Car battery instead of Variac - Page 1
Forum section: electronics / rf-microwave

author: sesht
author_rank: Newbie
timestamp: December 28, 2023, 07:17:15 am
content: Hi,I have a couple of solid state 12V transceivers that haven’t been powered in 25+ years. I don’t have a variac to hand but I do have a couple of flat car batteries. Could I use one half-charged to power up the rigs, then (say) 3/4 charged, and then if all is well, using a 12v PSU? And if that works on receive, what’s the best approach for TX?

author: coppercone2
author_rank: Super Contributor
timestamp: December 28, 2023, 07:24:58 am
content: if you wanna do a slow power up maybe its better to put a current limiting resistor because if it fails on a car battery it will put out that current thats used to cold crank you cari think the short circuit current will most definatly be worse then a shorted reasonable size variac

author: Andy Chee
author_rank: Super Contributor
timestamp: December 28, 2023, 08:25:13 am
content: I'd just stick an inline fuse on it and go for it.The transceiver manual should tell you the rated fuse, but if you don't have a manual, try 5 amp fuse for transceivers up to 25W, 15 amp fuse for 50W radio.

author: Professzore
author_rank: Contributor
timestamp: December 28, 2023, 08:25:54 am
content: Hi,The question is, how many transceivers can be killed during this procedure? I bet, less is the best. As I usually do (with different electronics), I may follow this guide:find the RX power, and the accepted voltage range in the specs,design a simple circuit to produce the minimum RX power within the accepted DC range (a used car batterywith a current limiting resistor and a properly sized fusemay*be a good solution),test all the units for receive, also set them to the lowest transmit power,find the TX power in specs, modify the circuit to this requirement (this will be the hardest part), connect a proper dummy load, and try TX on all units.* To clarify this: a car (starter) battery, even when old and degraded, can easily handle currents way over 50 Amps at 12(ish) V DC. If there is any problem within the units, possibly they'll be instantly killed. That's why both the fuse and the resistor are needed.Without the specs, it will be a clear trial-and-error game with some possibility of loss. Not to mention, you must think way over the fundamentals of the ohms-law when designing the TX protection circuitry.On the other hand. Without question, by far the most useful tool for this purpose is abench PSU, best with not just a current limit setting, but with overcurrent protection (which shuts the output when the current exceeds the limit).

author: SeanB
author_rank: Super Contributor
timestamp: December 28, 2023, 08:29:03 am
content: Use a 55W car headlight bulb in series with the positive lead. Bulb will be low resistance when cold, and if there is a fault on power on it will glow brightly, but if it is either cold or very dim ( barely visible red glow in the dark, around 2V across it, then it is fine to bypass the lamp and apply full voltage. Have a 30A fuse (or whatever the transciever uses) in series as well, just in case something lets go.For transmit you just use a dummy load, as most transceivers typically always apply power to the final stage transistors, just doing the PTT using switching of the drive, though if you have one that does not use 2 55W lamps in parallel, same fuse, and on transmit at low power setting they should be barely lit, with 5A of current flow and cold, they will slowly warm up over a second or so, enough to show the TX stage does not go bang. Then you bypass the lamps, and test at full power.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: December 28, 2023, 02:06:55 pm
content: I agree with ProfesszoreFor powering up old radios I use a PS with an overload protection and an amp meter. I watch the amp meter when I turn the radio on.If I am being very careful, I use a Lab grade PS and start with 14 Volts without load then Zero amps and turn up the amps slowly.If you do not have one, then you can use a mulltimeter in amp setting between the radio and the battery.Along with a low amp fuse, say 1-2 amps. for RX. The radio should not draw much amperage in Rx mode. If it does (look at the spec sheet) then you have a problem.Or just use the low amp fuse for RXwithout the multimeter. If the radio works on RX then good....You can then put in the recommended fuse for Tx testing and test that.Be careful with your multimeter, it may not tolerate high amps for very long.EDIT: Variacs are used on radios that have an AC line voltage to DC 14 volt PS built in to them. Or old tube type radios. You said your radios are 12 volt. Variac will not work.

author: Randy222
author_rank: Frequent Contributor
timestamp: December 28, 2023, 05:25:14 pm
content: Do solid state devices like to have lower than expected voltages applied? Like a FET that turns on in non-linear region?

author: p.larner
author_rank: Frequent Contributor
timestamp: December 30, 2023, 11:28:18 am
content: good point on the fets going into linear mode,why not use a dim bulb type tester on the line in of a linear psu and try that,maybe a few different wattage bulbs that can be switched in,only thing to have issues is teardrop tants i would guess.