The International Cessna^® 170 Association

Hello,

The 170 is the first model I've been interested in which has a generator. I've been able to answer most of my questions by searching on the forums here, but I have a few more questions.

1. Do generators cause voltage spikes during engine start and shutdown, as alternators can?

2. Does a higher-output generator (say, rated at 35 A) produce incrementally higher output than a lower-output generator (25 A) at low RPM (e.g. when you're at 1000 RPM waiting to take off)?

3. For folks who have modern radios and LED lighting, what's a typical max electrical load? If you still have a generator, what RPM does your generator need to be to meet that load?

4. Same questions as (3) but with the addition of pitot heat?

Thanks!

Bruce

bjan11 wrote:Hello,

The 170 is the first model I've been interested in which has a generator. I've been able to answer most of my questions by searching on the forums here, but I have a few more questions.

1. Do generators cause voltage spikes during engine start and shutdown, as alternators can?

NO

2. Does a higher-output generator (say, rated at 35 A) produce incrementally higher output than a lower-output generator (25 A) at low RPM (e.g. when you're at 1000 RPM waiting to take off)?

NO

3. For folks who have modern radios and LED lighting, what's a typical max electrical load? If you still have a generator, what RPM does your generator need to be to meet that load?

1700 RPM

4. Same questions as (3) but with the addition of pitot heat?

I do not have pitot heat. This airplane is not approved for operation in icing conditions even if it did have it.

Thanks!

Bruce

addendum: As to Question #3: The “coming in” speed/rpm of generators varies individually but is around 1200 rpm. The paired regulator is adjusted for that also, …and the “cut in/out” relay disconnects the generator from the electrical system at approximately that rpm (voltage actually) so that the battery does not discharge into the generator. (Alternators accomplish this using rectifier-diodes.) This is why am ammeter often shows a sharp/sudden discharge at low rpm rather than a mild/increasing discharge. As for electrical-load: After conversion to LED land/taxi lamps my load dropped considerably when the gen is “off-line”. (I have dual Land/Taxi lamps and the change from four incandescent to four LED was dramatic. At pattern power-settings..typically 1700 RPM minimum on downwind while preparing to reduce speed and deploy flaps…at which point RPM is increased again due to the flap drag… there is never a discharge until after landing/roll-out.)
My estimated c electrical load is 22A with all lights On.

A previous discussion I made is at: viewtopic.php?f=24&t=9128&p=81178&hilit ... oad#p81178

Short periods of battery discharging…such as waiting for take-off…. or at low RPM in-flight….. are neither harmful nor distressing….but is acceptable. The battery will re-charge after a similar period of time above 1700 RPM (the approximate RPM of generator full-capacity). I am slightly amused at what I perceive is a common concern about this unimportant subject. Even while flying around in the pattern practicing take-offs and landings with the land/taxi lights on continuously….. is not a problem for a healthy battery. If your battery cannot support such operations…then it’s time for a new battery…and this is a good way to make that discovery.

If I am practicing in the pattern…and even if I’m making full stops and returning to another take-off…. it’s good practice ..(not to mention Courteous to others)….to turn off the LAND light and taxi using the TAXI lamp. If awiting takeoff in the run-up area….it’s not only discourteous…. it’s poor-technique to have the land/taxi lights ON…because it blinds and distracts other operators nearby and who may be landing/taking-off. When stationary..turn them Off. When moving…turn them On. IMO

Hope that helps.

Lots of great info, thanks. Sounds like upgrading to all-LEDs means the 25 A generator (which I believe is stock on B models) is plenty adequate, but the 35 A upgrade would be required for pitot heat.

Minor battery discharge during taxi doesn't worry me, the battery is designed to handle that. And it seems like Concorde is able to put more capacity into the same size box nowadays, so no worries there.

The only thing that surprises me is that I think you're saying the bigger generator does not generate more output at low RPM. I would assume the main internal difference for a bigger generator is more windings, and it seems like more windings would equal more current at any RPM. But perhaps my understanding is wrong.

Thanks again.

bjan11 wrote:Lots of great info, thanks. Sounds like upgrading to all-LEDs means the 25 A generator (which I believe is stock on B models) is plenty adequate, but the 35 A upgrade would be required for pitot heat.

Bruce,
A 4509 landing light bulb uses 8 amps, about the same as a heated pitot. Unless you’re planning on doing CAT 3 approaches when would you need pitot heat and landing lights on at the same time? If you buy a 1948-1952 C-170 you’ll have the “piano key” switch panel. They use double-throw (ON-OFF-ON) switches that are mechanically limited to being single-throw switches with a switch limit clip. By removing the clip on the landing light switch you could add a circuit for the pitot heat. Selecting the switch to the up position would give pitot heat and pressing the switch down would activate the landing lights.

My two cents on this , is if your 25 amp gen is doing a great job leave it . LEDS are the new way these days . LED landing lights last longer and I prefer the “color” they produce on the surface . Presently I run a 60 amp alternator and it’s an over kill and thinking about a 35 amp alternator . [or even a 25 amp alternator ] I now run a “Earth X “ lithium battery and have power to spare . Even at idle for a period there isn’t any issues.

I have a 12A generator and it is working fine. I did switch every light I could to LED before I started upgrading avionics. I will likely upgrade to either a 35A or an alternator when I do my overhaul, but that's just because the accessories get rebuilt at that time as well.

I do highly recommend a digital voltmeter on your panel so you know the system voltage (I have this one https://www.aircraftspruce.com/catalog/ ... -03691.php). And always check that your generator comes online during your runup.

I have a 20 amp generator on a 1954 B model with a Zeftronics G1200N.

The system has not been charging and I flashed the generator in accordance with Zeftronics flash procedure.

When I flashed the generator (engine off), it sounded as if the generator was turning.

My question is if that is normal and if not, is there a sheer pin in the connection to the drive that may be broken allowing the generator to rotate?

Also, an old tractor is generally running when we flash the generator. If I understand this correctly.

Here are some documents I got from Zeftronics when I was working a different issue a few years ago.

I did call them and spoke to an engineer that was helpful as well. I would recommend calling them if these files don't help.

Doug

BWPlowman wrote: ↑Mon Jun 09, 2025 3:19 am ............When I flashed the generator (engine off), it sounded as if the generator was turning........

BW have a look at the link listed below as it shows the generator drive coupling which is suspect if the generator is able to turn with the engine shut down. It is recommended to change the rubber part of the connector every 500 hours.

Outstanding. Thank you both!

A Generator…and an electric motor…are the same device. If you turn a generator…it will produce current. If you supply current to the same device..it will motor.

I tsounds as if you have a Connection-Problem. (The fact the generator motors ..indicates the gen is good. The problem is likely you have either an electrical connection error…or you have made a mistake in “flashing” the gen.

<edit in response to Mile’s comment following this post> Miles is correct, if the generator is installed then it should not “motor” if power is applied unless the coupling has failed.
Also, in order to “motor” a generator, power is applied to the Armature (large terminal) and, with the Field (small terminal) grounded…and ther motor-frame grounded…. a good generator will be a motor.

If power is only applied to the field, as is done when flashing the field, the generator shouldn’t spin. Spinning a generator requires power applied to the armature, which can only happen if you somehow bypass the reverse current protection function in the regulator. If the generator spins while installed on a non-running engine, the coupling is broken.