Technical Avionics Question

Is there any reason a 28volt MD41-1748 GPS Annunciator couldn't be used in a 14volt system if a 14v-to-28v step up transformer was used to supply current to the annunciator only? It appears that the GPS supplies only the ground to the lights involved and the push buttons are momentary contact only - which isn't a voltage issue.

Welcome to VAF!

Phil, welcome to VAF

DC voltages

Well, DC is more difficult to 'step up' than AC. So you would need some sort of active power supply to make 28vdc from 12vdc.

I would check the annunciator first. Perhaps the only difference is the lamps??

jdeas said:

Well, DC is more difficult to 'step up' than AC. So you would need some sort of active power supply to make 28vdc from 12vdc.

I would check the annunciator first. Perhaps the only difference is the lamps??

Click to expand...

This listed feature is probably the only other part, aside from the lamps, that is voltage dependent -

• Annunciator lighting is automatically dimmed by means of a photocell feedback circuit

As opposed to this feature -

• Push-button lighting level is controlled by the instrument lighting rheostat

14vdc to 28vdc up converter

OK, I did the same as to what you want to do. The 14v units are more in demand and that makes them more expensive, so I will give the straight scoop.

I have taken a 14v unit apart and looked at the guts with a maintenance manual.

First, the 28v units will use 28v light, they will work, but will be dimmer than the 14v lights.

The relays are 28v, and will switch over.

So they bottom line is, it will work. Mine did, but all I wanted was the button switches to get my KLN90 to work and the lights to come on.

Anyway, it will be a real pain to switch the lights and relays out if you need them, so I installed the following unit which is cheaper in compare to others. Since you are just going to run the lights and relays, the current draw is very low.

They are made in Germany so you know the quality is good. I am happy with the one I installed.

http://www.jaycar.us/productView.asp?ID=AA0264&CATID=83&form=CAT2&SUBCATID=1002#12

Good luck, and if you need any more info, PM me and I will see if I can help.

Brian

They do make voltage converters that will step from 12/14V to 24/28V. Check with an avionics shop and they can point you in the right direction or even sell you one if they have it in stock.

28 vs 14

If it was me, I would stick with the unit voltage appropriate to your system voltage.
Yes you can get 14 to 28v converters, but is the extra expense, wiring and complexity worth it?

if you use the 28 volt unit in a 14 volt airplane, yes the relays may switch, but will they switch consistently when the voltage may sag a bit like on approach with the engine idling. Will the annunciator lights be bright enough in the sunlight?

for my money, I would stick with the appropriate voltage unit.

Inexpensive DC to DC converters are available on eBay. Sometimes the current capacity of converters from China are exaggerated. The one linked above claims 10 amp capacity; so it should be good for 2 or 3 amps. Shipping takes about a month.
Joe Gores