[ARC5] SCR-274N Dynamotor Question
Mark K3MSB
mark.k3msb at gmail.com
Mon Feb 7 16:39:55 EST 2022
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 at 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 at comcast.net>
> Date: 2/5/22 7:20 PM (GMT-07:00)
> To: ARC-5 Mail List <arc5 at mailman.qth.net>
> Cc: ARC-5 List <arc5 at 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 at 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 <https://go.microsoft.com/fwlink/?LinkId=550986> for
> Windows
>
>
> *From: *J Mcvey via ARC5 <arc5 at mailman.qth.net>
> *Sent: *Saturday, February 5, 2022 9:32 AM
> *To: *Mark K3MSB <mark.k3msb at gmail.com>
> *Cc: *ARC-5 List <arc5 at mailman.qth.net>
> *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 at 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
>
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> On Sat, Feb 5, 2022 at 2:29 AM Robert Downs <wa5cab at 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 at mailman.qth.net [mailto:arc5-bounces at 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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