[ARC5] SCR-274N Dynamotor Question
Mark K3MSB
mark.k3msb at gmail.com
Sat Feb 5 10:36:04 EST 2022
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 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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