mark K3MSB











Float Power Supply: 

This is what I'm using now, and it's not working out well.   I measure 26.8V power off. 

Power On Key Up:  23.8V    Power On Key Down:  23.5V.   This gives me an Ep of only 480V.

The above voltages are measured directly on the battery terminals.


On Mon, Feb 7, 2022 at 12:43 PM scottjohnson1 <scottjohnson1@cox.net> wrote:
I will reiterate what I said early on.  A wet or gelled battery will do the same thing better and cheaper. The biggest advantage is the lack of a huge surge attendant with charging a big bank of DLCs or electrolytic.  The floating battery provides a low impedance to the load, and more accurately represents the environment thein which equipment was designed to operate.  

Scott W7SVJ



Sent from my Verizon, Samsung Galaxy smartphone


-------- Original message --------
From: Doran Platt <jeepp@comcast.net>
Date: 2/5/22 7:20 PM (GMT-07:00)
To: ARC-5 Mail List <arc5@mailman.qth.net>
Cc: ARC-5 List <arc5@mailman.qth.net>
Subject: Re: [ARC5] SCR-274N Dynamotor Question

Have you considered a super capacitor bank and appropriate pre-charge circuit. They are great to flatten inrush currents.
On 02/05/2022 1:36 PM howard holden <holden7471@msn.com> wrote:


Mark, besides the wire losses, that 0.5 ohm resistor should also be shunted when you reach running speed. At the additional 3.5A under load, that’s 1.75V drop. Also make sure all your connections are tight, any connectors both crimped and soldered. Check for both AC and DC losses at every point in the path to the DM from the wall socket. You may be surprised at the many points of loss.


Howie WB2AWQ


Sent from Mail for Windows


From: J Mcvey via ARC5
Sent: Saturday, February 5, 2022 9:32 AM
To: Mark K3MSB
Cc: ARC-5 List
Subject: Re: [ARC5] SCR-274N Dynamotor Question


You have a lot of losses along the way. Increasing the wire thickness will help a bit.

 

A 28-0-28 full wave rectification should produce 39.6V ( less the diode drop) at full input power. The variac may be contributing to the sag as well.

A regulated supply would be the optimal solution.

My 50A is a switch mode power supply that was designed for a military radio rack , so it has RFI filters.

EP is the 590V it should be with 28V at the supply end.

 

On Saturday, February 5, 2022, 10:36:29 AM EST, Mark K3MSB <mark.k3msb@gmail.com> wrote:

 

 

GM Gents

 

Your comments are much appreciated.   I will attempt to answer all questions in a single response.

 

Dynamotor rebuild:  Yes, I rebuilt it when I put the dynamotor into service about 10 years ago.

All caps were replaced in the BC-456 modulator.

 

Float Power Supply: 

This is what I'm using now, and it's not working out well.   I measure 26.8V power off. 

Power On Key Up:  23.8V    Power On Key Down:  23.5V.   This gives me an Ep of only 480V.

The above voltages are measured directly on the battery terminals.

 

New Power Supply:

 

Wire Sizes:  This could be an issue and I realized it early on.  I'm using 18 AWG doubled up. 

That means each connection uses two wires in parallel. 

Not optimal, but a single 18 AWG wire wouldn't do.

Changing to a larger wire size isn't a problem (I just have to get some).

The dynamotor is spec'd at 5.8 A @ 28VDC so I figured paralleled 18 gauge wire should work.

 

28-0-28V 50A.   Using a full wave bridge rectifier.   Positive lead goes to 0.5 ohm resistor for current inrush.

That's connected to a 20,000 uF filter capacitor.  Using a 680 ohm 2W bleeder resistor.

I would prefer to use a 1 ohm, but didn't have one of sufficient wattage.

 

Using a IM-11 VTVM for all the following measurements (unless otherwise indicated)

I have panel meters for voltage and current in circuit -- voltages agree with VTVM readings.

 

With the Panel Meter set for 28 VDC:

Voltages / current measured at the filter capacitor terminals:

Key up:  28V    4A

Key Down:  24V  7.5 A.   510V Ep   30W output

 

Voltages measured at the bridge rectifier output:

Key up:  29 V DC

Key Down:  26 VDC

 

Voltages measured at the transformer output (using DMM)

Key up:  28.34 VAC

Key Down:  27.41 VAC 

 

If I increase the voltage to 30 VDC measured at the filter capacitor output:

Key up:  30V  4A

Key Down:  25V  7.5A    Ep 550V (which is what it should be)  40W output

 

Current spike on startup: Yup it is !!   As expected.

 

73 Mark K3MSB

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

On Sat, Feb 5, 2022 at 2:29 AM Robert Downs <wa5cab@cs.com> wrote:

No.  Set it for 28VDC key up.  32 VDC would put 16 VDC on each of the 12.6 V rated tube filaments which will significantly shorten their service life

 

Robert Downs

 

From: arc5-bounces@mailman.qth.net [mailto:arc5-bounces@mailman.qth.net] On Behalf Of Mark K3MSB
Sent: Friday, February 4, 2022 19:38
To: ARC5
Subject: [ARC5] SCR-274N Dynamotor Question

 

I'm working on a new 28V power supply for my SCR-274N system as well as other projects.

I'm using a 28-0-28 V 50A Transformer with a bridge rectifier (so I'm not using the CT).   I'm using a 220V 16A Variac to set the primary voltage.

When the setting set for 28V key up,  I'm getting about a 4 to 5 volt drop key down.    

I didn't expect that much voltage drop as the transformer is pretty stout for an SCR-274N.

Can I run the system on 30 to 32V key up without damaging the dynamotor?  

This would be a short term fix as I suspect I'll need a VR circuit.

73 Mark K3MSB

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